Skating - 1060.bloomfieldweb.com

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Skating

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Observations about Skating

When you’re at rest on a level surface, without a push, you remain stationary

with a push, you start moving that direction

When you’re moving on a level surface, without a push, you coast steady & straight

with a push, you change direction or speed

5 Questions about Skating

1. Why does a motionless skater tend to remain motionless?

2. Why does a moving skater tend to continue moving?

3. How can we describe the motion of a coasting skater?

4. How does a skater start, stop, or turn?

5. Why does a skater need ice or wheels in order to skate?

Question 1

Q: Why does a motionless skater tend to remain motionless?

A: A body at rest tends to remain at rest

This observed behavior is known as inertia

Question 2

Q: Why does a moving skater tend to continue moving?

A: A body in motion tends to remain in motion

This behavior is the second half of inertia

Newton’s First Law (Version 1)

An object that is free of external influences moves in a straight line and covers equal distances in equal times.

Note that a motionless object obeys this law!

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Question 3

Q: How can we describe the motion of a coasting skater?

A: The skater moves at a constant speed in a constant direction

Physical Quantities

1. Position – an object’s location

2. Velocity – its change in position with time

Both are vector quantities: Position is distance and direction from a reference

Velocity is speed and direction of motion, relative to a reference

Newton’s First Law (Version 2)

An object that is free of external influences moves at a constant velocity.

Note that a motionless object is “moving” ata constant velocity of zero!

Another Physical Quantity

3. Force – a push or a pull

Force is another vector quantity: the amount and direction of the push or pull

Net force is the vector sum of all forces on an object

Newton’s First Law

An object that is not subject to any outside forces moves at a constant velocity.

Question 4

Q: How does a skater start or stop moving?

A: A net force causes the skater to accelerate!

4. Acceleration – change in velocity with time

5. Mass – measure of object’s inertia

Acceleration is yet another vector quantity: the rate and direction of the change in velocity

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Newton’s Second Law

An object’s acceleration is equal to the net force exert on it divided by its mass. That acceleration is in the same direction as the net force.

Traditional form: net force = mass acceleration

F = ma

Cause

Effect Resistance to cause

About Units

SI or “metric” units: Position → m (meters)

Velocity → m/s (meters-per-second)

Acceleration → m/s2 (meters-per-second2)

Force → N (newtons)

Mass → kg (kilograms)

Newton’s second law relates the units:

Question 5

Q: Why does a skater need ice or wheels to skate?

A: Real-world complications usually mask inertia

Solution: minimize or overwhelm complications

To observe inertia, therefore, work on level ground (minimize gravity’s effects)

use wheels, ice, or air support (minimize friction)

work fast (overwhelm friction and air resistance)

Summary about Skating

Skates can free you from external forces

When you experience no external forces, You coast – you move at constant velocity

If you’re at rest, you remain at rest

If you’re moving, you move steadily and straight

When you experience external forces You accelerate – you move at a changing velocity

Acceleration depends on force and mass

Falling Balls


Observations about Falling Balls

When you drop a ball, it begins at rest, but acquires downward speed

covers more and more distance each second

When you tossed a ball straight up, it rises to a certain height

comes momentarily to a stop

and then descends, much like a dropped ball

A thrown ball travels in an arc

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6 Questions about Falling Balls

1. Why does a dropped ball fall downward?

2. How differently do different balls fall?

3. How would a ball fall on the moon?

4. How does a falling ball move after it is dropped?

5. How can a ball move upward and still be falling?

6. How does a ball's horizontal motion affect its fall?

Question 1

Q: Why does a dropped ball fall downward?

A: Earth’s gravity exerts a downward force on the ball

That force is the ball’s weight

That weight points toward earth’s center

Its weight causes the falling ball to accelerate downward—toward earth’s center

Question 2

Q: How differently do different balls fall?

A: Not differently. They all fall together!

A ball’s weight is proportional to its mass:

Question 2

Q: How differently do different balls fall?

A: Not differently. They all fall together!

A ball’s weight is proportional to its mass:

That ratio is equivalent to an acceleration:

Acceleration Due to Gravity

That ratio is the acceleration of a falling object!

It is called the acceleration due to gravity

Near earth’s surface, all falling balls accelerate downward at9.8 meter/second2

Question 3

Q: How would a ball fall on the moon?

A: It would fall more slowly.

Moon’s acceleration due to gravity is

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Question 4

Q: How does a falling ball move after it is dropped?

A: It accelerates downward, covering more distance each second

A falling ball experiences only its weight Its acceleration is constant and downward

Its velocity increases in the downward direction

When dropped from rest, the ball’s velocity starts at zero and

increases in the downward direction

the ball’s altitude decreases atan ever faster rate

Question 5

Q: How can a ball move upward and still be falling?

A: It may be moving upward, but it is still accelerating downward!

A falling ball accelerates downward, but its

initial velocity can be anything, even upward!

When thrown upward, ball’s velocity starts upward but

increases downward

ball’s altitude increases at an ever slower rate until…

velocity is momentarily zero

and then ball falls downward…

Question 6

Q: How does a ball's horizontal motion affect its fall?

A: It doesn’t. The ball falls vertically, but coasts horizontally.

Ball’s acceleration ispurely vertical (downward)

It falls vertically

It coasts horizontally

Its path is a parabolic arc

Summary About Falling Balls

Without gravity, an isolated ball would coast

With gravity, an isolated ball experiences its weight,

accelerates downward,

and its velocity becomes increasingly downward

Whether going up or down, it’s still falling

It can coast horizontally while falling vertically

Ramps


Observations About Ramps

It’s difficult to lift a heavy wagon straight up

It’s easer to push a heavy wagon up a ramp

How hard you must push depends on the ramp’s steepness

The gentler the slope of the ramp, the smaller the push you must exert on the wagon

the longer the distance you must push the wagon to raise it upward

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5 Questions about Ramps

1. Why doesn’t a wagon fall through a sidewalk?

2. Why does a sidewalk perfectly support a wagon?

3. How does a wagon move as you let it roll freely on a ramp?

4. Why is it harder to lift a wagon up than to lower a wagon down?

5. Why is it easier to pull a wagon up a ramp than to lift it up a ladder?

Question 1

Q: Why doesn’t a wagon fall through a sidewalk?

A: The sidewalk pushes up on it and supports it.

The sidewalk and the wagon cannot occupy the same space

The sidewalk exerts a support force on the wagon that prevents the wagon from penetrating the sidewalk’s surface

acts perpendicular to the sidewalk’s surface: it is directed straight up

balances wagon’s downward weight

Question 2

Q: Why does a sidewalk perfectly support a wagon?

A: The sidewalk and wagon negotiate by denting and undenting.

The wagon and sidewalk dent one another slightly the more they dent, the harder they push apart

the wagon accelerates up or down in response to net force

the wagon bounces up and down during this negotiation

The wagon comes to rest at equilibrium—zero net force

The Wagon and Sidewalk Negotiate

If the wagon and sidewalk dent one another too much, each one pushes the other away strongly

the wagon accelerates away from the sidewalk

If the wagon and sidewalk dent one another too little, each one pushes the other away weakly (or not at all)

the wagon accelerates toward the sidewalk

If the wagon and sidewalk dent one another just right, the wagon is in equilibrium (zero net force)

Newton’s Third Law

For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object.

Misconception Alert

The forces two objects exert on one another must be equal and opposite, but each force of that Newton’s third law pair is exerted on a different object, so those forces do not cancel one another.

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Question 3

Q: How does a wagon move as you let it roll freely on a ramp?

A: The wagon accelerates downhill.

The wagon experiences two forces: its weight and a support force

The sum of those forces is the ramp force: a small downhill net force

weight

support force

ramp force (vector sum)

Pushing the wagon up the Ramp

To start the wagon moving uphill push wagon uphill more than the downhill ramp force

net force is uphill, so wagon accelerates uphill

To keep the wagon moving uphill push wagon uphill just enough to balance ramp force

wagon continues uphill at constant velocity

To stop the wagon moving uphill, push wagon uphill less than the downhill ramp force

net force is downhill, so wagon accelerates downhill

Question 4

Q: Why is it harder to lift a wagon up than to lower a wagon down?

A: You do work on the wagon when you lift it.The wagon does work on you when you lower it.

Energy – a conserved quantity it can’t be created or destroyed

it can be transformed or transferred between objects

is the capacity to do work

Work – mechanical means of transferring energy

work = force · distance

(where force and distance are in same direction)

Transfers of Energy

Energy has two principal forms Kinetic energy – energy of motion

Potential energy – energy stored in forces

Your work transfers energy from you to the wagon Your chemical potential energy decreases

wagon’s gravitational potential energy increases

Question 5

Q: Why is it easier to pull a wagon up a ramp than to lift it up a ladder?

A: On the ramp, you do work with a small force over a long distance. On the ladder, you do work with a large force over a small distance.

For a shallow ramp: work = Force · DistanceFor a steep ramp: work = Force · Distance

For a ladder: work = Force · Distance

Mechanical Advantage

Mechanical advantage is doing the same work, using a different balance of force and distance

A ramp provides mechanical advantage You lift wagon with less force but more distance

Your work is independent of the ramp’s steepness

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Summary about Ramps

Ramp reduces the force you must exert to lift the wagon

Ramp increases the distance you must push to lift the wagon

You do work pushing the wagon up the ramp

The ramp provides mechanical advantage It allows you to push less hard

but you must push for a longer distance

Your work is independent of ramp’s steepness

Seesaws


Observations about Seesaws

A balanced seesaw rocks back and forth easily

Equal-weight children sitting on the seats balance a seesaw

Unequal-weight children don’t normally balance the seesaw

Moving the heavier child toward the pivot restores the balance

Moving both children closer to the pivot speeds up the motion

6 Questions about Seesaws

1. How does a balanced seesaw move?

2. Why does a seesaw need a pivot?

3. Why does a lone seesaw rider plummet to the ground?

4. Why do the riders' weights and positions affect the seesaw's motion?

5. Why do the riders' distances from the pivot affect the seesaw's responsiveness?

6. How do the seesaw's riders affect one another?

Question 1

Q: How does a balanced seesaw move?

