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Faraday’s Law
PH 203
Professor Lee Carkner
Lecture 19
Magnetic Fields and Current We have already seen that:
There should be a parallel effect where magnetic fields can produce currents Called induction
Constant magnetic fields do not produce currents
For the area of interest there must either more or less magnetic field with time
Magnetic Flux The magnetic force in that region depends on
the density and direction of the field lines that pass through it
= ∫ B dA Similar to the electric flux
Uniform Field
= BA cos
Unit of flux is the Weber (Wb), 1 Wb = 1 T m2
Note that is with respect to the normal = 90 means edge on
Magnetic Flux
Flux Through a Loop
What is force on particle?
aligned with B, F= 0
Same is true for flux
if = 90, = 0
=BA cos 0 =BA
= BA cos 90 = 0
Induction
What happens if you change the flux through a loop of wire?
A current is induced in the wire Current stops when the movement stops
Usually means something (either the loop or the magnet) must be moving
Induced emf
Faraday’s Law We can find the emf from Faraday’s Law:
= -N(d/dt)
To find the emf, we don’t want , but rather how fast changes
For a smooth, continuous change:
/t = (f-i)/(tf-ti)
/t is also the slope of a line on a -t diagram
Lenz’s Law The induced current produces a magnetic field of
its own
Lenz’s Law: The induced current will be in a direction such that
the magnetic field it produces will counteract the changes in the original B
Induced Current
Changing B
If you decrease the flux:
Decrease flux, induce same direction field
If you increase the flux:
Increase flux, induce opposite direction B
Lenz’s Law
Applied Induction
You connect a source of motion to a magnet The changing flux produces a changing current
Can easily amplify and move the current Many applications in music
Microphone
Electric Guitar
Induction Devices Microphone
Speaker
Electric guitar Pickup magnet magnetizes string, the motion of which
induces current
Tape recorders and players The tape is magnetized such that when it passes the tape
heads it induces a current
How Does Induction Work?
If we move the wire through a B field the
electrons now have a velocity
This deflection produces an imbalance of charge
Next Time
Read 30.5-30.7 Problems: Ch 30, P: 5, 6, 13, 27, 38
Which of the following would increase the field inside of a solenoid the most?
A) Increasing the length and increasing the number of turns
B) Increasing the length and decreasing the number of turns
C) Decreasing the length and increasing the number of turns
D) Decreasing the length and decreasing the number of turns
E) None of these would increase the field
A face up coil of wire has a current moving clockwise when seen from above. What direction is the dipole moment of the coil?
A) Up
B) Down
C) Left
D) Right
E) Clockwise
What direction will the dipoles rotate in?A) 1 and 2 clockwise, 3 and 4
counterclockwiseB) 1 and 2 counterclockwise, 3 and 4 clockwiseC) 1 and 3 clockwise, 2 and 4 counterclockwiseD) 1 and 3 counterclockwise, 2 and 4 clockwise E) All clockwise
Which dipole has the most torque?
A) 1
B) 2
C) 3
D) 4
E) all tie
Which dipole has the most potential energy?
A) 1 and 2
B) 3 and 4
C) 1 and 4
D) 2 and 3
E) all tie