Transcript Magnets and Electromagnets

Magnetism &
Electromagnetism
Magnets
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A special stone first discovered <2000 years ago in Greece, in
a region called “Magnesia”, attracted iron, they called it
“magnetite” hence the “magnet” name.
2. About 1000 years ago they noticed that a hanging magnet
always pointed to the North Star A.K.A “Lodestar”. Hence
the other name for naturally occurring magnets – “lodestone”
Magnetic Poles
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Magnetic Poles – the ends of the magnet, area where the magnetic effect is
the strongest.
If a bar magnet is suspended by a thread or string, it will align itself so that
one strong end points north and the other points south, hence the names for
the “North” and “South” poles of the magnet.
Like poles of separate magnets repel – push away from – each other
Unlike poles attract each other
Magnets
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If you snap a magnet in half, the inside pieces
become the opposite poles:
Magnetic Fields
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that region around a magnet that is affected by the
magnet. Strongest at the poles, the Force forms lines
that go out of the North Pole and wrap back around to
enter in at the South Pole.
Attract & Repel
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Magnets attract because force comes out of North Pole and
goes into the South Pole
Attraction
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Repulsion
Magnets repel because the forces are pushing away from each
other
Inside a Magnet
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At the atomic level, there are protons (+ charge) & neutrons
(neutral charge) in the nucleus, and electrons (- charge)
spinning in orbits around the nucleus. The moving electron
acts as a mini electrical charge and therefore has a magnetic
field associated w/ it.
In ferrous materials clusters of atoms align there atoms w/ one
another. A cluster of billions of atoms w/ magnetic fields
aligned is called a domain.
Inside a Magnet
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When domains are randomly arranged – forces cancel
each other out. – no net magnetic affect
When domains have their magnetic affect in
alignment - forces are additive and create a strong
magnetic affect
Making Magnets
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Since Magnetism and electricity are so closely related, it is relatively easy
to make magnets
Temporary magnets – materials that become magnetized while in contact
w/ strong magnets – ie a paperclip is able to pick up more paper clips when
stuck to a strong magnet
Permanent magnets – materials that maintain their magnetism when the
magnet is removed from it.
Electric Current & Magnetic Fields
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When electric charges run thru a wire they create an electric current – a
flow of charge thru a material
An electric current produces a magnetic field
An electric current through a coil of wire around a nail produces a magnet
Electric circuit – a complete path through which electric current can flow
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Each circuit has a source of electrical energy
Have devices that are run by the electric current
Connected by conducting wires and a switch
Conductors & Insulators
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Conductors allow current to flow easily
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Their electrons are loosely bound to their atoms
Metals – copper, silver, iron, superconductors
Insulator – do not allow current to flow easily
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Electrons are tightly bound to atom
Plastic, wood, rubber, sand, glass
Magnetic Earth
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Earth’s core is Iron – Earth is a giant magnet
Earth’s magnetic north pole is not the same as Earth’s axis north pole. It is
about 1250 km (776 miles) away from the true north pole
The angle between true north and magnetic north is the magnetic
declination.
Magnet Lab (22.1)
Part 1: Hold two magnets 1 cm apart, push together and record
results, repeat for all combinations N-S, N-N, S-S (3 total trials)
Part 2:Using a meter stick, slowly bring two magnets together in
all three combinations as part 1 and record to the nearest .5 mm
the distance that the other magnet moves (either direction).
Repeat each trial 3 times. (9 total trials)
Part 3: Place 5 magnets together, figure out north and south poles,
place your paper over and draw the projected magnetic field
lines. Then slowly pour out iron filings over the paper.
Comment on what was wrong and correct about your drawing.
(Slowly pour iron filings back into container) CLEAN UP!
Part 4: Attach a battery to the electromagnetic setup. Count how
many paperclips you can attach. Repeat with nail-wire
electromagnet, then explain the difference.
Electric Currents Produce
Magnetism (and vice versa)
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Magnetic field around long straight wire
I
Right hand rule
determines
direction of
magnetic field
Right Hand Rule(s)
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Long Straight Wire (Rule #1)
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Point thumb in direction of current
Fingers wrapped around wire point in direction of
magnetic field
Circular loop of Wire (Rule #2)
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Curl fingers around wire with tips in field direction
Thumb points in direction of current
Alternate (preferred) version of
Second RHR
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Put curled fingers in current direction around
loop or loops; thumb points in field direction
INSIDE loop or coil.
Force on Current Carrying Wire
F = BIL sinQ
Q
is angle between
field and wire I
Force is perpendicular to both
current and field direction
q