Note for next year:I don’t get Ampere’s law

Clean up this unit- see AP objectives

Magnetism

This train does not roll on wheels.All its weight floats on magnets.

Misc pictures to ebed later

Magnetism was discovered over 2,000 years ago.The first magnets were natural rocks called lodestones.

Magnetism was named for the region of Greece (Magnesia) where these rocks were found.

No matter the shape,Magnets have two sides or “poles”

N

S

North pole

South pole

NS

N

SN

S

N

S

If you break a magnet

N

S

N

S

N

S

NS

NS

NS

NS

You could keep breaking the magnet until you were down to a single atom

AND IT too would act as a magnet with a north and south pole.

N

S

As before LIKES POLES REPELOPPOSITE ATTRACT

N

S

N

S

N

S

N

S N

S

N

SREPEL REPEL ATTRACT

The earth is also a magnet.It has North and South Magnetic Poles

(a little off from geographic poles)

Compasses use a magnetized needle which points to the magnetic poles of the planet (a little off from geographic poles)

NS

EW

N

S

EW

Notice a compass would not work too well here.

The North Pole of a magnet is the one that is attracted to the North Pole of the earth

N

S

EW

N

S

Which really means that the north magnetic pole is by definition the south pole of a magnet since they attract. (But this is just trivia).

Until 1820, people thought electricity and magnetism were unrelated. Until Hans Christian Oersted made a discovery during a class.

N

S

EW

When the switch was closed and current flowed, the compass needle moved!!!!

N

S

EW

animation

ELECTRICITY AND MAGNETISM are VERY RELATED

MAGNETIC Fields are created by moving charged particles

Electricity & Magnetism

Likes repel

Opposites attract

Similar because:

Electricity & Magnetism

Different because:

Positive and negative charges exist independent of each other

P+ e-

YOU cannot have a N pole without a S pole t

N S N S N S+ ETC

Electricity & Magnetism

They both (like gravity) exert a force without contact.

Similar because:

That really bugs me.

Electricity & MagnetismSimilar because:

THEY BOTH CREATE FORCE FIELDS

Field lines arrows run from N to S poles.Is a north pole like a + or - charge?

Where is the field the strongest?

Magnetic Field Strength has units of :

TESLA (T)

Surface of a Neutron Star 100,000,000

Magnetic Field

Surface of a strong magnet 10

Near earth’s magnetic poles .00005

gauss (G) is also used sometimes 1 G = 10-4 T

The variable for magnetic field is B.

Magnetic field lines are mapped out by using another magnet like a compass

Phet magnet simulation

A magnet will align itself with field lines

A magnet will align itself with field lines, when the torques balance out around the needle pivot. Think back to torque, how does the large red force balance the small blue force.

N

S

NS

Why do magnetic field lines have to be mapped out with both a N and S pole?

Why can’t we just put a N pole in the field and map out the net force on it like we did with electric field with a + test charge?

Iron filings will trace out field lines because they become temporarily magnetized.

Magnetic Fields are really 3D

Magnetism originatesin the motion of the electrons in iron.  

Spinning electrons act like tiny magnets. Almost 100 % cancellation of this effect occurs in mostmaterials.

 Iron, nickel, cobalt areexceptions.

Last sub-level of

3d ___ ___ ___ ___ ___

Fe- ferromagnetic

3d ___ ___ ___ ___ ___

Zn- Non magnetic

image of domains (using a special method to show magnetic fields,domains are not visible even with a microscope)

Little regions of a ferro-magnetic material,have the unpaired electrons align

domains can be forced to align by a magnetic field.

heating or banging tends to unalign them again

they will temporarily stay aligned

This is why certain materials become magnetic when touching a magnet.

Because the domains align in the field(the coins must be made with Fe, Ni, or Co)

N

S

N

S

N

S

N

S

N

S

In order for the to be an electric force between 2 objects. Both objects must have charges or induced charges.

