Induction

Physics 102Professor Lee

CarknerLecture 19

Rail Gun

For each rail B = 0I/2r B = (4X10-7)(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

II

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 m2

= 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)/(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 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 21.3-21.6 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) downC) upD) circling the wire in a clockwise

directionE) 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, downC) north, southD) up, westE) 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) leftB) rightC) upD) downE) right at you