15-Aug-15 Physics 1 (Garcia) SJSU Chapter 25 Electromagnetic Induction

Apr 19, 2023 Physics 1 (Garcia) SJSU

Chapter 25Electromagnetic

Induction


WARNINGStrong magnets will be

passed around the room.

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


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.


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.

Note: Do this in lab too.


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


Electric Generator

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


Demo: Electric Generator

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

DC current is generated.

Magnet

Magnet

Coils

DC Output

SHAFT



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.


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.

SmallVoltage

MediumVoltage

LargeVoltage

Demo: Electromagnetic Oscillations

Connect to alternating current (AC)

OscillatingMagnetic

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

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

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.

N SN S

N S

N S

Current

Current


Demo: Generator Becomes Motor

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

RotorSpins

Magnet

Magnet

DC Input

Battery



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.

Current

Primary Magnetic Field

Self-InducedMagnetic Field


Demo: Lenz’s Law


OscillatingMagnetic

Field

Electro-Magnet

Metal ring islevitated byself-inducedsecondary

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.


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


Check Yourself


OscillatingMagnetic

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?


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.


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.

EddyCurrents

Primary Magnetic Field

Self-InducedMagnetic Field


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.


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.

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15-Aug-15 Physics 1 (Garcia) SJSU Chapter 25 Electromagnetic Induction