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 m 2  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 )/(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   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  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 counterclockwise B)1 and 2 counterclockwise, 3 and 4 clockwise C)1 and 3 clockwise, 2 and 4 counterclockwise D)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