27-Apr-20

Chapter 25

Electromagnetic Induction

Physics 1 (Garcia) SJSU

WARNING

Strong magnets will be passed around the room. Keep these magnets away from iPods and laptops (

especially my laptop

)!

27-Apr-20 Physics 1 (Garcia) SJSU

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.

27-Apr-20 Physics 1 (Garcia) SJSU

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. 27-Apr-20 Physics 1 (Garcia) SJSU 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).

27-Apr-20 Physics 1 (Garcia) SJSU Hard disk

Electric Generator

Electric generator moves a conductor in a magnetic field to produce voltage via electromagnetic induction 27-Apr-20 Physics 1 (Garcia) SJSU

Demo

: Electric Generator

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

DC current is generated.

27-Apr-20 Magnet Physics 1 (Garcia) SJSU DC Output

S H A F T

Coils Magnet Note: Do this in lab too.

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.

27-Apr-20 Physics 1 (Garcia) SJSU 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. 27-Apr-20 Small Voltage Medium Voltage Physics 1 (Garcia) SJSU Large Voltage

Demo

: Electromagnetic Oscillations Put alternating current into an electromagnet and you create an oscillating magnetic field.

Coil with bulb Oscillating Magnetic Field This oscillating magnetic field induces electrical currents by inducing electrical field oscillations.

Electro Magnet This is, effectively, a very low frequency electromagnetic antenna. Connect to alternating current (AC)

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

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

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

Demo

: Generator Becomes Motor Pass a current into the generator and it becomes an electric motor.

DC Input 27-Apr-20 Physics 1 (Garcia) SJSU Battery

Magnet Magnet

Rotor Spins Note: Do this in lab too.

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 Self-Induced Magnetic Field Current 27-Apr-20 Physics 1 (Garcia) SJSU

Demo

: Lenz’s Law

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 Connect to alternating current (AC) Electro Magnet Oscillating Magnetic Field 27-Apr-20 Physics 1 (Garcia) SJSU

C

heck 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 27-Apr-20 Physics 1 (Garcia) SJSU

C

heck 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?

Electro Magnet Oscillating Magnetic Field Connect to alternating current (AC) 27-Apr-20 Physics 1 (Garcia) SJSU

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 27-Apr-20

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 27-Apr-20 Physics 1 (Garcia) SJSU

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.

27-Apr-20 Physics 1 (Garcia) SJSU

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.

27-Apr-20 Physics 1 (Garcia) SJSU