Induction Physics 102 Professor Lee Carkner Lecture 19

Rail Gun  For each rail  B =  0 I/2  r  B = (4  X10 -7 )(100000) / 2  (0.005)  B = 4 T so sum of B = 8 T  Force on conductor:  F = BIL sin   F = (8)(100000)(0.01) sin 90  F = 8000 N I I X B field from each rail F

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 m 2    = 0 means face on   = 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  = 0,  = BA = maximum 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?  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 = (  f -  i )/(t f -t i )   /  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  Consider a loop in a B field   The induced current flows such that the induced B is in the same direction as the original field   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   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  Homework, Ch 21, P 7, 9, 11, 13

A wire is carrying a current straight towards you. What is the direction of the magnetic field of the wire? A)straight towards you B)down C)up D)circling the wire in a clockwise direction E)circling the wire in a counterclockwise direction

A horizontal wire is carrying a current straight north. What is the direction of the magnetic field directly above the wire and directly below the wire? A)east, west B)up, down C)north, south D)up, west E)It depends on the magnitude of the current

A horizontal wire is carrying a current to your right. What direction would a proton be deflected in if it traveling exactly below the wire also to the right? A)left B)right C)up D)down E)right at you