--

--

- +

++

++

+

Attract

+

++

+

+ +

---

---

Repel

NADA

so this is really an interaction between 2 electric fields

electron neutron

N

S

N

Srepel

N

S

S attract

N

two permanent magnets

A permanent and iron nail

(an induced magnet)

N

S

NADA

A permanent and copper wire

Magnet activity: things to see or try

put a pencil through all 3 magnets, can you make them levitate?

how does a magnet affect a compass

does a compass affect another compass

Place the magnet flat under the magnetic viewerPlace the magnet on an edge under the magnetic viewer(How does it differ, remember you are viewing field lines)

Next Topic -- Electric current produces a magnetic field

If a magnet and electron are both stationary, there is no force between them.

e-

because there is only: 1 magnetic field &

1 electric field

If the electron(s) are moving then…

FORCE

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

moving charged particles (like electricity) produce a magnetic field.

Magnetic Field lines around a wire carrying current.

current

Magnetic Field Lines follow the right hand rule

Which way is current flowing?

I

Which way would the magnetic field point inside a loop if the current is traveling in a loop?

I I

Common convention is to use x’s (into the page) and circles (out of the page) to show magnetic fields etc…. Shouldn’t have to memorize this

Out of the “page” Into the “page”

When the wire forms a loop the field concentrates at the center.We’ll come back to this.

B =o I

2 r

vacuum permeability = 4 x 10-7 T m/A

current (A)

distance from wiremagnetic fieldstrength around a straight wire

I

15 A of current flows through a wire to the left, what is the strength and direction of the magnetic field 15 cm above the wire.

I

15 A of current flows through both wires below. If they are seperated by a distance of 30 cm. What will the magnetic field strength be midway between the?

I

The field is strongest in the middle of the loop

the more the loops the stronger the field

Coils of wire are used to intensify a magnetic field within it.

Which would be the north pole of the magnet?

If you are just given a wire there are two ways to predict the north pole.

Use the 1st right hand rule at the top of a loop.

Sorry This picture is not very clear

Or a very similar right hand rule for solenoids

Fingers follow current, thumb points to N pole

The greater the number of wraps the greater the internal field strength

A better way of stating this is:

the field strength increases as the density of the wraps increases.

Use thin wire with thin insulation

Within the coil the field strength concentrated and pretty much uniform.

Outside the coil the field is weak and divergent

B = 0nIfield strength in the coil (T) permeability

of free space # of wraps

meter

current (A)

Charged particles which are moving in a magnetic field will feel a force

What is the direction of the field between the ends of the magnet

SN

If a stationary proton is placed between the poles what happens?

Nothing

SN

If the proton is moving.It experience a force to both the field and its motion

Field

Motion

Force

motion of particle

B

Force

Fingers following magnetic field.

Thumb following direction of movement

palm indicates direction of forceon a POSITIVE particle.

Use left hand or opposite RH for electron!

Applet 1

Fingers following magnetic field.

Thumb following direction of movement

palm indicates direction of force

SN

Field

Motion

Force

This is what steers the electrons to the right pixel in a CRT TV

applet TV electron beam

scan rate / persistence of vision (applet)

And also protects the earth from some solar radiation

F = qvB sin()

The force on a single moving charged particle, is it in or out of the page?

Force on object (N)

Charge on object (C)

Speed of object (m/s)

B

v

1 if Perpendicular0 if parallel

It is into the page here.

The amount of force felt by a charge particle is proportional to RATE at which is “cuts” through magnetic field lines

SpeedDirection of movementLocation

A proton feels a force of 6x10-12 N pointed to the left when it moves straight up at 45,000 m/s. What is the magnitude and direction of the magnetic field?

v

F

If the magnetic field is into the page and the proton moves to the right, what is the direction of the force on it?

FF

Charged particle in a magnetic field applet

v

F

How much work will this magnetic force do on the particle?

FF

NONE This force can never do work on a charged particle because it is always perpendicular to motion (right hand rule).

But remember everything is based on …….

