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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