Download 4/7 Intro to Magnetism

Today4/7
Lab “Current Balance”
Right Hand Rule for:
Magnetic Force
Magnetic field
North Pole of a Current Loop
Start reading Ch. 21
Magnetic Forces
Charges experience Magnetic Forces as well as
Electric Forces.
The Electric force is given by : F = qE
The Magnetic force is given by : F = qvB sin 
v is the charge’s velocity, B is the magnitude
(strength) of the Magnetic field, and  is the
angle between v and B.
Direction of the Magnetic force is perpendicular to both v
and B, and so there are two choices for the force direction
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 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?
The RHR makes no
F = qvB sin 
=0
F=0
v
B
sense and there is no
force on the charge.
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
B
qv becomes Il
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
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 caused by Currents
1
1
Current loops
3
1
2
4
2
1
4
4
2
4
3
3
3
1
4
2
2
3
B Fields caused by Currents
Current loops
The net field points
one way inside the
loop and the opposite
way outside the loop.
B Fields caused by Currents
Current loops
B Fields caused by Currents
Current loops are magnets!!! Behave like magnets!!!
N
S
B Fields caused by Currents
Current loops
N
S