A: It moves at a constant rotational speed about a fixed axis in space

…because the seesaw has rotational inertia!

Physical Quantities

1. Angular Position – an object’s orientation

2. Angular Velocity – change in angular position with time

3. Torque – a twist or spin

All three are vector quantities Angular Position is the angle and rotation axis, relative to a reference

Angular Velocity is angular speed and rotation axis, relative to a reference

Torque is amount and rotation axis of a twist or spin

Net torque is the vector sum of all torques on an object

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Newton’s First Law of Rotational Motion

A rigid object that’s not wobbling and that is free of outside torques rotates at constant angular velocity.

Question 2

Q: Why does a seesaw need a pivot?

A: To prevent translational motion (i.e., falling)

The pivot is placed at the seesaw’s natural pivot, its center of mass

The pivot allows rotational motion, but not translation motional Translational motion is a change in position

Rotational motion is a change in angular position

Question 3

Q: Why does a lone seesaw rider plummet to the ground?

A: That rider’s weight twists the seesaw and thereby alters its motion

The weight of a lone rider produces a torque on the seesaw

torque = lever arm · forceperp(where the lever arm is a vector from the pivot to the location of the force)

That torque causes the seesaw to experience angular acceleration

4. Angular Acceleration – change in angular velocity with time another vector quantity

the rate and rotation axis of the change in angular velocity

Newton’s Second Lawof Rotational Motion

An object’s angular acceleration is equal to the net torque exerted on it divided by its rotational mass. The angular acceleration is in the same direction as the torque.

5. Rotational Mass – measure of rotational inertia

Question 4

Q: Why do the riders' weights and positions affect the seesaw's motion?

A: To balance the seesaw, their torques on it must sum to zero.

Adding a second rider adds a second torque The two torques act in opposite directions

If net torque due to gravity is zero, seesaw is balanced

A rider’s torque is their weight times their lever arm A heavier rider should sit closer to the pivot

A lighter rider should sit farther from the pivot

To cause angular acceleration, a rider leans or touches the ground

Question 5

Q: Why do the riders' distances from the pivot affect the seesaw's responsiveness?

A: Moving the riders toward the pivot reduces the seesaw’s rotational mass more than it reduces the gravitational torque on the seesaw

Lever arm is a vector from the pivot to the rider Gravitation torque is proportional to lever arm

Rotational mass is proportional to lever arm2

Angular acceleration is proportional to 1/lever arm

Moving the riders toward the pivot increases angular acceleration

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Question 6

Q: How do the seesaw's riders affect one another?

A: They exert torques on one another and also exchange energy.

Each rider experiences two torques about pivot: A gravitational torque produced by that rider’s weight

A torque exert by the other rider

For a balanced seesaw, the torques on each rider sum to zero

The riders exert equal but opposite torques on one another

Newton’s third law of rotational motion

For every torque that one object exerts on a second, there is an equal but oppositely directed torque that the second object exerts on the first.

Summary about Seesaws

A balanced seesaw experiences zero net torque

A balanced seesaw has a constant angular velocity

A non-zero net torque causes angular acceleration of the seesaw

Heavier riders need smaller lever arms

Lighter riders need larger lever arms

Wheels


Observations about Wheels

Friction makes wheel-less objects skid to a stop

Friction can waste energy and cause wear

Wheels mitigate the effects of friction

Wheels can also propel vehicles

6 Questions about Wheels

1. Why does a wagon need wheels?

2. Why is sliding a box across the floor usually hardest at the start?

3. How is energy wasted as a box skids to a stop?

4. How do wheels help a wagon coast?

5. How do powered wheels propel a bicycle or car forward

6. How is energy present in a wheel?

Question 1

Q: Why does a wagon need wheels?

A: Friction opposes a wheel-less wagon’s motion

Frictional forces oppose relative sliding motion of two surfaces

act along the surfaces to bring them to one velocity

come in Newton’s third law pairs

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Question 2

Q: Why is sliding a box across the floor usually hardest at the start?

A: Static friction is usually stronger than sliding friction.

Static friction opposes the start of sliding varies in amount from zero to a maximum value

Sliding friction opposes ongoing sliding has a constant value proportional to support force

Static friction’s maximum usually exceeds sliding friction

Question 3

Q: How is energy wasted as a box skids to a stop?

A: That energy becomes thermal energy.

Only sliding friction wastes energy The two surfaces travel different distances

The missing work becomes thermal energy

The surfaces also experience wear

The Many Forms of Energy

Kinetic: energy of motion

Potential: stored in forces between objects Gravitational

Magnetic

Electrochemical

Nuclear

Thermal energy: disorder into tiny fragments Reassembling thermal energy is statistical impossible

Elastic

Electric

Chemical

Question 4

Q: How do wheels help a wagon coast?

A: Wheels can eliminate sliding friction.

Wheels & roller bearings eliminate sliding friction rollers eliminate sliding friction, but don’t recycle

simple wheels have sliding friction at their hub/axle

combining roller bearings with wheels is ideal

Question 5

Q: How do powered wheels propel a bicycle or car forward?

A: They use static friction to obtain a forward force from the ground.

As you or an engine exert torque on a powered wheel static friction from the ground produces an opposing torque

The two torques partially cancel, reducing the wheel’s angular acceleration

The ground’s static frictional force pushes the vehicle forward

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Practical Wheels

Free wheels are turned by the vehicle’s motion

Powered wheels propel the vehicle as they turn.

Question 6

Q: How is energy present in a wheel?

A: Kinetic energy, both translation and rotational.

For a translating wheel:

For a rotating wheel:

The wheel of a moving vehicle has both forms of kinetic energy!

Summary about Wheels

Sliding friction wastes energy Wheels eliminate sliding friction

A vehicle with wheels coasts well

Free wheels are turned by static friction

Powered wheels use static friction to propel car

Bumper Cars


Observations about Bumper Cars

Moving cars tend to stay moving

Changing a car’s motion takes time

Impacts alter velocities and angular velocities

Cars often appear to exchange their motions

The fullest cars are the hardest to redirect

The least-full cars get slammed during collisions

5 Questions about Bumper Cars

1. Does a moving bumper car carry a force?

2. How is momentum transferred from one bumper car to another?

3. Does a spinning bumper car carry a torque?

4. How is angular momentum transferred from one bumper car to another?

5. How does a bumper car move on an uneven floor?

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Question 1

Q: Does a moving bumper car carry a force?

A: No, the bumper car carries momentum.

Momentum is a conserved vector quantity cannot be created or destroyed, but it can be transferred

has a direction (unlike energy)

has no potential form and cannot be hidden (unlike energy)

combines bumper car’s inertia and velocity

momentum = mass · velocity

Question 2

Q: How is momentum transferred from one bumper car to another?

A: The bumper cars push on one another for a period of time.

Bumper cars exchange momentum via impulses

impulse = force · time

When car1 gives an impulse to car2, car2 gives an equal but oppositely directed impulse to car1.

Individual momenta change as the result of an impulse

The total momentum doesn’t change

Car with least mass changes velocity most, so the littlest riders get pounded

Question 3

Q: Does a spinning bumper car carry a torque?

A: No, the bumper car carries angular momentum

Angular momentum is a conserved vector quantity cannot be created or destroyed, but it can be transferred

has a direction (unlike energy)

has no potential form and cannot be hidden (unlike energy)

combines bumper car’s rotational inertia and velocity

angular momentum = rotational mass· angular velocity

Question 4

Q: How is angular momentum transferred from one bumper car to another?

A: The bumper cars twist one another for a period of time.

Bumper cars exchange angular momentum via angular impulsesangular impulse = torque · time

When car1 gives an angular impulse to car2, car2 gives an equal but oppositely directed angular impulse to car1.

Individual angular momenta change as the result of an angular impulse

The total angular momentum doesn’t change

Car with least rotational mass changes angular velocity most.

Rotational Mass can Change

Mass can’t change, so the only way an object’s velocity can change is if its momentum changes

Rotational mass can change, so an object that changes shape can change its angular velocity without changing its angular momentum

Question 5

Q: How does a bumper car move on an uneven floor?

A: It accelerates in the direction that reduces its potential energy as quickly as possible

Forces and potential energies are related! A bumper car accelerates in the direction that reduces its total potential

energy as quickly as possible

The bumper car accelerates opposite to the potential energy gradient

On an uneven floor, that is down the steepest slope

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Summary about Bumper Cars

During collisions, bumper cars exchange momentum via impulses

angular momentum via angular impulses

Collisions have less effect on cars with large masses

cars with large rotational masses

Static Electricity


Observations aboutStatic Electricity

Objects can accumulate static electricity

Clothes in the dryer often develop static charge

Objects with static charge may cling or repel

Static electricity can cause shocks

Static electricity can make your hair stand up

6 Questions aboutStatic Electricity

1. Why do some clothes cling while others repel?

2. Why do clothes normally neither cling nor repel?

3. Why does distance weaken static effects?

4. Why do clingy clothes stick to uncharged walls?

5. Why do clingy clothes crackle as they separate?

6. Why do some things lose their charge quickly?

Question 1

Q: Why do some clothes cling while others repel?

A: They carry electric charges that attract or repel.

Electric charges comes in two types: Charges of the same type (“like charges”) repel

Charges of different types (“opposite charges”) attract

Franklin named the types “positive” and “negative”

Charge is actually a single conserved quantity

Question 2

Q: Why do clothes normally neither cling nor repel?

A: Clothes normally have zero net charge.

Electric charge is intrinsic to some subatomic particles is quantized in multiples of the fundamental charge is +1 f.c. for a proton, –1 f.c. for an electron

Equal protons and electrons → zero net charge Objects tend to have zero net charge → neutral

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Charge Transfers

Contact can transfer electrons between objects Surfaces have different chemical affinities for electrons

One surface can steal electrons for another surface

Rubbing different objects together ensures excellent contact between their surfaces

significant charge transfer from one to the other.

