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