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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
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?
F = qvB sin 
Fingers with B, thumb
v
F=0
F=0
with v, force points in
the direction of your
palm.
F big,
out
F smaller, in
B
F big, in
 is the angle between v and B.
F smallest,
in
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
F = qvB sin 
B
F

v
with v, force points in
the direction of your
palm.
F is out of the page, still
perpendicular to the
velocity.
Magnetic Forces, Wires
Find the direction of the force on the wire placed in a
B field.
F = qvB sin
F = IlB sin
F
I
qv becomes Il
The unit for B is “tesla” ot T
eg: B = 4T
B
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
F
B into
the page
F = qvB sin
What kind of motion?
Uniform, circular
v
How big is the circle?
a = v2/r
(see circular motion)
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 nailpush current through the wireproduce a magnetic field
B Fields caused by Currents
Compass needle and a wire
RHR #2 for B-field due to current:
See fig. 21.25 in
text
Grip wire with thumb in direction of
current-fingers wrap around wire.
Can only give CW or CCW
B Fields caused by Currents
Two currents
1
Due to wire 2
2
Due to wire 1
Forces equal and opposite, 3rd law
B Fields caused by Currents
Two currents
1
Due to wire 2
2
Due to wire 1
Forces equal and opposite, 3rd law
B Fields - Current Loop
1
1
Current loops
3
1
2
4
2
1
4
4
2
4
3
3
3
1
Now add
4 vectors
2
2
3
B Fields - Current Loop
Current loops
The net field points
one way inside the
loop and the opposite
way outside the loop.
B Fields - Current Loop
Current loops
B Fields - Current Loop
Current loops are magnets!!! Behave like magnets!!!
N
S
B Fields - Current Loop
Current loops
Phantom Bar
Magnet
N
S
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
S
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?
The loop (magnet) will rotate so
it lines up with the field. (S up
and out of the page)
Magnetic Forces, Wires
What happens to loop?
Very complicated, turn the loop into a compass needle.
S
Loop is a magnet (needle)
centered in the loop with N
facing away from you.
N
Which way does the field point at
the magnet due to the wire?
The loop (magnet) will rotate so
it lines up with the field. (S up
and out of the page)