A mass spectrometer zaps a substance into fragment ions. These ions are then accelerated into a magnetic field where they curve at different rates based on the mass to charge ratio. It is used to determine the elemental composition of a molecule

mass spec video

mass spec applet

Moving charged particle in a magnetic field spiral video clip.

A proton is shot into a magnetic field.In what direction with the magnetic force be?

+

In what direction should an electric field be set to keep it moving straight?

v

FB

E

FE

-

How about here with an electron?Which way should the electric field point?

EF

E

FB

Next topic: Force on a current carrying wire in a magnetic field

If a wire with no current is placed in a magnetic field, which way does it get pushed?

It doesn’t

SN

If the current is flowing.It experience a force to both the field and current

FieldCurrent

Force

Fingers following magnetic field.

Thumb following direction of movement or current

palm indicates direction of force

Fingers following electric field.

Thumb following direction of current

palm indicates direction of force

SN

FieldCurrent

Force

If the moving charged particles are stuck in a wire. The whole wire feels a force on it

Use the right hand rule to determine the poles of the magnet.

NS

The force is greater if the motion (current) and field are perpendicularfor reasons explained before

current

current

less force

What factors will affect the Magnitude of Force the wire feels.

Amount of current

Angle of wire to field

Strength of magnetic Field

Force of a wire carrying current by a magnetic field is

F = BILSin()

Magnetic Field (T)

Current (A)

Length of wire (m)

1 if Perpendicular0 if parallel

I

B

L

A wire is 50 cm long and carries 8.0 A through a magnetic field strength of 10 T. What is the force on it assuming it is perpendicular to the field.

If the wire has a mass of 35 g, what current would be needed to levitate it?

Forces between two parallel wires

I

B

I

B

I

B

I

B

Two current carrying wires will either attract or repel because of the interaction of their magnetic fields

But it is MUCH easier to think of 1 wire as creating the magnetic field. The other wire is just a current carrying wire in a magnetic field.

Would the wires attract or repel?

I IFollowing the right hand rule the force will be towards the other wire.

ATTRACT

A wire carrying 25 A of current is 5 cm from another wire carrying 35 A of current. What is the force on the 35 A wire if it is 1.3 m long (include its direction)?

25 A 35 A

B =o I

2 r

F = B I L Sin()

Who cares about forces on wires carrying current?

N

S

current in

current out

Which way does the magnetic field point?

What direction is the force on the left side of the loop?

What direction is the force on the right side of the loop?

Why doesn’t the current end of the loop generate a force?

N

S

current in

current out

rotation

N

S

current in

current out

A DC motor works very similarly but there is a problem with this motor

rotationoutin

Counter Clockwise

N

S

current in

current out

After the coil makes ½ a full rotation

rotationinout Clockwise

N

S

commutator

The two halves are separated by an insulatoreach side connects to a wire. but not current flows yet….

How this is solved is by using a commutator

N

S

“Brushes” make the connection from the battery to the commutator

dc motor animation

N

S

Stationary “brushes” make the connection from the battery to the commutator and keep the current going in the right direction

N

S

The commutator and brushes keep everything turning

dc motor animation

I

What is the direction of the force on the right side of the loop?Left side?Top?

II

The loop will experience torque.

Torque on a loop in a magnetic field.Works for any shaped flat coil.

= N I A B Sin()

Torque (Nm)

# of loopsArea within loop (m2)

Current (A)

Magnetic Field

view of loop from behind it

I

I

What is the direction of the forces on the top and bottom

What is the direction of the forces on the top and bottom

torque will be...

What is the direction of the forces on the top and bottom

What is the direction of the forces on the top and bottom

why a commutator is needed

= N I A B Sin()

no lever armsin() = 0

full lever armsin() = 1

= N I A B Sin()

no lever armsin() = 0

full lever armsin() = 1

90o

0o

In terms of the coil and the magnetic field, you would need to look at a right angle to the coil face.