A dryer charges clothes via these effects

Question 3

Q: Why does distance weaken static effects?

A: Forces between charges decrease as 1/distance2.

These electrostatic forces obey Coulomb’s law:

Electric charge is measured in coulombs

One fundamental charge is 1.6 10-19 coulombs

Question 4

Q: Why do clingy clothes stick to uncharged walls?

A: The charged clothes polarize the wall.

When a negatively charged sock nears the wall, the wall’s positive charges shift toward the sock,

the wall’s negative charges shift away from it,

and the wall becomes electrically polarized.

Opposite charges are nearest → attraction dominates

Question 5

Q: Why do clingy clothes crackle as they separate?

A: Separating opposite charges boosts voltages.

Charge has electrostatic potential energy (EPE)

Voltage measures the EPE per unit of charge Work raises the voltage of positive charge

Work lowers the voltage of negative charge

Voltage is measured in volts (joules/coulomb)

Question 6

Q: Why do some things lose their charge quickly?

A: Charge can escape through electric conductors.

Insulators have no mobile electric charges

Conductors have mobile electric charges, usually electrons (metals), but can be ions (salt water)

that will accelerate (and flow) toward lowest EPE

Conductors allow charges to cancel or escape

Summary aboutStatic Electricity

All objects contain countless charges

Objects can transfer charge during contact

Clothes often develop net charges during drying

Oppositely charged clothes cling to one another

and spark as separation raises their voltages.

Conductivity tends to let objects neutralize.

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Xerographic Copiers


Observations About Copiers

Copiers consume colored powder or “toner”

After jams, you can wipe off the powder images

Copies are often warm after being made

Copies are sometimes clingy with static electricity

3 Questions aboutXerographic Copiers

1. How can light arrange colored powder on paper?

2. How does a copier spray charge onto a surface?

3. How does a copier make its copies permanent?

Question 1

Q: How can light arrange colored powder on paper?

A: That light can control static electricity.

In a xerographic copier or printer, charge sprayed onto insulating layer

opposite charge flows onto layer’s back

layer acts as a charged capacitor

light selectively erases charge

remaining charge attracts toner particles

toner particles are bonded to paper

Question 2

Q: How does a copier spray charge onto a surface?

A: It uses a corona discharge to charge the air

A fine wire having a large voltage ( or ) is covered with tightly packed “like” charges ( or )

repulsive forces are intense and push charges into air

this flow of charge into the air is a corona discharge

That discharge is caused by a strong electric field

Electric Field

Two views of electrostatic forces: Charge1 pushes on Charge2

Charge1 creates electric field that pushes Charge2

Electric field isn’t a fiction; it actually exists! a structure in space and time that pushes on charge

a vector field: a vector at each point in space and time

observed using a test charge at each point

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Voltage Gradient

A + test charge accelerates along electric field

path that reduces its total potential energy quickest

Voltage is electrostatic potential energy per charge decreasing voltage is decreasing potential energy

path of quickest potential decrease is voltage gradient

voltage gradient is essentially a slope of voltage

A voltage gradient is an electric field

Metals, Fields, & Corona Discharges

Inside a metal, charge can move At equilibrium: voltage is uniform, electric field is zero

Charge resides only on the metal’s surface

Outside a metal, charge cannot move At equilibrium: both voltage and electric field can vary

Near a thin wire or sharp point at large voltage, voltage varies rapidly with distance, so big electric field

charge is pushed into the air: a corona discharge

Question 3

Q: How does a copier make its copies permanent?

A: It fuses or melts the powder onto the paper.

Summary aboutXerographic Copiers

It sprays charge from a corona discharge

That charge precoats a special insulating surface

It projects a light onto surface

The charge escapes from illuminated regions

The remaining charge attracts toner particles

Those particles are fused to the paper as a copy

Flashlights


Observations about Flashlights

They emit light when you switch them on

Brighter flashlights often have more batteries

Flashlights grow dimmer as their batteries age

Sometimes hitting a flashlight brightens it

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6 Questions about Flashlights

1. Why does a flashlight need batteries and a bulb?

2. How does power flow from batteries to bulb?

3. How does a flashlight’s switch turn it on or off?

4. How can a battery be recharged?

5. Why does a short-circuited flashlight get hot?

6. How do lightbulbs differ?

Question 1

Q: Why does a flashlight need batteries and a bulb?

A: Batteries power the bulb, which then emits light.

Batteries: chemical energy → electrostatic energy Bulb: electrostatic energy → light energy Since this energy transfer is ongoing, think power

Power is energy per unit of time The SI unit of power: 1 watt is 1 joule/second

Battery

Battery is chemically powered pump for charge pumps charge from its end to end develops a voltage rise from end to end

typically 1.5 volts for alkaline AAA, AA, C, and D cells typically 3 volts for lithium cells

does work pushing charge to higher voltage turns chemical potential into electrostatic potential

In a typical flashlight with two alkaline-cells total voltage rise in the battery chain is 3.0 V total electric power provided by batteries is 6 watts

Lightbulb Filament

Lightbulb filament lets charges flow through it carries charge from its entry end to its exit end develops a voltage drop from entry end to exit end receives energy from charge flowing to lower voltage turns electrostatic potential into thermal energy

In a typical flashlight with two alkaline-cells total voltage drop in lightbulb filament is 3.0 V total electric power consumed by lightbulb is 6 watts

Question 2

Q: How does power flow from batteries to bulb?

A: Power is carried by a current of charge in wires.

Current measures the rate of charge motion, electric charge crossing a boundary per unit of time,

The SI unit of current: 1 ampere is 1 coulomb/second

Batteries provide power to electric currents

Lightbulbs consume power from electric currents

The Direction of Current

Current is defined as the flow of positive charge but negative charges (electrons) carry most currents.

It’s difficult to distinguish between: Negative charges flowing to the right

Positive charges flowing to the left.

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Electric Current in a Flashlight

Current in the flashlight pumped from low voltage to high voltage by batteries

power provided = current · voltage rise

flows through a wire to the lightbulb

flows from high voltage to low voltage in lightbulbpower consumed = current · voltage drop

flows through a wire to the batteries

repeat… the current is traveling around a circuit

The current’s job is to deliver power, not charge!

About Wires and Filaments

Metals are imperfect conductors electric currents can’t coast through them electric fields are needed to keep currents moving

For a current to flow through a metal, that metal must have an electric field in it caused by a voltage drop and the associate gradient

Currents waste power in metal wires & filaments Wires waste as little power as possible Filaments waste much power and become very hot

Question 3

Q: How does a flashlight’s switch turn it on or off?

A: It opens or closes the circuit.

Steady current requires a “closed” circuit so charge loops and doesn’t accumulate anywhere

A flashlight’s electric circuit is closed (complete) when you turn the switch on

open (incomplete) when you turn the switch off

Question 4

Q: How can a battery be recharged?

A: Push current through it backward.

Battery provides power when current flows forward from end to end

current experiences a voltage rise

Battery receives power (and recharges) when current flows backward from end to end

current experiences a voltage drop

Effects of Current Direction

Batteries typically establish the current direction

Current direction doesn’t affect wires, heating elements, or lightbulb filaments,

Current direction is critically important to electronic components such as transistors and LEDs

and some electromagnetic devices such as motors.

Question 5

Q: Why does a short-circuited flashlight get hot?

A: Current bypasses the bulb and heats the wires.

If a conducting path bridges the filament, current bypasses the filament → circuit is “short”

there is no appropriate destination for the energy

energy loss and heating occurs in the wires

Short circuits can cause fires!

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Question 6

Q: How do lightbulbs differ?

A: They have different electrical resistances.

Current undergoes a voltage drop in a conductor

That voltage drop is proportional to the current:

voltage drop = resistance · currentwhere resistance is a characteristic of the conductor.

This relationship is known as Ohm’s law.

Resistance and Filaments

Batteries determine the filament’s voltage drop

The smaller a filament’s resistance, the more current it carries

the more electric power it consumes

A lightbulb filament is chosen to have enough resistance to limit power consumption

enough surface area to dissipate the thermal power

Summary about Flashlights

Current carries power from batteries to bulb

The switch controls the flashlight’s circuit

Current flows only when the circuit is closed

The batteries raise the current’s voltage

The lightbulb lower the current’s voltage

Household Magnets


Observations aboutHousehold Magnets

They attract or repel, depending on orientation

Magnets stick only to certain metals

Magnets affect compasses

The earth is magnetic

Some magnets require electricity

5 Questions aboutHousehold Magnets

1. Why do any two magnets attract and repel?

2. Why must magnets be close to attract or repel?

3. Why do magnets stick only to some metals?

4. Why does a magnetic compass point north?

5. Why do some magnets require electricity?

21

Question 1

Q: Why do any two magnets attract and repel?

A: Each magnet has both north and south poles

Like magnetic poles repel, opposite poles attract

Magnetic pole is a conserved quantity North pole is a “positive” amount of pole.

South pole is a “negative” amount of pole.

The net pole on any object is always exactly zero!

Magnets

Unlike charges, free poles are never observed

A magnet always has equal north and south poles

It has magnetic polarization, but zero net pole A typical bar or button magnet is a magnetic dipole

A dipole has one north pole and one south pole

A fragment of a magnet has a net pole of zero it retains its original magnetic polarization

it is typically a magnetic dipole

Question 2

Q: Why must magnets be close to attract or repel?

A: Forces are weakened by distance and cancellation

The magnetostatic forces between poles are proportional to the amount of each pole

proportional to 1/distance2

Forces between Magnets

Each magnet has both north and south poles

Magnets simultaneously attract and repel

The net forces and net torques on magnets depend on distance and orientation

are typically dominated by the nearest poles

increase precipitously with decreasing distance

Question 3

Q: Why do magnets stick only to some metals?

A: Only a few metals are intrinsically magnetic.

