c25 induction

8/6/2019 C25 Induction

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5-Aug-11 Physics 1 (Garcia) SJSU

Chapter 25

ElectromagneticInduction


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WARNING

Strong magnets will be

passed around the room.Keep these magnets away

from iPods and laptops(especially my laptop)!


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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.

Note: Do this in lab too.


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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 media

moves 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 is

induced.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 electric

current.

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

LargeVoltage


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Demo: Electromagnetic Oscillations

Connect to

alternating

current (AC)

Oscillating

Magnetic

Field

Electro-

Magnet

Put alternating current into an

electromagnet and you create

an oscillating magnetic field.

This oscillating magnetic field

induces electrical currents by

inducing electrical field

oscillations.

This is, effectively, a very low

frequency electromagnetic

antenna.

Coil with bulb


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Demo: Electric Motor

Can create an electrical motor by passing a

current through a set of electro-magnets

mounted on a rotating shaft.

Current inCurrent out

Electro-magnets


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Electric Motor, Analyzed

Electromagnetmounted on ashaft with

opposing magnetson each side.

Current direction

always such thatelectromagnet isrepelled, causingshaft to turn.

N S

N S

Current

Current


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5-Aug-11Physics 1 (Garcia) SJSU

Demo: Generator Becomes Motor

Pass a current into the generator and it

becomes an electric motor.

Rotor

Spins

Magnet

Magnet

DC Input

Battery



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Self-Induction

When a current is

induced by a

changing magnetic

field, 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: Lenzs Law

Connect to

alternating

current (AC)

Oscillating

MagneticField

Electro-

Magnet

Metal ring is

levitated by

self-induced

secondary

magnetic field

Induced current

produces a

secondary magnetic

field that is alwaysopposed to the

primary magnetic

field that induced it,an effect called

Lenzs law.


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Check Yourself

Does the ring levitate if we connect

the electromagnet to direct

current (e.g., to a battery)?

Does the ring levitate if it has a

gap?Electro-

Magnet


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Check Yourself

Connect to

alternating

current (AC)

Oscillating

Magnetic

Field

Electro-

Magnet

Hold the ring down on the shaft and it

starts to get hot. Why?

What if we cool the ring in liquid

nitrogen and repeat the demo?


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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 AmericasD

rop Zonehas 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 currentsproduce self-induced

magnetic fields, which

by Lenzs law are in

opposition of the

primary 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.

c25 induction

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