102 Induction
Physics Professor Lee Carkner Lecture 19
Rail Gun
F
For each rail
B = 0I/2r B = (4X107)(100000) / 2(0.005) B = 4 T so sum of B = 8 T
Force on conductor:
X I B field from each rail I
F = BIL sin F = (8)(100000)(0.01) sin 90 F = 8000 N
Magnetic Fields and Current
We have already seen that:
There should be a parallel effect where magnetic fields can produce currents
Constant magnetic fields do not produce currents
For the area of interest there must either more or less magnetic field with time
Magnetic Flux
Consider a region with a magnetic field
We will define the magnetic flux as the component of the B field that passes through a region times the area of the region, or: = BA cos
Unit of flux is the Weber (Wb), 1 Wb = 1 T m2 = 0 means face on = 90 means edge on
Magnetic Flux
Flux Through a Loop
=BA cos 0 =BA
What is force on particle?
aligned with B, F= 0
Same is true for flux
= BA cos 90 = 0
if = 0, = BA = maximum flux if = 90, = 0
Induction What happens if you change
the flux through a loop of wire? Connect a loop to an ammeter and move a magnet through it
Current stops when the movement stops
Usually means something (either the loop or the magnet) must be moving
Faraday's Law
We can find the emf from Faraday's Law: = N(/t) To find the emf, we don't want , but rather how fast changes Can write as: /t = (fi)/(tfti)
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
Consider a in loop a B field
The induced current flows such that the induced B is in the same direction as the original field
Changing B
If you increase the flux:
The induced current flows such that the induced B is opposite the original field
Applied Induction
Induction is useful because it links physical motion to electric currents
The moving magnet produces a changing flux
The current thus preserves a record of the motion Many applications in music
Microphone
Electric Guitar
Induction Devices
Microphone
Speaker Electric guitar
Pickup magnet magnetizes string, the motion of which induces current The tape is magnetized such that when it passes the tape heads it induces a current
Tape recorders and players
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 21.321.6 Homework, Ch 21, P 7, 9, 11, 13
A wire is carrying a current straight towards you....