Electrons have intrinsic magnetic dipoles In atoms, much of that magnetism remains active

In solids, that magnetism is usually cancelled perfectly

In a few solids, the cancellation is incomplete Iron and most steels are ferromagnetic materials

Refrigerators and Magnets

Ferromagnetic materials have magnetic domains Those domains ordinarily cancel on another

A magnet can alter those domains → magnetization

A magnet can magnetize a steel refrigerator it causes some domains to grow and others to shrink

the steel develops a net magnetic polarization

it attracts the magnetic pole that magnetized it

Magnets thus stick to steel refrigerators

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Soft & Hard Magnetic Materials

Soft magnetic materials have domains the grow or shrink easily,

so they are easy to magnetize or demagnetize.

They quickly forget their previous magnetizations.

Hard magnetic materials have domains that don’t grow or shrink easily,

so they are hard to magnetize or demagnetize.

They can be magnetized permanently.

Question 4

Q: Why does a magnetic compass point north?

A: Earth’s magnetic field twists it northward.

The earth produces a magnetic field that pushes north poles northward, south poles southward

exerts torques on magnetic dipoles, such as compasses

A compass immersed in earth’s magnetic field aligns it so that its north pole points northward.

Magnetic Fields

A magnetic field is a structure in space and time that pushes on pole

a vector field: a vector at each point in space and time

observed using a north test pole at each point

Question 5

Q: Why do some magnets require electricity?

A: Electric currents are magnetic!

A current-carrying wire produces a magnetic field

A current-carrying coil mimics a bar magnet

An electromagnet typically uses an electric current to produce a magnetic field

to magnetize a ferromagnetic material

Electromagnetism (Version 1)

Magnetic fields are produced by magnetic poles and subatomic particles,

moving electric charges,

and changing electric fields [for later…].

Electric fields are produced by electric charges and subatomic particles,

moving magnetic poles [for later…],

and changing magnetic fields [for later…].

Summary aboutHousehold Magnets

They all have equal north and south poles

They polarize soft magnetic materials and stick

They are surrounded by magnetic fields

Can be made magnetic by electric currents

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Electric Power Distribution


Observations aboutElectric Power Distribution

Household electricity is alternating current (AC)

Household voltages are typically 120V or 240V

Power is distributed at much higher voltages

Power transformers are common around us

Power substations are there, but harder to find

4 Questions aboutElectric Power Distribution

1. Why isn’t power transmitted via large currents?

2. Why isn’t power delivered via high voltages?

3. What is “alternating current” and why use it?

4. How does a transformer transfer power?

Question 1

Q: Why isn’t power transmitted via large currents?

A: Too much power would be wasted in the wires.

Current-carrying wires consume and waste power power wasted = current · voltage drop in wire

voltage drop in wire = resistance · current (Ohm’s law)

power wasted = resistance · current2.

Large currents waste large amounts of power

Question 2

Q: Why isn’t power delivered via high voltages?

A: High voltage power is dangerous.

High voltages can produce large voltage gradients

Current may flow through unintended paths a spark hazard,

a fire hazard,

and a shock hazard.

The Voltage Hierarchy

Electric power delivered to a consumer ispower delivered = current · voltage drop

Large currents are too wasteful for transmission

High voltages are too dangerous for delivery

So electric power distribution uses a hierarchy: high-voltage transmission circuits in the countryside

medium-voltage circuits in cities

low-voltage delivery circuits in neighborhoods

Transformers transfer power between circuits!

24

Question 3

Q: What is “alternating current” and why use it?

A: Fluctuating current → so transformers will work

In alternating current, the voltages of the power delivery wires alternate

and the resulting currents normally alternate, too.

AC and Transformers

Alternating voltage in the US completes 60 cycles per second,

so voltage and current reverse every 1/120 second.

AC complicates the design of electronic devices

AC permits the easy use of transformers, which can move power between circuits:

from a low-voltage circuit to a high-voltage circuit

from a high-voltage circuit to a low-voltage circuit

Question 4

Q: How does a transformer transfer power?

A: Its changing magnetic fields induce currents.

The primary coil carries an alternating current that produces an alternating magnetic field

that produces an electric field

that pushes current through the secondary coil

Power moves from primary coil to secondary coil




and changing electric fields [more later…].


moving magnetic poles,

and changing magnetic fields.

Electromagnetic Induction

Moving poles or changing magnetic fields produce electric fields, which propel currents through conductors, which produce magnetic fields.

Changing magnetic effects induce currents Induced currents produce magnetic fields

Lenz’s Law

When a changing magnetic field induces a current in a conductor, the magnetic field from that current opposes the change that induced it

25

Transformers

Alternating current in one circuit can induce an alternating current in a second circuit

A transformer uses induction

to transfer powerbetween its circuits

but doesn’ttransfer any chargesbetween its circuits

Current and Voltage

A transformer must obey energy conservation

Power arriving in its primary circuit must equal power leaving in its secondary circuit

Since power is the product of voltage · current, a transformer can exchanging voltage for current

or current for voltage!

Step-Down Transformer

A step-down transformer has relatively few turns in its secondary coil

so charge is pushed a shorter distance

and experiences a smaller voltage rise

A larger currentat smaller voltageflows in thesecondary circuit

Step-Up Transformer

A step-up transformer has relatively many turns in its secondary coil

so charge is pushed a longer distance

and experiences a larger voltage rise

A smaller currentat larger voltageflows in thesecondary circuit

Power Distribution System

A step-up transformer increases the voltagefor efficient long-distance transmission

A step-down transformer decreases the voltagefor safe delivery to communities and homes

Inductor

Electric and magnetic fields both contain energy

Electromagnet has magnetic energy Stores energy as current increases

Releases energy as current decreases

Exhibits Lenz’s law Current change induces opposing current

Opposes any changes in current

Known as an inductor

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Summary aboutElectric Power Distribution

Electric power is transmitted at high voltages

Electric power is delivered at low voltages

Transformers transfer power between circuits

Transformers require AC power to operate

The power distribution system is AC

Radio


Observations about Radio

It can transmit sound long distances wirelessly

It involve antennas

It apparently involves electricity and magnetism

Its reception depends on antenna positioning

Its reception weakens with distance

There are two styles of radio: AM and FM

3 Questions about Radio

1. How can a radio wave exist?

2. How is a radio wave emitted and received?

3. How can a radio wave represent sound?

Question 1

Q: How can a radio wave exist?

A: Electric and magnetic fields create one another.

Radio waves are electromagnetic waves: structures made only of electric and magnetic fields

they are emitted or received by charge or pole

they are self-sustaining while traveling, even in vacuum

their electric and magnetic fields recreate one another




and changing electric fields.


moving magnetic poles,

and changing magnetic fields.

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Structure of a Radio Wave

Electric field is perpendicular to magnetic field

Changing electric fieldcreates magnetic field

Changing magnetic fieldcreates electric field

Polarization of the waveis associated with thewave’s electric field

Question 2

Q: How is a radio wave emitted and received?

A: Accelerating charge ↔ electromagnetic wave

Accelerating charge causes electromagnetic wave It makes an electric field that changes with time

It makes a magnetic field that changes with time

and the two fields can form an electromagnetic wave

Electromagnetic wave causes accelerating charge Its electric field pushes on the charge

A Tank Circuit

For bigger wave, slosh charge in a tank

Tank circuit is a harmonic oscillator It consists of a capacitor and an inductor

Charge cycles through the circuit

Tank’s energy alternates between magnetic field in its inductor

electric field in its capacitor

Tank circuit can accumulate energy

Frequency set by capacitor & inductor

An Antenna is a Tank Circuit

An antenna is a straightened tank circuit!

Antenna’s frequency is set by its length

Resonant when it is ½ radio wavelength long

A conducting surface can act as half the antenna

Above a conductingsurface, antenna isresonant when it is¼ wavelength long

Emitting and Receiving Waves

A transmitter uses a tank circuit to slosh charge up and down its antenna,which acts as a second tank.

A receiver uses a tank circuit todetect charge sloshing on itstank-circuit antenna.

Transmitter antenna chargeaffects receiver antenna charge

Antenna orientations matter!

Question 3

Q: How can a radio wave represent sound?

A: Vary the wave to send sound information.

AM or “Amplitude modulation” Fluctuating amplitude conveys sound information

FM or “Frequency modulation” Fluctuating frequency conveys sound information

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AM Modulation

Information can be encoded as a fluctuating amplitude of the radio wave

The air pressure variationsthat are sound cause changesin the amount of chargemoving on the antenna andthus the intensity of the wave

The receiver detects thesechanges in radio wave intensity.

FM Modulation

Information can be encoded as a fluctuating frequency of the radio wave

The air pressure variationsthat are sound cause slightshifts in the frequency ofcharge motion on the antennaand the frequency of the wave

The receiver detects thesechanges in radio wave frequency.

Summary about Radio

Accelerating charges cause electromagnetic waves

Electromagnetic waves cause accelerating charges

Those waves are only electric and magnetic fields Accelerating charge on a transmitting antenna

produces a radio wave

that causes charge to accelerate on a receiving antenna

Radio waves can represent sound information

Microwave Ovens


Observations About Microwaves

Microwave ovens cook food from inside out

They often cook foods unevenly

They don’t defrost foods well

You shouldn’t put metal inside them?!

Do they make food radioactive or toxic?

4 Questions aboutMicrowave Ovens

1. Why do microwaves cook food?

2. How does metal respond to microwaves?

3. Why do microwave ovens tend to cook unevenly

4. How does the oven create its microwaves?

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Question 1

Q: Why do microwaves cook food?

A: Water in the food responds to their electric fields.

Microwaves are a class of electromagnetic waves Long-wavelength EM waves: Radio & Microwave

Medium-wavelength: IR, Visible, UV light

Short-wavelength: X-rays & Gamma-rays

Microwaves have rapidly fluctuating electric fields.

Water Molecules

Water molecules are unusually polar

An electric field tends to orient water molecules

A fluctuating electric field causes water molecules to fluctuate in orientation

Microwave Heating

Microwaves have alternating electric fields

Water molecules orient back and forth

Liquid water heats due to molecular “friction”

Ice doesn’t heat due to orientational stiffness

Steam doesn’t heat due to lack of “friction”

Food’s liquid water content heats the food

Question 2

Q: How does metal respond to microwaves?

