Magnetic Induction

Transforming energy with magnetism

Lorentz Force Reminder

F = qv B

Source: Griffith, The Physics of Everyday Phenomena

F is directed out of the screen.

CPS Question

B

I

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

A.

B.

C.

D.

E.

F.

–

Moving Creates a Potential

v

B

induced potential+

Motional emf

• Moving a conductor through a magnetic field causes separation of charges.

• Back-emf (motional emf) in conductor cancels potential from motion

= lvB

• when l, v, B all orthogonal

Rotating Loop Creates Circuit

B

V

V

Rotating Loop Creates Circuit

B

V V

Group Work

a. At which position(s) does the most magnetic flux pass through the loop?

b. At which position(s) does the magnetic flux through the loop change the fastest?

a b c d e

Group Work

c. What is the current direction at these points?

a b c d e

Faraday’s Law

• Greatest flux when perpendicular • Fastest change in flux /t when parallel

a b c d e

Rotation Powers a Generator

v

v

Motion

VVinduces a potential

which generates a

currentthat charges the

battery

B

through the field

Lenz’s Law Explains Generator

B

changing field

opposing field

needed I

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

Group Work

If the magnetic field intensifies, in what direction will the induced current be?

B

Which direction is the flux change?

What current would oppose it?

What is the Lorentz force on each side?

How a Microphone Works

microphone

geophone

Flux inside loop becomes more

How a Loudspeaker Works

loudspeakerinduced motion of magnet

currentcurrent-generated field attracts or repels magnet

Back Potential

• As the motor turns, back-potential B/t opposes

• Faster rotation more back potential

I

B