Intro to Magnetism
4-22
Today
• Magnetism
> Bar Magnet
> Earth's Magnetic Field
> Magnetic Field, B (Tesla, T)
> Magnetic Force, FB (N)
– on a moving charge
– on a current-carrying wire
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Intro to Magnetism
4-22
Bar Magnets
This diagram indicates the Magnetic Field that
surrounds a simple bar magnet. Note that all bar
magnets have both a "North" and "South" pole,
and that Magnetic Field Lines go from North to
South.
This is similar to the behavior of positive and
negative charges.
When two bar magnets are brought near one
another, we observe that they exert forces on
each other, such that North repels North, South
repels South, and North attracts South.
This is similar to the behavior of positive and
negative electric charges.
If two bar magnets are placed together, with
North to South, you will get a new single magnet.
Also, if you snap a bar magnet in half, you get
two smaller magnets, each of which will have a
North and South pole. **There is no way to
isolate a single North or South pole;
magnetic "Monopoles", unlike electric
charges, do no exist.
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Intro to Magnetism
4-22
Earth's Magnetic Field
The Earth's Magnetic Field
can be modeled as that of a
bar magnet, such that the
"North" magnetic pole is in the
Southern hemisphere, and the
"South" magnetic pole is in the Northern hemisphere.
*Note: compass needles point "North" because the needle's North pole
is attracted towards the Earth's "South" magnetic pole.
• A unique feature of Earth's magnetic field is that it is inconstant; it
changes in both magnitude and direction over time. Sailors used to
have to carefully measure the direction of the magnetic field in order
to navigate without benefit of the position of the Sun or stars at night.
• In fact, the actual Polarity (i.e., where N and S are) of the Earth's
magnetic field has Flip-flopped numerous times in the Earth's geologic
history. The causes of these flips is generally unknown, although they
have tended to be preceded by a significant weakening of the the
magnetic field. These flips have historically occurred every few
hundred thousand years.
• Geologic records indicate that the last flip was several thousand years
ago; you could say that we are overdue for a field reversal.
> *You may be interested to know that evidence suggests that
Earth's magnetic field has been weakening over the past few
decades.
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Intro to Magnetism
4-22
Magnetic Forces
- on a moving electric charge: When a charge q moves with a speed
v through a magnetic field B, it will experience a magnetic force FB
such that
FB = qvBSinθ
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