Board Work

What are the directions of the forces on the opposite charges moving with velocity v through magnetic field B?

B

v

+−

Moving conductors

through magnetic fields

Poll Question

B

I

What is the direction of the Lorentz force on the charges moving along this wire?

A.

B.

C.

D.

E.

F.

Current in a Field Feels a Force

F

B

I

Loop in a Field Has Torque

B

I I

Torque Turns a Motor

I

I

current I

force F

B

–

Motion Creates a Potential

v

B

induced potential+

Rotating Loop Creates Circuit

B

V

V

Rotating Loop Creates Circuit

B

V V

Rotation Powers a Generator

v

v

Motion

VVinduces a potential

which generates a

currentthat charges the

battery

B

through the field

Magnetic Induction

Transforming energy with magnetism

Objective

• Explain the electric potential created by a changing magnetic field.

What’s the Point?

• This is how electric current is harnessed to do mechanical work.

• It’s another way to think about the Lorentz force. ☺

Lorentz Force Reminder

F = qv B

Source: Griffith, The Physics of Everyday Phenomena

F is directed out of the screen.

Magnetic Flux

• Field through a surface

= BA

• Effectively the number of field lines enclosed by a loop

Flux Change Creates Potential

Faraday’s Law

The rate of magnetic flux change through a loop equals the electric potential induced around the loop.

V =

t

Magnetic Flux

• Flux changes as loop turns

a b c d e

• At a, c, e: Greatest flux , least change

• At b, d: Zero flux, fastest flux change /t

Induced current direction

Lenz’s Law

When a changing magnetic flux induces a current, the induced current’s magnetic field opposes the flux change.

Lenz’s Law Example

• From a to b: – flux decreases; into face of loop ←– Loop current increases– Induced field increases out of face of loop

a b c d e

Induced current

Lenz’s Law Explains Generator

B

changing field

opposing field

needed I

Board Work

If the loop inside this magnetic field expands, in what direction will the induced current around it be?

B

What is the Lorentz force on each side?

Which direction is the flux change?

What current would oppose it?

How a Microphone Works

microphone

geophone

Flux inside loop becomes greater

Wave-Powered Generator

Originally appeared in Smithsonian magazine.

How a Loudspeaker Works

loudspeakerinduced motion of magnet

currentcurrent-generated field attracts or repels magnet

Reading for Next Time

• Electromagnetic Waves