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Chapter 25 Electromagnetic Induction. WARNING. Strong magnets will be passed around the room. Keep these magnets away from iPods and laptops ( especially my laptop )!. Electromagnetic Induction.

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Chapter 25 electromagnetic induction l.jpg

Chapter 25ElectromagneticInduction

Physics 1 (Garcia) SJSU


Warning l.jpg
WARNING

Strong magnets will be passed around the room.

Keep these magnets away from iPods and laptops (especially my laptop)!

Physics 1 (Garcia) SJSU


Electromagnetic induction l.jpg
Electromagnetic Induction

Voltage is induced whether the magnetic field of a magnet moves near a stationary conductor or the conductor moves in a stationary magnetic field.

Physics 1 (Garcia) SJSU


Demo magnet induces current l.jpg
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 magnet’s motion, the greater the induced current.

Physics 1 (Garcia) SJSU

Note: Do this in lab too.


Magnetic recording l.jpg
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

Physics 1 (Garcia) SJSU


Electric generator l.jpg
Electric Generator

Electric generator moves a conductor in a magnetic field to produce voltage via electromagnetic induction

Physics 1 (Garcia) SJSU


Demo electric generator l.jpg
Demo: Electric Generator

DC Output

Turn the shaft by hand and as the coils pass the magnets a voltage is induced.

DC current is generated.

SHAFT

Magnet

Magnet

Coils

Note: Do this in lab too.

Physics 1 (Garcia) SJSU


Induction no free lunch l.jpg
Induction: No Free Lunch

Takes work to turn the generator crank to produce electric current.

The faster we turn the crank to produce more current, the more difficult it is to turn.

More difficult to push the magnet into a coil with more loops because the magnetic field of each current loop resists the motion of the magnet.

Physics 1 (Garcia) SJSU


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Faraday’s 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.

Large

Voltage

Medium

Voltage

Small

Voltage

Physics 1 (Garcia) SJSU


Demo electromagnetic oscillations l.jpg
Demo: Electromagnetic Oscillations

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

Oscillating

Magnetic

Field

Electro-

Magnet

Connect to alternating current (AC)


Demo electric motor l.jpg
Demo: Electric Motor

Can create an electrical motor by passing a current through a set of electro-magnets mounted on a rotating shaft.

Current out

Current in

Electro-magnets


Electric motor analyzed l.jpg

N

S

N

S

Electric Motor, Analyzed

Electromagnet mounted on a shaft with opposing magnets on each side.

Current direction always such that electromagnet is repelled, causing shaft to turn.

Current

N

S

N

S

Current


Demo generator becomes motor l.jpg
Demo: Generator Becomes Motor

Pass a current into the generator and it becomes an electric motor.

DC Input

Battery

Magnet

Magnet

Rotor

Spins

Physics 1 (Garcia) SJSU

Note: Do this in lab too.


Self induction l.jpg
Self-Induction

When a current is induced by a changing magnetic field, that current itself produces its own magnetic field. This effect is called self-induction.

Primary Magnetic Field

Current

Self-Induced

Magnetic Field

Physics 1 (Garcia) SJSU


Demo lenz s law l.jpg
Demo: Lenz’s Law

Oscillating

Magnetic

Field

Induced current produces a secondary magnetic field that is always opposed to the primary magnetic field that induced it, an effect called Lenz’s law.

Metal ring is

levitated by

self-induced

secondary

magnetic field

Electro-

Magnet

Connect to alternating current (AC)

Physics 1 (Garcia) SJSU


C heck yourself l.jpg
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

Physics 1 (Garcia) SJSU


C heck yourself17 l.jpg
Check Yourself

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?

Oscillating

Magnetic

Field

Electro-

Magnet

Connect to alternating current (AC)

Physics 1 (Garcia) SJSU


Demo magnetic brakes l.jpg
Demo: Magnetic Brakes

Strong magnet dropped into a copper pipe falls slowly due to secondary magnetic field induced by its motion.

Great America’s Drop Zone has a 22 story freefall, lasting four seconds, decelerated by magnetic braking.

Physics 1 (Garcia) SJSU


Eddy currents l.jpg
Eddy Currents

Changing magnetic field induces eddy currents within any conductor.

These internal currents produce self-induced magnetic fields, which by Lenz’s law are in opposition of the primary magnetic field.

Primary Magnetic Field

Eddy

Currents

Self-Induced

Magnetic Field

Physics 1 (Garcia) SJSU


Metal detectors l.jpg
Metal Detectors

Can detect the presence of metals by using a transmitter coil to create an oscillating primary magnetic field.

This creates a secondary magnetic field due to eddy currents in the metal.

Can detect this secondary magnetic field by using a receiver coil.

Physics 1 (Garcia) SJSU


Demo back electromotive force l.jpg
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.

Physics 1 (Garcia) SJSU


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