0

N S

This is how a ammeter works (measures current)also called a galvanometer

with no current a spring holds the needle at zero

510

510

A spring resists the turning of the coil

A galvanometer or ammeter

0

N S

When current runs through the wire.Which way is the current flowing?

the electromagnet twists against a springbecause of the torque

510

510

0

N S

When the current increases so does the torque on the magnet

so the spring stretches further.

510

510

Why does the needle line up like this?(think field lines)

A circular coil of wire has a diameter of 20.0 cm.The current running through the loops is 3.00 A, and the coil is placed in a uniform 2.00 T magnetic field. What is the minimum and maximum torque on the loop?

minimum = 0maximum = 1.88 N m

Electromagnets can turned on or off.Just place a ferromagnetic material in a coil of wire and flow current.

Which end would be the north pole?

bar magnetelectromagnet

A ferromagnetic material inside the coil of wire is called a core.It greatly increases the magnetic field strength. You can think of all the domains lining up inside.

B0 = 0nI

BM = MnI

Btot = BM + B0

field from coil alone

field from core

magnetic permeability of the substance

Mmaterial

mu-metalan alloy of Ni, Cu, Fe, Mo

M/0

2.5×10−2 20,000

Steel 8.8×10−4 700

Nickel 1.3×10−4 100

vacuum 1.3×10-6 1

T mA

Demo- air solenoid with cenco low voltage power supply

map field using compasses

draw in a piece of metal “core”

A solenoid valve

-

A sheet of metal is connected to a battery. Current flows due to the difference in voltage between the two ends

A little more realistic

-

-

-

-

-

If the plate of metal is place in an magnetic field,which way will the electron be pushed as if travels to the right?

-

As electrons flow to the right, they get pushed downward creating a negative side and a positive side.

-

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

+ + + + + + + + +

- - - - - - - - - - -

-

If I connected a wire between the two sides, would current flow?

-

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

+ + + + + + + + +

- - - - - - - - - - -

Yes

-

Because there is a voltage difference (electric potential) between the two side

-

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

+ + + + + + + + +

- - - - - - - - - - -

-

The difference in voltage on the two sides is called the HALL EFFECT.

-

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

+ + + + + + + + +

- - - - - - - - - - -

-

Why don’t all the current electrons all go to the bottom as they make across?

-

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

+ + + + + + + + +

- - - - - - - - - - -

They repel

+

An electron traveling to the right, is mostly indistinguishable from a proton traveling to the left.

Would the plate below be charged the same as above?

-

+ + + + + + + + +

+ + + + + + + + +

- - - - - - - - - - -

- - - - - - - - - - -

The hall effect first revealed that it was the electrons travelling.

The voltage difference max’s out when the electric force repelling the electrons is equal to the magnetic force pushing them down.

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

Fmag

Felectric

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

--

-

-

----

+ + + + + + + + +

- - - - - - - - - - -

Which way does the electric field point?

Hall effect sensor can cheaply measure magnetic fields

Moving charged particles makes a magnetic field.

(electricity makes magnetism)

Can the reverse be true?

ABSOLUTELY!!!

A wire and a magnet moving relative to each producesvoltage or current.

-

If the grey metal conductor is moved to the right, which way will the electrons be pushed?

-

+

-

The motion of the conductor through the field creates a potential / voltage difference / EMF. Very much like the hall effect.

-

+

-

-

+

+

+

+

+

+-

-

A very crude representation of charge distribution but you should get my drift

-

+

-

The imbalance of charge max’s out when the magnetic push is balanced by the electrostatic repulsion.

Fmag

Felectric

v

-

+

-Fmag

Felectric

v

Electric Force on a charge particle F, E, q?

E = qF

F = q E

Magnetic force on a moving charged particle

F = q v B

= q vd Bq E

-

+

-Fmag

Felectric

v

E = v B if we assume a uniform electric field (a decent approximation here)

V = E d The potential difference between the two ends is the electric field x the length of the rod.