A: Currents flow back and forth in the metal.

Non-conductors polarize in the microwaves

Conductors carry currents in the microwaves Good, thick conductors reflect microwaves

Poor or thin conductors experience resistive heating

Sharp conductors initiate discharges in the air

Question 3

Q: Why do microwave ovens tend to cook unevenly?

A: Interference produces nonuniform electric fields.

Interference is when the fields add or cancel Adding fields are constructive interference

Canceling fields are destructive interference

Reflections lead to interference in a microwave

Most ovens “stir” the waves or move the food

Question 4

Q: How does the oven create its microwaves?

A: A magnetron tube radiates microwaves.

Magnetrons are vacuum tubes invented in WWII

Electrons travel through empty space obtaining power from a strong electric field

bent by a strong magnetic field and the Lorentz force

delivering power to an electromagnetic field

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Generating Microwaves

Magnetron tube has tank circuits in it

Streams of electrons amplify tank oscillations

A loop of wire extracts energy from the tanks

A short ¼-wave antenna emits the microwaves

Summary aboutMicrowave Ovens

They cook food because of its water content

Polar water molecules heat in microwave fields

Thin or sharp metals overheat or spark

The microwaves are produced by a magnetrons

Sunlight


Observations about Sunlight

Sunlight appears whiter than most light

Sunlight makes the sky appear blue

Sunlight becomes redder at sunrise and sunset

It reflects from many surfaces, even nonmetals

It bends and separates into colors in materials

5 Questions about Sunlight

1. Why does sunlight appear white?

2. Why does the sky appear blue?

3. How does a rainbow break sunlight into colors?

4. Why are soap bubbles and oil films so colorful?

5. Why do polarizing sunglasses reduce glare?

Question 1

Q: Why does sunlight appear white?

A: We perceive 5800 K thermal light as “white”

Light is a class of electromagnetic waves Single-frequency light has a rainbow color

A thermal mixture of rainbow colors can look white

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Spectrum of Sunlight

Sunlight is thermal radiation—heat from the sun Charges in the sun’s hot photosphere jitter thermally

Accelerating charges emits electromagnetic waves

The sun emits a black-body spectrum at 5800 K

We perceive thermal light at 5800 K as “white”

Question 2

Q: Why does the sky appear blue?

A: Air particles Rayleigh-scatter bluish light best

Rayleigh scattering occurs when passing sunlight polarizes tiny particles in the air,

this alternating polarization reemits light waves, so

air particles scatter light—they absorb and reemit it.

Rayleigh Scattering

Air particles are so small that they are much less ½ the wavelength of light

poor antennas for light

scatter long-wavelengths (reds) particularly poorly

scatter short-wavelengths (violets) somewhat better

Rayleigh scattered sunlight appears bluish

Unscattered sunlight (solar disk) appears reddish

Effect is strongest at sunrise and sunset

Question 3

Q: How does a rainbow break sunlight into colors?

A: Rainbow colors take different paths in raindrops

Sunlight slows while it passes through matter Light waves electrically polarize the matter

That polarization delays and slows the light wave

Index of refraction = reduction factor for light’s speed

Index of refraction varies slightly with color

Light at Interfaces

When light changes speed at an interface, relationship between electric & magnetic fields changes

it refracts—its path bends as it cross the interface it bends toward the perpendicular if it slows down

it bends away from the perpendicular if it speeds up

it reflects—part of it bounces off the interface the reflection is almost perfect for metal surfaces

the reflection is partial for insulator surfaces

Light and Dispersion

The rainbow colors of light in sunlight have different frequencies

polarize material slightly differently

and therefore travel at slightly different speeds

Index of refraction depends slightly on color

Violet light usually travels slower than red light violet light usually refracts more than red

violet light usually reflects more than red

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Rainbows

Occur when sunlight encounters water droplets and undergoes refraction, reflection, and dispersion.

Question 4

Q: Why are soap bubbles and oil films so colorful?

A: They display color-dependent interference effects

Light waves following different paths can interfere

The two partial reflections froma soap or oil film can interfere

Different colors of light caninterfere differently

Question 5

Q: Why do polarizing sunglasses reduce glare?

A: Glare is mostly horizontally polarized light

Sunlight is a uniform mix of polarizations

When sunlight partially reflects at a shallow angle its different polarizations reflect differently

it becomes polarized—it is no longer a uniform mix

Polarizing sunglasses block horizontal polarization

Reflection of Polarized Light

Polarization affects angled reflections

When light’s electric field is parallel to a surface there is a large fluctuating surface polarization

and thus a strong reflection.

When electric field is perpendicular to a surface there is a small fluctuating surface polarization

and thus a weak reflection.

Glare is mostly polarized parallel to the surface

Polarization and Sunlight

Polarizing sunglasses block horizontally polarized light

and thus block glare from horizontal surfaces.

Rayleigh scattering has polarizing effects, so much of the blue sky is polarized light, too.

Polarizing sunglasses darken much of the sky

Summary about Sunlight

Sunlight is thermal light at about 5800 K

It undergoes Rayleigh scattering in the air

It bends and reflects from raindrops

It interferes colorfully in soap and oil films

It reflects in a polarizing fashion from surfaces

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Discharge Lamps


Observations aboutDischarge Lamps

They often take moment to turn on

They come in a variety of colors, including white

They are often whiter than incandescent bulbs

They last longer than incandescent bulbs

They sometimes hum loudly

They flicker before they fail completely

5 Questions aboutDischarge Lamps

1. Why phase out incandescent lightbulbs?

2. How can colored lights mix so we see white?

3. Why does a neon lamp produce red light?

4. How can white light be produced without heat?

5. How do gas discharge lamps produce light?

Question 1

Q: Why phase out incandescent lightbulbs?

A: Because they waste too much electric power.

Incandescent lightbulb is a thermal light source with a relatively low filament temperature of 2700 K

It emits mostly invisible infrared light

Less than 10% of its thermal power is visible light

Non-thermal light sources can be more efficient

Question 2

Q: How can colored lights mix so we see white?

A: Primary colors of light trick our vision.

We have three groups of light-sensing cone cells Their peak responses are to red, green, and blue light

Those are therefore the primary colors of light

Mixtures of primary colorscan make us see any color

Question 3

Q: Why does a neon lamp emit red light?

A: Neon’s quantum structure dictates light emission

Electrons obey the rules of quantum physics In matter, electrons exist as quantum standing waves

three-dimensional patterns of nodes and antinodes

each wave “cycles” in place—it does not change with time

In atoms, those standing waves are called orbitals

In solids, those standing waves are called levels

Quantum structure dictates atom’s light emission

34

Quantum Physics

Classical physics (pre-1900) thought that everything in nature is a particle or a wave

electrons, atoms, and billiard balls are particles

light and sound are waves

Modern physics (post-1900) recognizes that everything in nature is both particle and wave

things are most wave-like when they are left alone

things are most particle-like when they interact

Electrons in Matter

Electrons exist in matter as quantum standing waves

have energies that are set by their specific waves

obey the Pauli exclusion principle:

No two indistinguishable Fermi particlesever occupy the same quantum wave

have two distinguishable states: spin-up or spin-down

can occupy each wave in 1’s or 2’s, but no more than 2

tend to occupy lowest energy waves, 2 per wave

Electrons in a Neon Atom

Electrons in any atom tend to settle into the lowest energy orbitals

cannot be more than 2 to an orbital

Lowest energy arrangement is atom’s ground state

Higher energy arrangements are atom’s excited states

In a neon atom, the nucleus has 10 protons

electrical neutrality requires it to have 10 electrons

ground state has electrons in 5 lowest-energy orbitals

Neon Discharge Lamps

Neon lamp metal electrodes inject free charges into dilute neon

plasma forms—a vapor of charged particles

electric field causes current to flow in the plasma

current is mostly electrons streaming toward positive

electrons often collide violently with neon atoms

Neon Lamps and Excited States

Collisions in the plasma occasionally ionize neon atoms, sustaining the plasma

cause electronic excitations of the neon atoms

In a neon atom, electrons normally occupy ground state orbitals

collisions can shift electrons to higher energy orbitals

light emission can return them to lower energy orbitals

Atoms interact with light via radiative transitions

Radiative transition that emits light is fluorescence

Light from Atoms

The quantum physics of light: Light travels as a wave (diffuse rippling fields)

Light is emitted or absorbed as a particle (a photon).

A photon carries a specific amount of energyPhoton energy = Planck constant · frequency

An atom’s orbitals differ by specific energies Orbital energy differences set the photon energies

Excited atom emits a specific spectrum of photons

35

Atomic Fluorescence

Photon energy is the difference in orbital energies Small energy differences infrared (IR) photons

Moderate energy differences red photons

Big energy differences blue photons

Even bigger energy differences ultraviolet (UV) photons Each atom has its own fluorescence spectrum

Neon’s fluorescence spectrum is dominated by red light

Question 4

Q: How can white light be produced without heat?

A: Synthesize the proper mixture of primary colors.

Fluorescent tubes use a discharge in mercury gas to produce UV light

UV light causes phosphors on the tube wall to glow

phosphors synthesize white light from primary colors

Phosphors

A mercury discharge emits mostly UV light A phosphor can convert UV light to visible

Absorb a UV photon, emit visible photon. Missing energy usually becomes thermal energy.

Fluorescent lamps use white phosphors They imitate thermal whites at 2700 K, 5800 K, etc.

Specialty lamps use colored phosphors Blue, green, yellow, orange, red, violet, etc.

