Magnetic Induction
Transforming energy with magnetism
Lorentz Force Reminder
F = qv B
Source: Griffith, The Physics of Everyday Phenomena
F is directed out of the screen.
CPS Question
B
I
What is the direction of the Lorentz force on the charges moving in this wire?
A.
B.
C.
D.
E.
F.
–
Moving Creates a Potential
v
B
induced potential+
Motional emf
• Moving a conductor through a magnetic field causes separation of charges.
• Back-emf (motional emf) in conductor cancels potential from motion
= lvB• when l, v, B all orthogonal
Rotating Loop Creates Circuit
B
VV
Rotating Loop Creates Circuit
B
V V
Group Work
a. At which position(s) does the most magnetic flux pass through the loop?
b. At which position(s) does the magnetic flux through the loop change the fastest?
a b c d e
Group Work
c. What is the current direction at these points?
a b c d e
Faraday’s Law
• Greatest flux when perpendicular • Fastest change in flux /t when parallel
a b c d e
Rotation Powers a Generator
v
v
Motion
V Vinduces a potential
which generates a
currentthat charges the
battery
B
through the field
Lenz’s Law Explains Generator
B
changing field
opposing field
needed I
Current in a Field Feels a Force
F
B
I
Loop in a Field Has Torque
B
I I
Torque Turns a Motor
I
I
current I
force F
B
Group Work
If the magnetic field intensifies, in what direction will the induced current be?
B
Which direction is the flux change?
What current would oppose it?
What is the Lorentz force on each side?
How a Microphone Works
microphonegeophone
Flux inside loop becomes more
How a Loudspeaker Works
loudspeakerinduced motion of magnet
currentcurrent-generated field attracts or repels magnet
Back Potential
• As the motor turns, back-potential B/t opposes
• Faster rotation more back potential
I
B