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Quiz 5

Reminder: Emily’s Office Hours next week are different:Wednesday 11/12 from 1-2pm in TB114 Monday 11/10 OH are cancelled

Physics 7C Fall 2008Lecture 6: Field model

Electric Force & Electric Field,

Magnetic Force & Field

If time permits, begin

Electric Potential Energy & Electric Potential

Dipole field

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Tesla Coil

Powered by electricity Observe the top--what do you

see? Observe as a fluorescent

lamp is brought near--what do you see?

Image: http://tommcmahon.typepad.com/photos/uncategorized/2007/08/16/teslacoil2.jpg

Takeaway message: the device is generating something electrical throughout space in the vicinity of the coil.

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Field Model: What is a field?

…some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same.

-Physics 7C Course Notes In physics, a “field” refers to a quantity that

has a value for every point in space.”-homework from DLM 10

Is temperature a field?

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Temperature Field

What do places with the same color have in common?

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True or False

Gravity is more on Earth than the moon.

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True or False

Gravity is more for a feather on Earth than for an elephant on the moon.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

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Field Model: Gravitation

A source mass m1 creates a gravitational field in a direction toward the source mass with a value g=Gm1/r2

The net Gravitational Field is the sum of all the source fields.

A test mass m2, placed in a gravitational field, experiences a gravitational force Magnitude given by F=m2g Direction of force: Attractive

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Electric Phenomenon

Like gravitational force, the electrical interaction between objects acts at a distance.

Electrical interaction depends on charges

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New Models:

Electric Field and Forces Field, Forces, Potential Energy, and Potential

Magnetic Field and Forces

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Applying Field Model to Electrical Phenomena

A charge Q generates an Electric Field EQ

Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ.

How might we draw the electric field?

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Applying Field to Electrical Phenomena

A charge Q generates an Electric Field EQ

Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ. For + test charge, force points

in the same direction as field. For - test charge, force points

in opposite direction of field

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Phenomenon: Van de Graaff Generator

Observe what happens as the generator builds charge.

What happens when a neutral conductor is brought near the generator?

a) No affect

b) Pulled toward

c) Pulled away

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Phenomenon: Van de Graaff Generator

Next: Touch the conducting sphere to the generator…what will happen now?

a) No affect

b) Pulled toward

c) Pulled away

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Understanding Fields & Forces

True or False: The generator creates an

electric field whether or not another object is placed near it.

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Understanding Fields & Forces

True or False: The generator creates an

electric force whether or not another object is placed near it.

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Applying Field to Electrical Phenomena

A charge Q generates an Electric Field EQ

Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ. For + test charge, force points

in the same direction as field. For - test charge, force points

in opposite direction of field

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Superposition of Fields

The electric field is a vector field To find the field from multiple sources, add

the vectors!

Which way is the electric field at the marked spot?

-

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Superposition of Fields

The electric field is a vector field To find the field from multiple sources, add

the vectors!

a b

cd

1) Which vector might be the electric field from the top charge?

-

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Superposition of Fields

The electric field is a vector field To find the field from multiple sources, add

the vectors!

a b

cd

2) Which vector might be the electric field from the bottom charge?

-

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Superposition of Fields

Which direction does the net Electric Field point?

-

E+E-

c Etot = 0d Neither a nor b

(but not 0)

b

a

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Checking Understanding:

If I put a charge at the marked location, which way will the force be?

