CH 22-1

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CH 22-1
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Electric Dipole
An electric dipole is made of equal magnitude,
opposite charges. In an electric field, an
electric dipole will twist so that it aligns with
the E-field with the q toward the E-field and
the -q opposite the E-field.
If you release the electric dipole from rest,
what will it do?
q
?q
E-field
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Magnetic Dipole
A compass needle is a magnetic dipole. It will
twist so that it is aligned with the magnetic
field, with the N side of the needle toward the
magnetic field and the S side away from the
magnetic field.
If you release the magnetic dipole from rest,
what will it do?
N
S
B-field
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No magnetic monopoles
If you break a magnetic dipole in half, how will
it respond to a magnetic field? It still twists
just like before! Thus, if you break a magnetic
dipole in half, you will still have a magnetic
dipole. You cannot have just a N pole, without
also having a S pole. There are no magnetic
monopoles!
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Compass
A compass points in the direction of the net
magnetic field.
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Magnetic field can be created by current in a
wire

1. Place the compass on the edge of the table. It
   will twist so it is aligned with Earths magnetic
   field.
2. Connect a wire to a battery and lay the wire on
   top of the compass with the N-S axis of the
   compass.
3. Which way does the needle twist and why?

4. Disconnect the wire, what happens to the
needle? 5. How do you know that current produces
a magnetic field?
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Direction of magnetic field due to current --
The Right-hand Rule
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Poll
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Poll
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Poll
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Magnetic field due to moving electron
The magnetic field due to a moving electron is in
the opposite direction and is given by the
left-hand rule.
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Poll
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Poll
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Magnetic field due to a long wire
The magnetic field at a distance r from a long
wire with current I has a magnitude
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Magnetic field due to current in a loop
What is the direction of the magnetic field at
each of the points shown, due to current in the
loop?
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Magnetic field due to current in a loop, along
the axis of the loop
battery not shown
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Magnetic field diminishes with distance from the
loop
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Loop of current acts like a magnetic dipole -- we
call it an electromagnet
S N
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Poll
What is the direction of the magnetic field along
the axis of the loop? 1. downward 2. upward
I
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Poll
Which is the N side of the electromagnet? 1. the
top 2. the bottom
I