Download Chapter 36 – Magnetism

Chapter 36 – Magnetism
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Historically speaking our understanding of magnets comes from the use of lodestones by the Chinese to
navigate ships. Lodestones contain iron ore, which has a scientific name of magnetite.
For most of human history electricity and magnetism were believed to be unrelated. This changed in the
19th century when Oersted noticed that electric current in a wire near a compass moved the compass
needle. This discovery paved the way for many more, including electric power, radio and television.
36.1 – Magnetic Poles
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Magnets are very similar to electric charges: they exert forces on one another without touching; they can
attract and repel; and the strength of the force depends on the distance between the magnets. Electric
forces are produced by charges, but magnetic forces are produced by areas called magnetic poles.
o We define the poles as north and south - if you hang a bar magnet from a string it turns so that
the north-seeking end points north and the south-seeking end points south. The first end is called
the North Pole and the second is called the South Pole; a horseshoe magnet is a bent bar magnet.
Just like electric charges – likes repel and unlikes attract.
The one major difference between electric charges and magnets is that electric charges can be separated
(electrons and protons are not attached) but the north and South Pole of a magnet cannot be separated. If
a bar magnet is cut in half two separate magnets are created that each have a north pole and a south pole.
Even if you broke the magnet in half until you had one atom, it would still have a north and South Pole.
36.2 – Magnetic Fields
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The space around a magnet in which it exerts a force is called the magnetic field. The shape of this
field can be mapped with magnetic field lines.
o The direction of the field outside the magnet is from the North Pole to the South Pole.
o Field strength is greater where the lines are close together – it’s strongest near the poles.
o If we place magnet or a small compass in the field it will generally line up with these lines.
36.3 – The nature of a Magnetic Field
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Magnetism and electricity are very closely related to each other. An electric charge will always be
surrounded by an electric field, but if it is moving it will be surrounded by a magnetic field also.
o Magnetic fields are formed by “distortions” in the electric field that are caused by the electric
field’s motion. We can say that a magnetic field is produced by the motion of electric charges.
A magnet is formed when the electrons in the material spin in the same direction. Electrons spinning in
opposite directions cancel each other out. In one iron atom there are 4 electrons whose spins are not
cancelled out by another electron spinning the opposite direction, this is what allows it become
magnetized. (Other magnetic elements, like nickel and cobalt, have similar situations)
36.4 – Magnetic Domains
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The magnetic fields around iron atoms are so strong that they interact and line up with each other.
Clusters of aligned atoms are called magnetic domains.
o In iron that is not magnetized these domains don’t line up with each other, but when you bring a
magnet near iron that is not magnetized two things happen.
The domains that go in the direction of the magnetic field of the magnet grow bigger.
The domains that are not aligned with the magnetic field rotate around to become aligned.
o When we bring a magnet near unmagnetized iron it will become a magnet, however it is not a
permanent magnet and can be compared to water when you bring a charged rod near it – the
water molecules are dipoles so they line up temporarily because they interact with the electric
field of the charged rod, but when you remove the rod the molecules go back to normal.
To make a permanent magnet put iron or iron alloys in a strong magnetic field (soft alloys are easier to
magnetize). Tapping the iron while it is in the field it will help the domains to line up, however if you
drop, tap or heat that piece of iron after it is a magnet, you knock domains out of alignment making it
weaker.
36.5 – Electric Currents and Magnetic Fields
• A wire with a current running through it is surrounded by a circular magnetic field. If you surround that
wire with compasses they will align themselves with that magnetic field (that is how we know that it is
circular in shape). If you change the direction of the current, the compasses will change direction also.
• If you take a current carrying wire and bend it into a loop, the magnetic field bunches up in the center.
If you bend it again into a second loop effect is stronger. As the number of loops increases field
intensity in the region inside the loops increases. We call a current-carrying coil of wire with many
loops and electromagnet. (See figure 36.13 on page 569 for a visual of this change in field strength).
• When we use a superconductor in the current carrying wire we can create very large magnetic field
intensity in the center. There are many practical applications of this superconducting electromagnet
technology including research at the Fermi lab (where they do nuclear research with high speed and high
energy atomic particles), MRI technology in hospitals and the possibilities of high-speed transportation.
36.6 – Magnetic Forces on Moving Charged Particles
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In order for a magnet to exert a force on a charged particle the particle has to be moving. This force is
perpendicular to both the electrons movement and the magnetic field, and it exists when the charged particle
is moving perpendicular to the magnetic field. The force acts as a sideways deflecting force.
o The perpendicular nature of this force makes it very different from the force of gravity between masses,
the electrostatic force between charges and the force between magnetic poles.
o This deflection of charge particles by magnetic fields has many practical applications: it forces the
spread of electrons inside a T.V. which provides the picture; and it deflects charged particles coming to
earth from outer space (which protects us from cosmic radiation).
36.7 – Magnetic Forces on Current Carrying Wire
The same way that an individual charged particle experiences a sideways deflecting force in a magnetic
field, a current carrying wire experiences a force also. When the current travels in one direction the
wire will bend one way and when the current is reversed the wire will bend the opposite way.
Practical applications of this varying from sensitive electric meters to strong electric motors.
36.8 – Meters to Motors
An instrument that indicates the present of an electric current is a galvanometer. A galvanometer can
take many forms but there is always a current carrying wire bent into a loop and a magnet. These two
items can be set up so that the magnet is a needle that moves in the presence of current or so that the
magnet is stationary forcing the wire to move in the presence of current.
A galvanometer that has been calibrated to measure current is called an ammeter.
A galvanometer that has been calibrated to measure volts is called a voltmeter.
• A simple modification to the design of a galvanometer will produce an electric motor. A motor uses a
galvanometer where the magnet is stationary and the current carrying wire is moving. It is set up so that
when the coil makes a half of a revolution the current reverses. This process continues in a cyclic
fashion to produce continuous rotation. (see a simple picture of a DC motor on page 573 in figure 36.20)
Larger motors usually use an electromagnet instead of a permanent magnet, and many loops that are usually
wound around an iron cylinder (called an armature).
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36.9 – Earth’s Magnetic Field
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The earth is a giant magnet (although the magnetic north and south pole of the earth do not align with the
geographical North and South Pole of the earth) with an unstable magnetic field that has varied greatly over
time, and has hat at least 20 complete reversals over the past 5 million years.
o The earth acts like a giant bar magnet, but cannot possibly be one because the center of the earth is too hot
for the magnetic domains of a large amount of iron to stay aligned.
o It is possible that currents in the molten part of earth beneath the curst are causing the magnetic field
because they act like a loop of magnetic charge traveling around the earth.
o Another possibility is convection currents from the rising heat of the earth’s core combined with the
rotational effects of the earth are producing the magnetic field…however more study would be necessary
to show this.