Starting Fluorescent Lamps

Starting a discharge requires electrons in the gas

Those electrons can be injected into the gas by heated filaments with special coatings

or by high voltages

Once discharge starts, it can sustain the plasma

Starting the discharge damages the electrodes Atoms are sputtered off the electrodes

Damage limits the number of times a lamp can start

Stabilizing Fluorescent Lamps

Gas discharges are electrically unstable Gas is initially insulating

Once discharge is started, gas become a conductor

The more current it carries, the better it conducts

Current tends to skyrocket out of control

Stabilizing discharge requires ballast Inductor ballast (old, 60 Hz, tend to hum)

Electronic ballast (new, high-frequency, silent)

Question 5

Q: How do gas discharge lamps produce light?

A: The discharge emits atomic fluorescence light

36

Low-Pressure Discharge Lamps

Mercury gas has its resonance line in the UV Low-pressure mercury lamps emit mostly UV light

Some gases have resonance lines in the visible

Low-pressure sodium vapor discharge lamps emit sodium’s yellow-orange resonance light,

so they are highly energy efficient

but extremely monochromatic and hard on the eyes.

High Pressure Effects

High pressures broaden each spectral line Collisions occur during photon emissions,

so frequency and wavelength become smeared out.

Collision energy shifts the photon energy

Radiation trapping occurs at high atom densities Atoms emit resonance radiation very efficiently

Atoms also absorb resonance radiation efficiently

Resonance radiation photons are trapped in the gas

Energy must escape discharge via other transitions

High-Pressure Discharge Lamps

At higher pressures, new spectral lines appear High-pressure sodium vapor discharge lamps

emit a richer spectrum of yellow-orange colors, are still quite energy efficient, but are less monochromatic and easier on the eyes.

High-pressure mercury discharge lamps emit a rich, bluish-white spectrum, with good energy efficiency. Adding metal-halides adds red to improve whiteness.

Summary aboutDischarge Lamps

Thermal light sources are energy inefficient

Discharge lamps produce more light, less heat

They carefully assemble their visible spectra

They use atomic fluorescence to create light

Some include phosphors to alter colors

LEDs and Lasers


Observations about LEDs and Lasers

LEDs and Lasers often have pure colors

LEDs can operate for years without failing

Lasers produce narrow beams of intense light

Lasers are dangerous to eyes

Reflected laser light has a funny speckled look

37

6 Questions aboutLEDs and Lasers

1. Why can’t electrons move through insulators?

2. How does charge move in a semiconductor?

3. Why does a diode carry current only one way?

4. How does an LED produce its light?

5. How does laser light differ from regular light?

6. How does a laser produce coherent light?

Question 1

Q: Why can’t electrons move through insulators?

A: Electrons can’t easily change levels in insulators.

Electrons obey the rules of quantum physics In matter, electrons exist as quantum standing waves

three-dimensional patterns of nodes and antinodes

each wave “cycles” in place—it does not change with time

In solids, those standing waves are called levels

To move, electrons must be able to switch levels

In an insulator, electrons can’t easily change levels

Electrons in Solids

Electrons settle into a solid’s lowest-energy levels

Levels clump into energy bands, leaving gaps

Fermi level: between last filled and first unfilled

Metals

In a metal, the Fermi level has empty levels just above it in energy

electrons near the Fermi level can change levels easily

electrons can move in response to electric fields

The electrons are like patrons in a partly filled theatre

Current can flow through a metal

Insulators

In an insulator, the Fermi level has no empty levels nearby

electrons can’t move in response to electric fields

The electrons are like patrons in a completely filled theatre

Current can’t flow through an insulator

Question 2

Q: How does charge move in a semiconductor?

A: A semiconductor is an insulator with a small gap

Pure semiconductors are “poor insulators” empty conduction band is just above full valence band

The electrons are like patrons in a theater with a full ground floor but a low empty balcony

Light or heat can allow current in pure semiconductor

38

Doped Semiconductors

Dopant atoms can add or remove electrons

A p-type semiconductor has fewer electrons than normal

has some empty valence levels

An n-type semiconductor has more electrons than normal

has some filled conduction levels

Question 3

Q: Why does a diode carry current only one way?

A: Its p-n junction is asymmetric and special

When p-type and n-type semiconductors touch they have different affinities for electrons

electrons migrate from the n-type to p-type

leaving a depletion region with no mobile charges

That junction can conduct current only one way

pn-Junction (before contact)

Before p-type semiconductor meets n-type, each material can conduct electricity

each material is electrically neutral throughout

pn-Junction (after contact)

After p-type semiconductor meets n-type, electrons migrate from the n-type to the p-type

an insulating depletion region appears at junction

depletion region develops an electric field

Forward in the Diode

When electrons enter n-type and exit p-type, depletion region’s electric field is cancelled away

depletion region shrinks

diode can conduct current

Reverse in the Diode

When electrons enter p-type and exit n-type, depletion region’s electric field is reinforced

depletion region grows

diode cannot conduct current

39

Question 4

Q: How does an LED produce its light?

A: Electrons emit light while changing bands

LEDs are Light-Emitting Diodes LED is a diode and has a pn-junction

Electrons cross junction in the conduction band

Electrons dropping into the valence band emit light Electron briefly orbits the empty valence level

Electron drops into valence level via a radiative transition

The larger the band gap, the bluer the light

Question 5

Q: How does laser light differ from regular light?

A: Laser light is a single electromagnetic wave.

Most light sources produce photons randomly Each photon usually has its own wave

Laser light involves duplicate photons Laser amplification duplicates an initial photon

Each photon becomes part of a single giant wave

Spontaneous Emission

Excited atoms normally emit light spontaneously

These photons are uncorrelated and independent

Each photons has its own wave mode

These independent waves are incoherent light

Stimulated Emission

Excited atoms can be stimulated into duplicating passing light

These photons are correlated and identical

The photons all have the same wave mode

This single, giant wave iscoherent light

Question 6

Q: How does a laser produce coherent light?

A: A laser amplifier duplicates an initial photon.

Excited atom-like systems can act as laser medium Duplicate photons they’re capable of emitting

Duplication is perfect: the photons are true clones

Laser Oscillation

A laser medium can amplify its own light A laser medium in a resonator acts as an oscillator

It duplicates its one of its own spontaneous photons

Duplicated photons leak from semitransparent mirror

The photons from this oscillator are identical

40

Summary about Lasers and LEDs

Lasers produce coherent light by amplification

Coherent light contains many identical photons

Laser amplifiers and oscillators are common

LEDs are incoherent, light-emitting diodesCameras


Observations about Cameras

They record a scene on an image sensor

Good cameras need focusing, cheap ones don’t

Many cameras have zoom lenses

Some cameras have bigger lenses than others

They have ratings like focal length and f-number

5 Questions about Cameras

1. Why does a camera need a lens?

2. Why do most camera lenses need focusing?

3. Why are lenses telephoto or wide-angle?

4. Why do fancy lens’s have internal apertures?

5. Why is a good camera lens so complex inside?

Question 1

Q: Why does a camera need a lens?

A: Lens bends rays from one point to one point.

An illuminated object reflects or scatters light The object’s light produces diffuse illumination

A converging lens bends light rays via refraction Light rays spreading from a point converge to a point

Real Images

An image forms in space on far side of the lens The image is a pattern of light in space that exactly resembles the object,

except for size and orientation

The image is “real” – you can put your hand in it

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Question 2

Q: Why do most camera lenses need focusing?

A: So that the real image forms on the image sensor.

The sensor records the pattern of light it receives

When focused, the real image forms on the sensor

Focusing

Distant object’s light diverges slowly Real image forms near to the lens

Nearby object’s light diverges quickly Real image forms far from the lens

Lens can only focus perfectlyon one object at a time

Lens Diameter and Focusing

Larger lens gathers more light so the image is brighter

but focus is more critical

and there is less depth of focus.

Smaller lens gathers less light so the image is dimmer

but focus is less critical

and there is more depth of focus.

Question 3

Q: Why are lenses telephoto or wide-angle?

A: Lens’ focal length (FL) determines image size

Focal length measures lens’s converging strength Long FL: long image distance, large dim image

Short FL: short image distance, small bright image

Question 4

Q: Why do fancy lenses have internal apertures?

A: To vary image brightness and depth of focus

f-number is focal length divided by lens diameter Small f-number: bright image, small depth of focus

Large f-number: dim image, large depth of focus

Sophisticated lenses have adjustable f-numbers

Question 5

Q: Why is a good camera lens so complex inside?

A: To allow zooming and to correct image flaws.

Zooming: adjustable focal length using complex lenses

Dispersion different colors focus differently

Reflections fog in photographic images

Spherical aberration imperfect focus of spherical lens

Poor focusing off axis coma distortions

Spherical focus projected on flat film astigmatism

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Zoom Lenses

A zoom lens typically forms three images overall Its first lens group produces a real image

Its second lens groupprojects a resized image

Its third lens groupprojects an image ontothe image sensor

Summary about Cameras

They use converging lenses to form real images

Lens focal length sets image size

Lens f-number sets image brightness

The image sensor records the pattern of light

Optical Recording and Communications


Observations about Optical Recording and Communications

Optical disks can store lots of audio or video

That audio or video is of the highest quality

Optical disks continue to play perfectly for years

Playback of optical disks involves lasers

Lasers and fibers are used in communication

5 Questions about Optical Recording and Communication

1. How is information represented digitally?

2. How is information recorded on an optical disk?

3. How is information read from an optical disk?

4. How can light carry information long distances?

5. Why does light follow an optical fiber’s bends?

Question 1

Q: How is information represented digitally?

A: As a sequence of digital symbols

Audio or video info is a sequence of numbers Each number is represented as digital symbols

Offers good noise-immunity Permits error correction to further reduce noise

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Digital Audio

The air pressure in sound is measured thousands of times per second

Each measurement is represented digitally using about 16 binary digits, each a 0 or a 1.

Question 2

Q: How is information recorded on an optical disk?

A: As reflective symbols in a spiral track.

Symbols are lengths/spacing of reflective regions

Symbols are as small as can be detected optically Laser wavelength determines symbol density

Question 3

Q: How is information read from an optical disk?

A: A focused laser beam reflects from the disk.

