Today49

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Today4/9

• Lab Current Balance
• Right Hand Rule for
• Magnetic Force
• Magnetic field
• North Pole of a Current Loop
• Start reading Ch. 21
• HW 4/9 Magnetism Due Friday, 4/11

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Magnetic Forces RHR
A positive charge moves in a B-field that points
to the right. What direction is the force on the
charge for each case?
Fingers with B, thumb with v, force points in the
direction of your palm.
F qvB sin ?
v
F 0
F 0
B
? is the angle between v and B.
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Magnetic Forces RHR
A positive charge moves to the right in a
B-field that points in the direction shown. What
direction is the force on the charge?
Fingers with B, thumb with v, force points in the
direction of your palm.
B
v
F is out of the page, still perpendicular to the
velocity.
4
Magnetic Forces, Wires
Find the direction of the force on the wire
placed in a B field.
F qvB sin?
F IlB sin?
qv becomes Il
The unit for B is tesla ot T eg B 4T
5
Motion in B Field (for HW)
A positive charge moves to the right in a
B-field that points into the page. What is the
acceleration?
F ma
What kind of motion?
Uniform, circular
B into the page
v
How big is the circle?
a v2/r (see circular motion)
F qvB sin?
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B Fields caused by ?
The discussion so far has been about how charges
respond to Magnetic fields, but what causes the
Magnetic field?
B-Fields are caused by moving charges, or
currents.
Wrap a wire around a nail-push current through
the wire-produce a magnetic field
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B Fields caused by Currents
Compass needle and a wire
RHR 2 for B-field due to current Grip wire with
thumb in direction of current-fingers wrap around
wire. Can only give CW or CCW
See fig. 21.25 in text
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B Fields caused by Currents
Two currents
1
2
Forces equal and opposite, 3rd law
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B Fields caused by Currents
Two currents
1
2
Forces equal and opposite, 3rd law
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B Fields - Current Loop
Current loops
1
Now add vectors
4
2
3
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B Fields - Current Loop
The net field points one way inside the loop and
the opposite way outside the loop.
Current loops
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B Fields - Current Loop
Current loops
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B Fields - Current Loop
Current loops are magnets!!! Behave like
magnets!!!
S
N
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B Fields - Current Loop
Current loops
Phantom Bar Magnet
N
S
15
Magnetic Forces, Wires
What happens to loop?
Very complicated, turn the loop into a compass
needle.
Loop is a magnet (needle) centered in the loop
with N facing away from you.
I2
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Magnetic Forces, Wires
What happens to loop?
Very complicated, turn the loop into a compass
needle.
Loop is a magnet (needle) centered in the loop
with N facing away from you.
Which way does the field point at the magnet?
S
The loop (magnet) will rotate so it lines up
with the field. (S up and out of the page)
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Magnetic Forces, Wires
What happens to loop?
Very complicated, turn the loop into a compass
needle.
Loop is a magnet (needle) centered in the loop
with N facing away from you.
S
Which way does the field point at the magnet due
to the wire?
N
The loop (magnet) will rotate so it lines up
with the field. (S up and out of the page)