V = v B l

l

= v B lEmf / voltage difference due to motion (V)

Speed of motion (m/s)

Length of metal in field (m)

As far as I can tell, voltage and emf are the same thing except that emf ignores any internal resistance.

(I remember being annoyed with this in college physics)

A metal bar slides over a wire,Why is the direction of current (I) in the wire?CW or CCW

+

How fast do you need to move 15 meters of wire perpendicular to a 8.0 tesla field to generate 120 volts.

If a proton is shot through the field as shown it experiences an upward force following the right hand rule

SN

Field

Motion

Force

Field

Motion

Force (current)

If a wire is moved so that it moves only in the same direction as beforethe charged particles inside feel a force and the mobile ones move

The result is electric current

SN

Fingers point to the field,Thumb points in the direction of the wire’s movementYour palm gives you the direction of conventional current

SN

If the wire is moved up, what direction is current

Field

Motion

current

Fingers point to the field,Thumb points in the direction of the wire’s movementYour palm gives you the direction of conventional current

SN

If the wire is moved down, what direction is current

Field

Motion

current

A generator is A LOT like a motor, but you put work in and get electricity out!!!

N

S

Which way will current flow, if the loop is rotated as shown.

Field

Motion

For the right side of the loopleft side of the loop

current

Field

Motion current

N

S

Which way will current flow, if the loop is rotated as shown.

The current is picked up by brushes and sent to your home

N

S

As the wires trade places….. the current direction will flip

current

0A

+A

-A

0A

+A

-A

When the wire is moving parallel to the magnetic field, the current is the ZERO

OR the faster a wire “cuts through” magnetic field lines the greater the current

Animation of AC or DC generator

An AC generator

Faraday’s law of induction(the math behind the scenes of what we have already seen)

Michal Faraday(1791 – 1867)

The 3rd child of a blacksmith was “given” the most basic education…. the rest he worked for having been born lower class. He discovered relationships the really made electricity useful on a wide scale basis as well as many discoveries in chemistry. The unit of capacitance is named for him.

1st off the concept of flux.

It is the rate of flowthrough a boundary.

Think of the arrows as water…

Which ring would getthe most water flowing through it

Phi is the variable for flux (the greek f )

Magnetic flux is the amount of magnetic field “flowing” through something (like a loop wire)

Magnetic flux

B

Cos (

B B

Magnetic flux

Magnetic field (T) Area of

“boundary” (m2)

Field to loop orientation

Cos (1 Cos ( 0

B

What is the magnetic flux through a loop of wire forming a radius of 10 cm in an magnetic field of 3 T if it is at an angle of 45o to the field?

B

t

Voltage is proportional to the rate of change in magnetic flux vs. time. (the rate at which the wire cuts through field lines)_

B

Don’t write this equation down yet

Flux FluxRotated by 10o

Rotated by 10o

Where during the spin is the greatest rate of change of flux

As the ring rotates around at a constant rate, the rate of change of flux is not

constant.

The easiest way is again, the faster the wire cuts through field lines the more current / voltage is produced.

t

Faraday’s law of induction

Induced EMF(Volts)

Number of loops

Rate of flux

change

The negative sign has to do with Lenz’s law which we will see coming up

A loop with a radius of 7.0 cm is initially perpendicular to a 4.0 tesla field. It is rotated

such that it is parallel with the field in 0.01 seconds. Determine the average emf in the coil.

If there had been 10 loops?If it had rotated in 0.1 s?

= v B l sin ()remember

Emf produced by a wire moving in a magnetic field

NN S

v

v

a

b

= 2NBlv sin()

number of loops

distance from a to b

N S

Instead of moving the wire, you can move the magnet.Which direction of motion would generate the most current

or

N S

What kind of motion of the magnet would generate the most current

NS

A magnet is brought closer to a loop of wire which way does the current flow in the loop?

2nd pick a point that is easy to visualize direction of the field

1st draw the magnetic field

NS

We haven’t learned a right hand rule for magnets moving but we have learned one for moving wires/charged particles….