-

db

c

ae Insufficient

Information

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Field vs. Force

How many objects are required to create a electrical field? At least…

a) 0

b) 1

c) 2

d) 3

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Field vs. Force

How many objects are required to create a electrical force? At least…

a) 0

b) 1

c) 2

d) 3

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Models of Electric Phenomena: Electric Field and Forces

Each source charge Q generates an Electric Field EQ Direction convention shown at right

The net Electric Field is the sum of all the source fields

Charge q, placed in an electric field Etot, experiences a force Felec on q=qEQ. For + test charge, force points in the same

direction as field. For - test charge, force points in opposite

direction of field Field, Forces, Potential Energy, and Potential

PE & Forces--started in 7A Potential--started in 7B We’ll develop more in DLM 13 & 14

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Field Model:

A source (A) creates a (B) _ field in a direction _. The net (B) _ field is the sum of all the

source fields. A test (A) , placed in a (B) _

field, experiences a (B) _ force Magnitude given by _____ Direction of force: _____

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Field Model: Magnetism

A source ____________ creates a magnetic field in a direction given by _______. The net magnetic field is the sum of all the source

fields. ___________, placed in a magnetic field,

experiences a magnetic force Magnitude given by _____ Direction of force given by _____

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A little background

Compasses or bar magnets, if allowed, will always orient north-south

Why?

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A little background

Compasses orient in the same direction as the magnetic field.

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A little background

Iron fillings also orient in the same direction as the magnetic field.

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Magnetic Field from a wire

If we allow iron fillings freedom to rotate, and put them near a current-carrying wire, this is how they align:

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Magnetic Field from a wire

If we place compasses around a long current-carrying wire, this is how they align

(view is looking down wire)

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Field Model: Magnetism

A source moving charge creates a magnetic field in a direction given by _______. The net magnetic field is the sum of all the source

fields. A test moving charge, placed in a magnetic

field, experiences a magnetic force Magnitude given by _____ Direction of force given by _____

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Phenomenon: Magnet near an electron beam

The beam is composed of electrons--moving charges

Observe the effects of a large magnet on the beam…

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Phenomenon: jumping wires

Two wires Initially no current (observe wires) Connect both wires to a generator, making

current flow. Observe: What happens to the wires? What happens if I reverse the direction of the current

in one wire (compared to first time)? What happens if I reverse the direction of the current

in both wires (compared to the first time)? What would happen if I could put current in only

one wire?

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Reviewing what you’ve previously studied…

Gravitational Potential Energy

1

2

3

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Relationship between Potential Energy and Force

Pote

ntia

l Energ

y

r

-

0

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Relationship between Potential Energy and Force

1

2

3

Pote

ntia

l Energ

y

r

-

0

Negative means decrease of PE with decreasing r

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Relationship between Potential Energy and Force

1

2

3

Pote

ntia

l Energ

y

r

-

0

More slope closer to earth means F is greater there

F = - PE/r, the - slope

Force increases with greater slope

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Reviewing what you’ve previously studied…

Relationship between Potential Energy and Force.

1

2

3

4

• Magnitude of Force = slope of PE vs. r graph.

•

€

F = −dPE

dr

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Defining a new quantity

Gravitational Potential: How much Potential Energy would a mass m have if placed (x,y)?

1

2

3y

x

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Electric Field and Potential: Constant Electric Field

Slope of the potential constant as a function of

distance. negative

Electric field is constant as a function of

distance positive

1 :

ˆ ˆ ˆ3 :

dVD E

dxdV dV dV

D E x y zdx dy dz

→

→

=−

=− − −

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Electric Potential of a point charge:Positive and Negative Charge.

Not all potentials are straight lines!

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Equipotential Surfaces: Lines where V is the same.

Equipotential surfaces for a point charge. Circles are 0.5 V apart. Distance between

circles is NOT uniform! Circles get closer and

closer toward center Potential grows like 1/r

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Putting it all together…

Which quantities depend only on source charge(s)?

a) Electric Field (E)

b) Electric Force (F)

c) Electric Potential Energy (PE)

d) Electric Potential (V)

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Putting it all together…

Which are vector quantities?a) Electric Field (E)

b) Electric Force (F)

c) Electric Potential Energy (PE)

d) Electric Potential (V)

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Putting it all together…

Which quantities are related by slopes (that is, if you take the slope of one, you get the other)

a) Electric Field & Electric Force

b) Electric Potential Energy & Electric Potential

c) Electric Force & Potential Energy

d) Electric Field & Electric Potential