Diode laser light is focused on the shiny layer

Reflected light is detected by photodiodes

Playback Issues

Laser light must focus exactly on the symbols

Light wave forms a “waist”—its minimum width Wave limits on focusing are known as diffraction

Waist can’t be much smaller than a wavelength

Symbols can’t be much smaller than a wavelength

Question 4

Q: How can light carry information long distances?

A: Changes in that light can represent information.

Analog representations such as AM modulation often used for remote process monitoring.

Digital representations such as pulse codes pulses with discrete amplitudes used as symbols

provides noise-immunity, error correction, compression, and channel-sharing.

Question 5

Q: Why does light follow an optical fiber’s bends?

A: The light is trapped by total internal reflection.

As light enters a material with alower index of refraction, it bendsaway from the perpendicular

If bend exceeds 90°, light reflectsperfectly: total internal reflection.

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Optical Fibers

High-index glass core in a low-index glass sheath Light in core encounters interface at shallow angle

Light experiences total internal reflection

Light is trapped; it bounces endless through the core

Light travels for many kilometers in ultrapure fibers

Pulse spreading can limit information bandwidth Minimize light path difference: single mode fibers

Minimize dispersion: operate at dispersion minimum

Optical Fiber Types

Summary about Optical Recording and Communication

Optical disks store information as pits and flats

Focused laser light reads that information

Digital representations allow perfect playback

Optical fibers carry information as lightAudio Players


Observations about Audio Players

They are part computer, part sound system.

They require electric power, typically batteries.

They reproduce sound nearly perfectly.

They are sensitive to static charge.

4 Questions about Audio Players

1. How does an audio player “store” sound?

2. How does it move sound information around?

3. How does the audio player’s computer work?

4. How does the audio player’s amplifier work?

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Question 1

Q: How does an audio player “store” sound?

A: It represents that sound as digital information

It uses representations of sound information, sequences of air pressure measurements

that contain everything needed to recreate the sound.

Recording and recreating are done in analog form

Storing and retrieving are done in digital form

Analog Representation

One physical quantity represents one number

Any continuous physical quantity can be used: the voltage on a wire,

the current in a circuit,

the strength of a permanent magnet.

This direct representation is sensitive to noise

Analog representations are “imperfect.”

Digital Representation

A group of “symbols” represents a number

A symbol can be any discrete physical quantity: a positive or negative charge on a capacitor

an integer value of voltage on a wire

a north or south magnetic pole on a magnet

This indirect representation is insensitive to noise

Digital representations can be “perfect.”

Question 2

Q: How does it move sound information around?

A: It uses MOSFET electronic switches.

A MOSFET Transistor consists of two back-to-back pn-junctions

with a nearby “gate” surface that can store charge.

Gate charge controls current flow in MOSFET

MOSFET Transistor Off

A typical MOSFET Transistor normally has a vast depletion region in its “channel”

normally can’t conduct electric current

MOSFET Transistor On

Charging that MOSFET’s gate alters the filling of electron levels in the channel

so the depletion region vanishes

and the device can conduct electric current.

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Question 3

Q: How does the audio player’s computer work?

A: It uses MOSFETs to form logic elements.

Computers perform logical operations with bits A bit is a base-two digit

It can hold one of only two symbols: 0 or 1.

Bit-wise representation of numbers is called binary

MOSFET logic elements manipulate bits

Logic Elements

Inversion (the NOT logic element) One input bit, one output bit

Output bit is inverse of input bit

Not-And (the NAND logic element) Two input bits, one output bit

Output bit is inverse “and” of input bits

Any function and thus any computercan be built from these two logic elements

CMOS Logic Elements

Bits are represented by charge (1 is +, 0 is zero)

Uses complementary MOSFETs n- and p-channel MOSFETs are paired

CMOS Inverter has 2 MOSFETS

CMOS NAND has 4 MOSFETS

Question 4

Q: How does the audio player’s amplifier work?

A: It uses MOSFETs as analog amplifiers.

MOSFET lets a tiny charge control a big current

Amplifier has three circuits: Input current represents sound

Output current is amplified version

Power current provides power

Summary about Audio Players

Represent sound in digital and analog forms

Use MOSFETs to work with sound information

Digital computer comprised of CMOS logic

Analog amplifier based on MOSFETs.Nuclear Weapons


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Observations aboutNuclear Weapons

They release enormous amounts of energy

They produce incredible temperatures

They produce radioactive fallout

They are relatively difficult to make

They use chain reactions

4 Questions aboutNuclear Weapons

1. Where is nuclear energy stored in atoms?

2. Why are some atomic nuclei unstable?

3. How does a nuclear chain reaction work?

4. Why do nuclear explosions produce fallout?

Question 1

Q: Where is nuclear energy stored in atoms?

A: In each atom’s central nugget, its nucleus.

Each atom has an ultra-dense core or nucleus Extremely small (1/100,000th of atom’s diameter)

Contains about 99.95% of the atom’s mass

Contains most of the atom’s potential energy Evidence is related to: E=mc2

Structure of Nucleus

Nucleus contains two kinds of nucleons Protons are positively charged

Neutrons are electrically neutral

Two forces are active in a nucleus Electrostatic repulsion between protons

Nuclear force attraction between touching nucleons

At short distances, the nuclear force dominates

At long distances, the electric force dominates

Question 2

Q: Why are some atomic nuclei unstable?

A: They can release potential energy by breaking

In a stable nucleus, all objects remain in place For classical stability: stable equilibrium is required

For quantum stability: potential energy minimum

Objects quantum-tunnel out of unstable nuclei

Quantum Tunneling

Each object in a nucleus is in equilibrium

Classical object must climb potential hill to escape Without extra energy, object is trapped

Classical nucleus would be stable

Quantum object can escape without climbing hill Heisenberg uncertainty principle makes

position of wave-particle uncertain

Particle-wave can “tunnel” through potential hill

Objects can tunnel out of unstable nuclei

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Radioactivity

Large nuclei have two possible problems: Too many protons: too much electrostatic potential

Too many neutrons: isolated neutrons are unstable

Balance between protons and neutrons is tricky

Large nuclei tend to fall apart spontaneously Known as radioactive decay

Such decay is statistical, with a characteristic half-life

May include a splitting process called fission

Question 3

Q: How does a nuclear chain reaction work?

A: Decaying nuclei can shatter one another.

Radioactive decay releases tiny projectiles

Projectiles can collide with and agitate other nuclei

Agitated nuclei can decay, releasing new projectiles

and this can repeat, over and over again

Induced Fission

A large nucleus may break when struck Collision knocks its nucleons

out of equilibrium

Collision-altered nucleusmay undergo induced fission

Since a neutron isn’t repelledby nucleus, it makes an idealprojectile for inducing fission

Chain Reaction

Since neutrons can induce fission and induced fission releases neutrons,

this cycle can repeat,a chain reaction!

Each fission releases energy Many fissions release

prodigious amounts of energy

Sudden energy releaseproduces immense explosion

Requirement for a Bomb

A fission bomb requires 4 things: An initial neutron source

a fissionable material (undergoes induced fission)

each fission must release additional neutrons

material must use fissions efficiently (critical mass)

235U and 239Pu are both fissionable materials, but 235U is rare and must be separated from 238U

and 239Pu is made by exposing 238U to neutrons.

The Gadget & Fat Man

Each of these fission bombs started as a 239Pu sphere below critical mass (6 kg)

It was crushed explosively to supercritical mass

and promptly underwentan explosive chain reaction.

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Little Boy

This bomb started as a 235U hollow sphere below critical mass (60 kg)

A cannon fired a 235U plug through that sphere so that it exceeded critical mass

and it promptlyunderwent anexplosive chainreaction.

Question 4

Q: Why do nuclear explosions produce fallout?

A: Many fission-fragment nuclei are radioactive.

Large nuclei are neutron-rich High proportion of neutrons dilutes proton repulsion

Fragments of large nuclei are still neutron-rich They have too many neutrons for their size

They tend to be radioactive

Radioactive Fallout

Fission-fragment nuclei gather electrons Electrons accumulate until neutral atoms form

Atoms are chemically identical to ordinary atoms

But atoms have unstable nuclei—too many neutrons

We incorporate those atoms into our bodies Iodine-131 (8-day half-life)

Cesium-137 (30-year half-life)

Strontium-90 (29-year half-life)

Radioactive atoms decay, damaging molecules

Summary aboutNuclear Weapons

Nuclear energy is stored in atomic nuclei

Nuclear fission released electrostatic potential

Each fission releases an astonishing energy

Induced fission permits a chain reaction

Fission bombs explode via a chain reaction

Fission fragment nuclei form radioactive atoms

Nuclear Reactors


Observations aboutNuclear Reactors

They provide enormous amounts of energy

They consume nuclear fuel

They produce radioactive waste

Their nuclear fuel is sometimes reprocessed

They have safety issues

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5 Questions aboutNuclear Reactors

1. How can a nuclear reactor use natural uranium?

2. How do thermal fission reactors work?

3. How can a fission chain reaction be controlled?

4. How is energy extracted from a nuclear reactor?

5. What are the safety issues with nuclear reactors?

Question 1

Q: How can a nuclear reactor use natural uranium?

A: Slow the fission neutrons to thermal speeds.

Slow-moving (thermal) neutrons can fission the fissionable portion of uranium

are ignored by the nonfissionable portion of uranium

Slow-moving neutrons can fission natural uranium

A chain reaction is possible in natural uranium

Uranium Isotopes

Uranium-235 (235U - 92 protons, 143 neutrons) is radioactive – fissions and emits neutrons

fissionable – breaks when hit by neutrons

a rare fraction of natural uranium (0.72%)

Uranium-238 (238U - 92 protons, 146 neutrons) is radioactive – emits helium nuclei, some fissions

nonfissionable – absorbs fast neutrons without fission

a common fraction of natural uranium (99.27%)

Natural Uranium

Fissioning uranium nuclei emit fast neutrons Fast neutrons can fission 235U,

Fast neutrons are strongly absorbed by 238U.