The magnet moving to the right would be JUST like the coil moving to the….

NS

Field

Motion

current

Current is into the page

NS

As the magnet move back to the left it would be like the coil moving to the…..

NS

Field

Motion

current

And current will flow…

Current is out of the page

Current reverses!!!

The greater the length of wire in the changing magnetic field, the greater the current.

A simple way to do this is to coil the wire.

The more coils, the more effective the “generator” is.

This is how a “shaker” light works. A decent one will have a lot of coils

The field lines “cut” through the wire as the magnet moves

A CHANGING MAGNETIC FIELD PRODUCES CURRENT

This is called electromagnetic induction

A stationary magnet and wire produce NO Voltage or CURRENT

“Shaker” light.

A generator produces electric voltage & current which means it makes energy?

NO of course,Energy is not created or destroyed it just changes forms.

A generator converts energy usually supplied by coal/gasoline into electrical energy.

Phet generator applet

Slower Faster

It makes sense that moving the magnet faster (more KE), produces more electrical energy.

More Energy IN = More Energy OUT

Moving a magnet at the same speed through more coils also produces more energy

HOW does that make sense with conservation of energy?

It is harder to push a magnet through the middle of more coils Work (energy) = Force x Distance

Pushing & Pulling the magnet, produces an electric current.

A current flowing through a wire produces a…..

Magnetic Field

LENZ’s Law:

When a moving magnetic field creates a current, that current produces a magnetic field which opposes the motion of the magnet that created it.

Lenz’s law demo’s

Magnetic Aluminum coins?

Lenz’s Pendulum

Slowing down gravity?

B

A wire moves down through the magnetic field.In what direction will current flow?

What direction will the magnetic field be around the wire?

The more current produced, the harder you have to push the magnetic to move it because the lenz magnetic field is stronger

Magnet

A magnet produces eddy currents when moving near a conductor.

These eddy currents produce a magnet field which slows the magnet

velocity force from eddy current magnetic field

Applications of Lenz’s law

What slows you down on Six Flags Superman Ride

You know how long it takes a triple beam balance to stop moving?Not if….

A vacuum motor consists of just a lot wire coiled and with each end connected to 120 V outlet.

What do you think would happen if you took lets say 100 feet of wire and put the ends into an outlet?

How does the wire in a motor not short out the circuit or burn up the wire

Even 200 or 300 feet of copper would only have a resistance of a few ohms meaning ~ 60 Amps of

current.

Needed for 60 A

Motor loop wire

The current through the loops is impeded not by the resistance of the wire but by a backwards voltage/emf created by the rotation of the loop.

Explanation follows……

N

S

current in

current out

Picture a simple motor (commutator omitted for clarity)

The magnetic field will create torque on the current carrying loop of wire as before

N

S

BUT Don’t forget the wire is rotating

For a second ignore the direction that the “protons” are moving in the wire due to current flow. Look at the direction a “proton” is moving movement of the now rotating wire with the wire.

What direction would the force be on the proton?

Field

Motion

Force

On the right side

Field

MotionForce

On the left side

N

S

current in

current out

The rotation of the wire, creates a force a force which opposes the current in the wire.

Field

Motion

Force

Field

MotionForce

current

current

The force pushing back current is called Back EMF.It acts like resistance in the wire (since it is slows current).

N

S

current in

current out

Field

Motion

Force

Field

MotionForce

current

current

N

S

Motion

current

Back EMF

When the motor first start up, the coil is not rotating. There is no back EMF and

current is briefly very BIG.

N

S

Motion

current

Back EMF

As the coil speeds up, back EMF increases and

current slows.

N

S

Motion

current

Back EMF

Eventually the motor reaches a speed where the back EMF balances out with the voltage driving

current and friction etc…and the motor maintains a

constant speed.

N

S

Motion

current

Back EMF

Now the motor starts to lift something heavy, which causes itsSpeed to …Back EMF…Current to…

Why will a motor that gets jammed trip a breaker?

