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ElectromagneticInduction

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Electromagnetic Induction

Voltage is induced whether the magnetic field of a

magnet moves near a stationary conductoror

the conductor moves in a stationary magnetic

field.

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Demo: Magnet Induces Current

Voltage is induced when a magnet moves towards

or away from a coil, inducing a current in the coil.

Faster the magnets motion, the greater the

induced current.

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Magnetic Recording

Write data by magnetizing

recording media (e.g.,

video tape, hard disk)using electromagnets.

Data is read back using the

induced current produced

when magnetized mediamoves past receiver coils

(reverse of writing data).Hard disk

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Electric Generator

Electric generator moves a conductor in a

magnetic field to produce voltage via

electromagnetic induction

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Demo: Electric Generator

Turn the shaft byhand and as

the coils passthe magnets avoltage isinduced.

DC current isgenerated.

Magnet

Magnet

Coils

DC Output

S

HA

F

T

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Induction: No Free Lunch

Takes work to turn

the generator crank

to produce electriccurrent.

The faster we turn

the crank to

produce morecurrent, the more

difficult it is to turn.

More difficult topush the magnetinto a coil with

more loopsbecause themagnetic field ofeach current loopresists the motionof the magnet.

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Faradays Law

The induced voltage in a coil is proportional to the

product of the number of loops and rate at which

the magnetic field changes within the loops.

Small

Voltage

Medium

Voltage

Large

Voltage

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Self-Induction

When a current is

induced by a

changing magneticfield, that current

itself produces its

own magnetic field.

This effect is calledself-induction.

Current

Primary Magnetic Field

Self-Induced

Magnetic Field

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Demo: Magnetic Brakes

Strong magnet dropped into

a copper pipe falls slowly

due to secondary magneticfield induced by its motion.

Great AmericasDrop Zone

has a 22 story freefall,

lasting four seconds,

decelerated by magnetic

braking.

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Eddy Currents

Changing magnetic field

induces eddy currents

within any conductor.

These internal currents

produce self-induced

magnetic fields, which

by Lenzs law are in

opposition of theprimary magnetic field.

Eddy

Currents

Primary Magnetic Field

Self-Induced

Magnetic Field

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Metal Detectors

Can detect the presence ofmetals by using atransmitter coil to create

an oscillating primarymagnetic field.

This creates a secondarymagnetic field due to eddycurrents in the metal.

Can detect this secondarymagnetic field by using areceiver coil.

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Demo: Back Electromotive Force

Changing magnetic field in an electromagnet

induces a voltage in the electromagnet

itself.

When we disconnect a large

electromagnet the magnetic field

rapidly collapses.

This changing magnetic field

self-induces a large voltage in the

coil of the electromagnet.

Large spark occurs at the switch.