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Induction
Physics 102Professor Lee
CarknerLecture 19
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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
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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
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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
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Magnetic Flux
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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
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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
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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
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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
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Induced Current
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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
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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
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Microphone
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Electric Guitar
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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
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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
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Next Time
Read 21.3-21.6 Homework, Ch 21, P 7, 9, 11, 13
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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
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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
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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