Natural uranium contains mostly 238U, with some 235U

chain reactions won’t work in natural uranium

Thermal (Slow) Neutrons

Slow neutrons affect uranium isotopes differently Slow neutrons can still fission 235U

Slow neutrons don’t interact with 238U

A 235U chain reaction can occur in natural uranium if its neutrons are slowed by a moderator

Moderator nuclei small nuclei that don’t absorb colliding neutrons

extract energy and momentum from the neutrons

slow neutrons to thermal speeds

Question 2

Q: How do thermal fission reactors work?

A: Using moderators, they “burn” ordinary uranium.

Reactor core is natural or slightly enriched uranium

Moderator is interspersed throughout core

Moderator slows neutrons to thermal speeds

Nuclear chain reaction occurs only among 235U

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Choosing Moderators

Hydrogen nuclei (protons) Near-perfect mass match with neutrons

Outstanding energy and momentum transfer

Slight possibility of absorbing neutrons

Deuterium nuclei (heavy hydrogen isotope) Excellent mass match with neutrons

Excellent energy and momentum transfer

No absorption of neutrons

More about Moderators

Carbon Adequate mass match with neutron

Adequate energy and momentum transfer

Little absorption of neutrons

Choosing a moderator Deuterium is best, but it’s rare (used as heavy water)

Hydrogen is next best (used as ordinary water)

Carbon is acceptable and a convenient solid

Question 3

Q: How can a fission chain reaction be controlled?

A: Use feedback to operate right at critical mass.

Critical mass is governed by size and structure of reactor core

type of nuclear fuel (extent of 235U enrichment)

location and quality of moderator

positions of neutron-absorbing control rods

accumulation of fission fragment nuclei

Controlling Chain Reactions (Part 1)

Critical mass governs chain reaction rate Below it, fission rate diminish with each generation

Above it, fission rate increases with each generation

Generation rate of prompt neutrons is very short

Controlling prompt-neutron fission is difficult!

Delayed neutrons make reaction controllable Some fissions produce short-lived radioactive nuclei

These radioactive nuclei emit neutrons after a while

Delayed neutrons contribute to the chain reactions

Controlling Chain Reactions (Part 2)

There are two different critical masses Prompt critical: prompt neutrons sustain chain reaction

Delayed critical: prompt and delayed neutrons required

Reactors operate Below prompt critical mass

Above delayed critical mass

Control rods can adjust above or below criticality

Question 4

Q: How is energy extracted from a nuclear reactor?

A: It becomes heat and operates heat engines.

A nuclear reactor releases thermal power Fissions release thermal energy into the reactor core

Thermal energy is extracted by a coolant

Coolant is used to power a heat engine (e.g., steam)

Heat engine is used to generate electrical power

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Question 5

Q: What are the safety issues with nuclear reactors?

A: Preventing the release of radioactive nuclei.

Fission reactors produce radioactive nuclei (from fissions)

plutonium (from neutron capture by 238U)

Containing those nuclei forever is a challenge

Long Term Safety Issues

Radioactive nuclei are a radiation hazard They cannot be made safe, they can only be stored

They must be sequestered securely for eons

Plutonium is a nuclear proliferation hazard It can be used as a nuclear fuel in reactors

It can also be used in weapons

Short Term Safety Issues

To avoid catastrophes, chain reactions must never get out of control

reactors and spent fuel must never overheat

Reactors must be designed so that they are stable—chain reaction stops if overheating occurs

redundant—no single failure can cause a disaster

easy to quench—chain reaction stops immediately

Nuclear Accidents (Part 1)

Windscale Pile 1 (Britain) Carbon moderator burned while being annealed

Three Mile Island (US) Cooling pump failed and core overheated (while off)

Chernobyl Reactor 4 (USSR) Coolant boiled in overmoderated graphite reactor

Exceeded prompt critical and partially vaporized

Tokia-mura Accident (Japan) Critical mass was reached while processing uranium

Nuclear Accidents (Part 2)

Fukushima Daiichi (Japan) Earthquake caused reactors to be shut down

Tsunami disabled cooling pumps

Cooling water in reactors and spent fuel storage boiled

Reactor cores and spent fuel overheated and melted

Explosions damaged containment structures

Radioactive nuclei leaked into air and water

Summary about Nuclear Reactors

They slow fission neutrons using a moderator

They can use natural or slightly enriched uranium

They control their chain reactions carefully

They produce radioactive waste and plutonium

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Medical Imaging and Radiation


Observations AboutMedical Imaging and Radiation

They do their jobs right through your skin

Imaging involves radiation of various sorts

Some imaging radiation is itself hazardous

Radiation can make you well, sick, or neither

Some radiation involves radioactivity

Some radiation involves accelerators

5 Questions about Medical Imaging and Radiation

1. How are X-rays produced?

2. Why do X-rays image bones rather than tissue?

3. How does CT scanning create a 3D image?

4. How do gamma-rays kill cancerous tissue?

5. Why does MRI image tissue, not bone?

Question 1

Q: How are X-rays produced?

A: By accelerating electrons or atomic fluorescence

X-rays are high-frequency electromagnetic waves An X-ray photon carries a large amount of energy

X-rays are produced by very energetic events Extremely rapid accelerations of electrons

Extremely energetic radiative transitions in atoms

Bremsstrahlung X-rays

When a fast-moving electron whips around a massive nucleus electron accelerates extremely rapidly

may emit much of its energy as an X-ray photon

Characteristic X-rays

When a fast-moving electron hits a massive atom it may knock an electron out of a tightly bound orbital

and leave the atom (now an ion) ina highly excited state.

Atom then undergoes a radiativetransition to a less excited state

and emits an X-ray photon.

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Producing X-rays

X-rays are produced with high-energy electrons Accelerate electrons to 10kV - 100kV

Let electrons hit heavy atoms

Some X-rays emitted via bremsstrahlung and some as characteristic X-rays

X-ray tube filters awaythe lowest energy photons,because they’re uselessand cause skin burns.

Question 2

Q: Why do X-rays image bones rather than tissue?

A: The larger atoms in bone can absorb X-rays.

X-rays interact with atoms in two principal ways Rayleigh scattering—absorption and reemission

Photoelectric effect—absorption and electron ejection

All atoms participate in Rayleigh scattering

Large atoms participate in the photoelectric effect

X-rays Rayleigh Scattering

Rayleigh scattering is what makes the sky blue An atom absorbs a photon and then reemits it

This “scattered” photon travels in a new direction

All atoms participate in X-ray Rayleigh scattering

X-ray Photoelectric Effect

Photoelectric effect is a radiative transition X-ray photon shifts electron from orbital to free wave

Free electron carries photon’s residual energy

Effect most likely for small free-electron energies so effect most likely for atoms with many-electrons

X-ray Imaging

An atom that blocks X-rays casts a shadow Many-electron atoms produce strong shadows

Few-electron atoms cast essentially no shadows

X-ray imaging observes shadows of large atoms

Unfortunately, all atoms Rayleigh scatter X-rays Rayleigh scattering causes a distracting haze

Haze is filtered away by collimating structures

Question 3

Q: How does CT scanning create a 3D image?

A: Many X-ray images are analyzed by computer

X-rays are taken from different angles Shadows overlap differently at each angle

Computer analyzes these X-ray images Reconstructs 3D arrangement of parts

Displays cross sections of the body

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Question 4

Q: How do gamma-rays kill cancerous tissue?

A: These high-energy photons destroy molecules.

Gamma-rays are extremely high energy photons cause widespread damage to molecules and tissues

Gamma-rays are produce by high-energy events radiative transitions within atomic nuclei

high-energy bremsstrahlung (particle accelerators)

Producing Gamma Rays

Radioactive decay can produce gamma rays Decay may leave nucleus in an excited state

Radiative transition in nucleus emits gamma photon

Decaying cobalt-60 (Co60) emits two gamma photons

Electron accelerators can produce gamma rays Electrons are accelerated to extreme energies

Near heavy nuclei, emit Bremsstrahlung gamma rays

Gamma-rays and Matter

Gamma rays interact with individual charges

Compton scattering: photon hits electron They exchange energy & momentum

Both particles leave the atom

Atom, molecule, tissue are damaged

Pair production: photon → electron & positron Positron is anti-matter version of electron

Positron and another electron soon annihilate

Resulting gammas damage atoms, molecules, tissue

Radiation Therapy

Gamma rays are highly penetrating in tissue Little Rayleigh scattering and photoelectric effect

Much Compton scattering and pair production

Each gamma ray event damages many molecules Gamma rays can cause enough damage to kill cells

Approaching tumors from many angles minimizes collateral damage to healthy tissue

Question 5

Q: Why does MRI image tissue, not bone?

A: MRI images hydrogen nuclei, common in tissue.

Magnetic Resonance Imaging images hydrogen Hydrogen nuclei are protons Protons are magnetic—they are tiny dipole magnets Protons tend to orient in an external magnetic field Radio waves can influence that orientation

MRI is based on proton-orienting effects

Nuclear Magnetic Resonance

MRI grew out of Nuclear Magnetic Resonance Nuclei in a magnetic field interact with radio waves Yields information about atoms and their environments

Why NMR works: many nuclei are magnetic dipoles In magnetic field, dipole’s energy depends on orientation Nuclei have quantized orientations (quantum physics) In magnetic field, nuclei have quantized energies Nuclear orientations can change via radiative transitions

NMR studies atoms via those radiative transitions

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Magnetic Resonance Imaging

NMR typically studies hydrogen nuclei (protons) Protons have two quantized orientations H-NMR flips protons between their two orientations

Magnetic resonance imaging is based on H-NMR Person is placed in a carefully designed magnetic field That magnetic field varies with location and time Protons are probed with sophisticated radio waves MRI machine senses where and when protons respond MRI machine assembles image a person’s hydrogen

MRI images hydrogen and its tissue environment.

Summary about Medical Imaging and Radiation

X-rays and gamma-rays are high energy photons

X-rays scatter from heavy atoms, for imaging.

Gamma-rays disrupt cells, for therapy.

MRI detects and locates hydrogen nuclei.

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