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

Faraday Michael Faraday ( ) 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

Magnetic Flux

Faraday’s Law of Induction The instantaneous induced emf equals the rate of change of the magnetic flux through the circuit

Lenz’s Law opposes The polarity of the induced emf is such that it produces a current whose magnetic field opposes 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.

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

Motional emf

Generators

The emf varies sinusoidally Depends on the number of loops, the magnetic field, the size of the loop and the angular velocity

Generators

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) self- inductance This opposition to change is called the self- inductance of the circuit Self-inductance is like inertia: it opposes any change in the current. Applet:

Self-inductance

Unit of self-inductance is the Henry

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

Energy Energy is stored in an inductor

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