Transcript L26_Faraday

Board Work
What are the directions of the forces on the
opposite charges moving with velocity v
through magnetic field B?
B
−
+
v
Moving conductors
through magnetic fields
Poll Question
What is the direction of
the Lorentz force on the
charges moving along
this wire?
A.
D.
B.
E.
C.
F.
B
I
Current in a Field Feels a Force
B
F
I
Loop in a Field Has Torque
B
I
I
Torque Turns a Motor
current I
I
force F
I
B
Motion Creates a Potential
–
B
v
+
induced potential
Rotating Loop Creates Circuit
B
V
V
Rotating Loop Creates Circuit
B
V
V
Rotation Powers a Generator
Motion
through the field
induces a potential
which generates a
current
that charges the
battery
v
V
V
B
v
Magnetic Induction
Transforming energy with
magnetism
Objective
• Explain the electric potential created by a
changing magnetic field.
What’s the Point?
• This is how electric current is harnessed to
do mechanical work.
• It’s another way to think about the Lorentz
force. ☺
Lorentz Force Reminder
F = qv  B
Source: Griffith, The Physics of Everyday Phenomena
F is directed out of the screen.
Magnetic Flux
• Field through a surface
F = BA
• Effectively the number of field lines
enclosed by a loop
Flux Change Creates Potential
Faraday’s Law
The rate of magnetic flux change
through a loop equals the electric
potential induced around the loop.
V=
DF
Dt
Magnetic Flux
a
b
c
d
e
•
Flux changes as loop turns
•
•
At a, c, e: Greatest flux F, least change
At b, d: Zero flux, fastest flux change DF/Dt
Induced current direction
Lenz’s Law
When a changing magnetic flux
induces a current, the induced
current’s magnetic field opposes the
flux change.
Lenz’s Law Example
Induced current
a
b
c
d
e
• From a to b:
– flux decreases; DF into face of loop ←
– Loop current increases
– Induced field increases out of face of loop
Lenz’s Law Explains Generator
changing field
e
e
opposing field
B
needed I
Board Work
What is the
Lorentz force on
each side?
B
Which direction is
the flux change?
What current
would oppose it?
If the loop inside this magnetic field
expands, in what direction will the
induced current around it be?
How a Microphone Works
microphone
geophone
Flux inside loop
becomes greater
Wave-Powered Generator
Originally appeared in Smithsonian magazine.
How a Loudspeaker Works
loudspeaker
currentgenerated field
attracts or
repels magnet
induced motion
of magnet
current
Reading for Next Time
• Electromagnetic Waves