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Physics 7C SS1, Lecture 9:Field Model & EM Waves

Magnetic Field

Electromagnetic Waves

Polarization

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Final Exam Review Sessions Definitely some on Monday. Which

other day would you like most?a) Thursday

b) Friday

c) Saturday

d) Sunday

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Magnetic Forcev

BRHR2 (for positive charge): your thumb points in the direction of the moving charge, B is along your index finger, and F is the middle finger.

Very Bad Finger

F out of the screen

F = qvBsinwhereis the angle between B and v

F

vB

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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

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Magnetic Force Suppose a large magnetic field points downward at

every point in the room. Which direction should the particle be traveling in to experience a force to the left?

1) Into the board

2) Out of the board

3) Left

4) Right

5) Down

6) Up

7) No Force

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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 upwards, toward the ceiling?

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

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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 Reverse direction for negative test

charge

F

vB

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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:t=0, before 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:As 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:As entering the field

Repeat for charges in the top, bottom, and red wire:

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

Why?

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Applying RHR2:As entering the field

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

Draw the magnetic field from the induced current. (focus on the inside the loop)

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

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Applying RHR2: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: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

Why?

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Applying RHR2:As leaving the field

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

Draw the magnetic field from the induced current. (focus on the inside the loop)

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

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A new way to analyze situations with changing B Magnetic Flux: the “amount of B-field through

an area Depends on…

B: strength of B-field A: area bound by conductor : orientation of loop with respect to B-field.

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

B

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Applying Magnetic Flux 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

When was there an induced current?

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A changing B-field induces a current that creates another B-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?

21Image 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

23Image from http://www.lbl.gov/MicroWorlds/teachers/polarization.pdf

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What happens in each of the following cases? (use RHR2)

S

S

N

N

(a) Wire carrying current is placed in a B-field

(b) A wire (without current) is moved up and down as shown

X

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What happens in each of the following cases? (use RHR2)

S

S

N

N

(c) A loop of wire (no current) is turned within a magnetic field

(b) A loop of wire carrying current is placed in a B-field

I

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What happens in each of the following cases? (use B)

S N

(c) A loop of wire (no current) is turned within a magnetic field