The amount of back EMF is proportional to the speed at which the motor rotates

Field

Motion

back EMF

current

Field

Motion

back EMF

current

Field

Motion

back EMFcurrent

Motor starts up, no movement initially. No motion or back EMF.Large current.

Motor spins up. Speed and back EMF Increase. Current Decreases

Under no load the motor will reach a maximum speed, when the Back EMF reduces the current to just overcome Friction and Heat Loss

Field

Motion

back EMF

current

Max Speed no load (no work)

Field

Motion

back EMF

current

When the motor does work against a load. It slows down.Back EMF drops, current goes up and it does work.

This is how a motor “knows” how hard to push (within limits)

Field

Motion

back EMF

current

Back EMF will flow current the backwards as it continues to rotate.

When a the current is shut off (current is cut)

When a motor is shut “off” its rotational momentum keeps it moving for a time and it acts like a generator.

The current produced can back feed current to any device connected. This can be bad.

Motors and generators are the same device just used differently

BMW regenerative braking video clip

Faraday’s switch applet

Current is produced in a wire only when the magnetic field around the wire CHANGES!

Field lines cut through it

Faraday applet

Transformer change the voltage of electricity

but not for free

Currentis fed to this side

A current is produced on this side

Current in

When a current flows through the primary coil wrapped around a core it acts like an electromagnet.This causes the entire core to become magnetized.

Which way will the field lines point?

Current in

If the current is constant, they core will remain strongly magnetized

Will the secondary coil have more, less, or the same amount of current as the primary?

NONE!

Remember: a stationary magnet (constant magnetic field) and wire produce NO Voltage or

CURRENT

In order to make current, the magnetic field has to change. Like moving the magnet in and out of the coil

Current in

This is like holding a stationary magnet in the secondary coil.

Even a strong magnet makes no currentIF it does not move

Current in

This VEXED early scientists trying to produce current in the other coil. UNTIL…...

No current

They noticed right when the switch shut off the primary current.

xxx

A light would light!

xxx

But only for a split second.

xxx

But when the switch was opened again…..

xxx

the light lit again.

xxx

The light would only light RIGHT when the switch was opened or closed

xxx

OPEN

To keep it lit, the switch must be

xxx

CLOSED

To keep it lit, the switch must be

xxx

OPEN

To keep it lit, the switch must be

xxx

CLOSED

To keep it lit, the switch must be

xxx

OPEN

To keep it lit, the switch must be

xxx

CLOSED

To keep it lit, the switch must be

xxx

Because it is not a magnetic field that makes current in a wire but a magnetic field that is….

Moving or changing.

Important:

A magnet and wire only make electricity if they are moving relative to each other.

a magnet sitting on top of a wire doesn’t make your toaster work. DUH...

xxx

IT WORKS

xxx

If only there was a way to get electricity that moved back and forth on its own!!!

Like alternating current!!!!!!!!!!!!!!!!!!!!!

0A

+A

-A

The current jiggles back and forth automatically!!!

Early inventors only had batteries, which made DC

xxx

Steady current, constant magnet = No lightDRATS!@!

xxx

From AC Generator AC does the work for me this is great!!

I’m getting paid for nothing.But who is paying me?

You are getting ready to go to college where you will be FRESHMEN again. Keep working…..

And don’t do anything stupid with your new found freedom!

NO ONE….. I’m sorry do you want fries with that?

= BLvThe magnetic field strength is the same

More wire means….

The magnetic field strength is the same

More wire means….

More wire = More Voltage

= BLv

More coils in the secondary means the voltage is increased

Less coils in the secondary means the voltage is decreased

6,000 V

120V

8000 V

240,000 V

A typical power “grid” why bother with the voltage increase????

   Steps down from 240,000 V to 8000 V

Steps down from 8000 V to 240 V

Nin Nout=Vin Vout

A primary coil of 500 loops fed from household current is connected via a transformer to 38 loops. What is the output voltage?

