Electro-magnetic Induction

Lecture 3 AP Physics

Class Demo

Producing a voltage with a magnetic field and a wire coil.

Result

A voltage is produced by a changing magnetic field in a coil of wire

We will call this voltage an emfemf (electro-motive-force) to remind us that it is induced by magnetism.

Faraday

Michael Faraday (1791-1867)

The experimentalist of 1800’s

Invented motor, generator, transformer, electrolysis and the theory of electromagnetic induction

Magnetic Flux

The voltage created depends on the magnetic field and the area

B A ����������������������������

Magnetic Flux

cos

B A

BA

Faraday’s Law of Induction

The instantaneous induced emf equals the rate of change of the magnetic flux through the circuit

Nt

Lenz’s Law

The polarity of the induced emf is such that it produces a current whose magnetic field opposesopposes the change in the magnetic flux.

Lenz’s Law

Example

200 turn square coil, 18 cm on a side had a resistance of 2 . Find the emf and current in the loop if the perpendicular magnetic field changes from 0 to 0.5 T in 0.8 sec.

4.05 V

2.03 A

Nt

IR

Motional emf

An emf will be produced any time there is relative motion between the circuit and the field.

Motional emf

B v

Generators

Generators

The emf varies sinusoidally

Depends on the number of loops, the magnetic field, the size of the loop and the angular velocity

Generators

sinNBA t

max NBA

The Motor

A motor converts electrical energy to mechanical energy

A generator converts mechanical energy to electrical energy

Eddy Currents

Demo: falling magnet

Self Inductance

Self-inductance

A coil will oppose any change in the magnetic flux.

If there is a change, there will be an induced emf that will oppose the change. (Lenz’s Law)

This opposition to change is called the self-self-inductanceinductance of the circuit

Self-inductance is like inertia: it opposes any change in the current. Applet: http://www.walter-fendt.de/ph14e/osccirc.htm

Self-inductance

Induced voltage is related to the rate

the current is changing: I

Lt

The inductance is the constant of

proportionality: N

LI

Self-inductance

Unit of self-inductance is the Henry

V s1 H 1

A

Self-inductance

For a solenoid, N turns and length l, the self inductance is

2oN A

L

Energy

Energy is stored in an inductor

21

2PE L I

Links

Additional Links:http://www.physics.sjsu.edu/becker/physics51/induction.htm

http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/magcon.html#c1