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Magnetic Induction. Transforming energy with magnetism. F = qv  B. Lorentz Force Reminder. Source : Griffith, The Physics of Everyday Phenomena. F is directed out of the screen. CPS Question. What is the direction of the Lorentz force on the charges moving in this wire?. B. A. D. B.

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Magnetic induction

Magnetic Induction

Transforming energy with magnetism


Lorentz force reminder

F = qv  B

Lorentz Force Reminder

Source: Griffith, The Physics of Everyday Phenomena

Fis directed out of the screen.


Cps question
CPS Question

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

B

A.

D.

B.

E.

C.

F.

I


Moving creates a potential

+

induced potential

Moving Creates a Potential

B

v


Motional emf
Motional emf

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

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

    e = lvB

  • when l, v, B all orthogonal


Rotating loop creates circuit

V

V

Rotating Loop Creates Circuit

B



Group work

a

b

c

d

e

Group Work

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

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


Group work1

a

b

c

d

e

Group Work

  • What is the current direction at these points?


Faraday s law

a

b

c

d

e

Faraday’s Law

  • Greatest fluxF when perpendicular

  • Fastest change in flux DF/Dt when parallel


Rotation powers a generator

Motion

V

through the field

v

V

induces a potential

v

B

Rotation Powers a Generator

which generates a current

that charges the battery


Lenz s law explains generator

changing field

e

e

opposing field

B

needed I

Lenz’s Law Explains Generator




Torque turns a motor

current I

I

force F

I

B

Torque Turns a Motor


Group work2
Group Work

Which direction is the flux change?

What current would oppose it?

What is the Lorentz force on each side?

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

B


How a microphone works

Flux inside loop becomes more

How a Microphone Works

microphone

geophone


How a loudspeaker works
How a Loudspeaker Works

loudspeaker

induced motion of magnet

current-generated field attracts or repels magnet

current


Back potential

B

m

I

Back Potential

  • As the motor turns, back-potentialDFB/Dt opposesm

  • Faster rotation  more back potential


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