Physics 7C Fa 2008Lecture 8: Electricity &

Magnetism

Magnetism: RHR 1 & 2

Light as EM wave

Polarizers

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Field Model of Magnetism

A source moving charge creates a magnetic fields in a direction given by RHR1.

Another moving charge, placed in a magnetic field, experiences a magnetic force Magnitude given by F=qvBsin Direction of force given by

RHR2

F

vB

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Which direction is the magnetic field at the following points? Where is it biggest?1) Left2) Right3) Up4) Down 5) Into screen6) Out of screen7) Away from wire8) Toward wire9) Something else

I

A B

C D

Answers added as slides at end of lecture.

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Which direction is the magnetic field at the following points? Where is it biggest?1) Left2) Right3) Up4) Down 5) Into screen6) Out of screen7) Away from wire8) Toward wire9) Something else

I W

X

Z

Y

Answers added as slides at end of lecture.

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Which direction is the magnetic field at point C?

1) Left2) Right3) Up4) Down 5) Into screen6) Out of screen7) Away from wire8) Toward wire9) Something else

I1

C

I2

You may assume I1=I2

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Magnetic Force

Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling along the chalkboards, to your left? 1) Into the board

2) Out of the board

3) Left (along particle path)

4) Right (opposite path)

5) Down

6) Up

7) No Force

v

B

F = qvBsinwhereis the angle between B and v

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Magnetic Force

Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling out of the board, to the back of the room? 1) Into the board

2) Out of the board

3) Left

4) Right

5) Down

6) Up

7) No Force

v

B

F = qvBsinwhereis the angle between B and v

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Magnetic Force

Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling upward, toward the ceiling?

1) Into the board

2) Out of the board

3) Left

4) Right

5) Down

6) Up

7) No Force

v

B

F = qvBsinwhereis the angle between B and v

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Magnetic Induction

Magnetic Flux: the “amount of B-field through an area” Stronger B-field means more field passes through Larger area means more field passes through Different orientations permit more field to pass

through

How should B-field be oriented for maximum magnetic field to pass through the loop?

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Inducing current Imagine a region with a magnetic field away

from you in some regions (into the screen) and zero in other regions, as shown below.

Right wire is blue wire. Left wire is red wire. At t=0, loop is outside the field. Our goal:

1) What happens as the loop enters the magnetic field?

2) What happens while the loops moves within B.

3) What happens as the loop exits the magnetic field?

4) Connecting to chaning fields.

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Applying RHR2: 0) Before we enter the field:

Describe the force at the instant shown on positive charges in the blue wire:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

Why?

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Applying RHR2: 1) Entering the field:

Describe the force at the instant shown on positive charges in the blue wire:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

Why?

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Applying RHR2: 1) Entering the field:

Repeat for red, top and bottom wires:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

Why?

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Applying RHR2: 1) Entering the field:

Draw the current the results from the forces we just describes as loop enters field.

Draw the magnetic field from the induced current.

Would this analysis change if I had asked for the forces on the electrons?

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Applying RHR2: 2) Within the field:

Describe the force at the instant shown on positive charges in the blue wire:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

Why?

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Applying RHR2: 2) Within the field:

Repeat for red, top and bottom wires:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

Why?

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Applying RHR2: 2) Within the field:

Draw the current the results from the forces we just describes as loop moves within field.

Draw the magnetic field from the induced current.

Would this analysis change if I had asked for the forces on the electrons?

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Applying RHR2: 3) Leaving the field:

Describe the force at the instant shown on positive charges in the blue wire:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

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Applying RHR2: 3) Leaving the field:

Repeat for red, top and bottom wires:

1) Left2) Right3) Up4) Down5) Into Screen6) Out of screen7) Zero8) Other

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Applying RHR2: 3) Leaving the field:

Draw the current the results from the forces we just describes as loop leaves field.

Draw the magnetic field from the induced current.

Would this analysis change if I had asked for the forces on the electrons?

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Connecting to changes in amount of field through loop

In which of the previous times was the amount of field passing through the loop changing?

a) Before entering field

b) While entering field

c) Within field

d) Leaving field

In which cases was a current induced?

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Changing fields induces a current that creates a field

Induced current makes a field opposite to the change in amount of field through loop:

1) Entering field

ti

No field

tf

Ext field into page

Iind

3) Leaving field

ti

No field

tf

Ext field into page

Induced field out of page Induced field out of page

Iind

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Consequences of changing magnetic fields

Anything that changes the flux through a conductor causes a current to flow in the conductor.

Before: cause of current flow is a voltage difference (like from a battery).

New model: changing magnetic flux induces voltage differences (which cause induced currents and induced magnetic fields)

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Switching Gears: Rethinking Light

What “waves” in light?

What propagates?

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Image from http://www.monos.leidenuniv.nl/smo/index.html?basics/light.htm

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A vertical wave traveling through a vertical fence passes unimpeded. The second fence also lets the wave pass.

If we place the second fence with horizontal slats, the vertical vibrations cannot pass through the fence.

Image from http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/light/u12l1e.html

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Image from http://www.lbl.gov/MicroWorlds/teachers/polarization.pdf

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Which direction is the magnetic field at the following points? Where is it biggest?1) Left2) Right3) Up4) Down 5) Into screen6) Out of screen7) Away from wire8) Toward wire9) Something else

I

A B

C D

BC>BD>BA>BB

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Which direction is the magnetic field at the following points? Where is it biggest?1) Left2) Right3) Up4) Down 5) Into screen6) Out of screen7) Away from wire8) Toward wire9) Something else

I W

X

Z

Y

BW=BX>BY>BZ

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Which direction is the magnetic field at point C?

1) Left2) Right3) Up4) Down 5) Into screen6) Out of screen7) Away from wire8) Toward wire9) Something else

I1C

I2

You may assume I1=I2

B2

B1

Bnet

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Force on particles in uniform B

B

v

Movement left,Force out of page

B

vFF

Movement out of page,Force right

B

v

v up, B down, so angle is 180º, and there is no force.

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