Physics 7C SS1Lecture 6: Electricity

Analogies with Gravity:

Electric Force, Electric Field,

Electric Potential Energy,

Electric Potential,

As you wait for class to start, ponder what the following statement means…Gravity is bigger on Earth than the moon.

2

New Model(s):

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

3

True or False

Gravity is more on Earth than the moon.

4

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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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 due today

6

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 due today

True/False: Temperature is a field

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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 due today

True/False: Temperature is a field

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Temperature Field

What do places with the same color have in common?

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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 due today

True/False: Height (as measured relative to Roessler Hall entrance) is a field.

10

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 due today

True/False: Height (as measured relative to Roessler Hall entrance) is a field.

11

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 due today

True/False: The gravitational field we defined for Earth is a vector 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.

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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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 field? At least…

a) 0

b) 1

c) 2

d) 3

•Field depends only on source charge!•Field does not change depending on what type of charge is placed in it.

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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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Field vs. Force

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

a) 0

b) 1

c) 2

d) 3

•Force depends on both source charge and test charge--as before, you need two objects to have a force!•Force direction and magnitude both can change depending on what type of charge is placed in the 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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Checking Understanding:

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

-

db

c

a

e Insufficient Information

But…if I tell you the type of charge, you can tell me the direction of the force.

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Models of Electric Phenomena: Electric Field and Forces

Each source charge Q generates an Electric Field EQ

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=qEtot. 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

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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

Potential

Energy

r

-

0

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Relationship between Potential Energy and Force

1

2

3

Potential

Energy

r

-

0

Negative means decrease of PE with decreasing r

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Relationship between Potential Energy and Force

1

2

3

Potential

Energy

r

-

0

More slope closer to earth means F is greater there

F = - PE/r, the - slopeForce 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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Quiz 3: Reflection, Refraction, Lenses

You should bring a straight edge You may bring and index card (3’x5’) You will not be allowed a calculator

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Ray Model in Review

Hitting a boundary can lead to reflection or refraction, often both. We can calculate the angles for any outgoing ray-

always with respect to normal Lenses are constructed carefully to bend light

in particular ways to create images We can locate images with the thin lens equation

or a ray tracing, and describe them in many ways

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Light passes from water (n=1.3) into an unknown substance

water

?

At larger angles, will there be total internal reflection?

a) Yes, past the critical angleb) No, not possiblec) Not enough information

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Light passes from water (n=1.3) into an unknown substance

water

?

At larger angles, will there be total internal reflection?

a) Yes, past the critical angleb) No, not possiblec) Not enough information

34

You have a 2-lens system. You know only what the information shown below:

? f=-25cm

Image created by first lens

25cm

100cm

T/F: There is enough information to locate the image produced by the second lens.

35

You have a 2-lens system. You know only what the information shown below:

? f=-25cm

Image created by first lens

25cm

100cm

T/F: There is enough information to determine what type of lens the first lens must be.

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You have a 2-lens system. You know only what the information shown below:

? f=-25cm

Image created by first lens

25cm

100cmT/F: There is enough information to determine what type of lens the first lens must be.

Note: Only true because I told you in class that the light originated from the left, so the original object was left of the lens.

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Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?

38

Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?

Stumped? 1) Try to locate the original object, making a guess about the focal length

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Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?

Stumped? 2) Next: how will focal length change if index of refraction increases?3) Draw new image.

40

Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?