Transcript Summary Lecture 12

Faraday’s Law of
Induction
Magnetic flux  = A B cos

B
A
A changing magnetic flux
generates an induced voltage
(emf = electromotive force)
 = (ABcos )final – (ABcos )initial
V = [emf] = - N  / t
Motional EMF
Voltage induced in a moving
conductor
Velocity
Vmax = [emf] = -Blv
Length
Induced emf depends on angle between
the Magnetic field and the velocity
direction.
Lenz’s Law
An induced emf gives rise to a
current whose magnetic field
opposes the original change in
flux
new, larger B
Induced current
Induced B
Lenz’s Law Exercises
Applications of Faraday’s Law
Eddy Currents
Transformer:
• Primary and secondary coils
• Iron yoke (to contain flux)
• AC voltages only !
• No loss ->
Input power = output power
V2
N2
I1
V1 = N1 = I2
Electric Generator:
angular
velocity
w = 2pf
Alternating (AC) voltage
 = BAcos(wt)

t
V
t
Electromagnetic Waves
0
0
0
Speed of light (Constant of Nature)
c = 299 792 458 m/s
(3 x 108 m/s)
Also: c = frequency times wavelength
c=lf