BUT no escaping the :Law of conservation of energy

Which means:

energy in = energy out

Time Time

power in = power out

For electrical energy:

Power = current x voltage

P = IV

power in = power out

P = IV

IinVin = IoutVout

When voltage is decreased….

current…..

power…..

High voltage = less current for the same energy transferred.

-minus the energy lost in the tranformers

A 12V transformer delivers 1.5 W, how much current is drawn from the 120V outlet it is connected to?

  How does this power-stealing device work?

Next set of slides are already done delete next year.

I

15 A of current flows through both wires below. If they are seperated by a distance of 30 cm. What will the magnetic field strength be midway between the?

I

t

Faraday’s law of induction

Induced EMF(Volts)

Number of loops

Rate of flux

change

The negative sign has to do with Lenz’s law which we will see coming up

A loop with a radius of 7.0 cm is initially perpendicular to a 4.0 Tesla field. It is rotated

such that it is parallel with the field in 0.01 seconds. Determine the average emf in the coil.

If there had been 10 loops?If it had rotated in 0.1 s?

A proton is shot into a magnetic field.In what direction with the magnetic force be?

+

In what direction should an electric field be set to keep it moving straight?

v

FB

E

FE

-

How about here with an electron?Which way should the electric field point?

EF

E

FB

A charge like an electron produces an electric field

e-

Moving an electron produces changing electric field.( only 1 field line shown for clarity)

Moving charge applet

animation

This moving ELECTRIC field also moves which creates a...

Magnetic FIELD….e-

This moving magnetic field also moves which creates a...

ELECTRIC FIELD….e-

Which makes a….

magnetic field which makes anelectric field which makes a magnetic field which makes anelectric field which makes a magnetic field which makes anelectric field which makes a magnetic field which makes anelectric field which makes a etc………...

Electromagnetic wave applet

An oscillating charge produces and electromagnetic wave

Electromagnetic wave applet

All that is needed to produce an EM wave is some electrons moving back in forth in a conductor

AC SOURCE

This type of device is called an…

Antenna

If another antenna is near

AC SOURCE

oscillating electric charge applet

It electrons will tend to oscillate too...

AC SOURCE

oscillating electric charge applet

If a meter is attached, AC current would be detected.

AC SOURCE

Some people in the late 1800’s people thought that the wires that delivered electricity to houses would be gone soon…

That didn’t work out

Nikola Tesla had this crazy idea in the early 1900’s that this could be used to send information without wires.

"As soon as [the Wardenclyffe plant is] completed, it will be possible for a business man in New York to dictate instructions, and have them instantly appear in type at his office in London or elsewhere. He will be able to call up, from his desk, and talk to any telephone subscriber on the globe, without any change whatever in the existing equipment.. . . ." — Nikola Tesla

An antenna is used for sending and receiving a signal.The optimum length of the antenna depends on the wavelength being sent

Sending information in the EM waves did however catch on a bit...

How information is sent in a wave.

Miscellaneous not used stuff

Placing ferromagnetic material in the center of the coils makes it even stronger. Iron is called a CORE.

Soft iron is used because its domains don’t stay aligned after the current is turned off. Steel would remain magnetized.

The direction of the magnetic field can be found by curling your fingers (right hand) following the current.

Your thumb shows the direction of the field (pointing to N)

If the current was reversed. The poles would switch.

What are the poles?

NS

The magnet on the boom of this crane can be turned on and off.

A quick note about “conventional current”

Vs “actual current”

the rules we deal with upcoming are based on conventional current

The magnetic force is due to the electron moving through a magnetic field. so

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Fmag

F = qvB sin()

Vdrift

Fmag = qe vd B

E = qF

F = q E

Fe = qe E Fmag = qe vd B

= qe vd Bqe E

E = vd B

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+ + + + + + + + +

- - - - - - - - - - -

Kind of like parallel plates so we can use…

V = E d

d

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+ + + + + + + + +

- - - - - - - - - - -

d

E = vd B

V = E d

VHall = vd B lThe book uses “l” instead of d