Summary for Chapters 24 and 25: Magnetism
Objects that do not have a net electrical charge can still exert forces on each other due to
magnetism.
Magnetic Forces
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These forces are similar to electrical forces, but arise from magnetic poles instead
of electrical charge. There are two types of magnetic poles, North Poles and South
Poles.
North Poles attract South Poles and visa versa. However two North Poles repel
each other and two South Poles repel each other. Like poles repel each other and
unlike poles attract each other. (Note that this rule is similar to the rule for electric
charges.)
We do not find isolated poles in nature, they come in pairs, every North Pole is
associated with a South Pole, and visa versa. Therefore a bar magnet has two
poles, a North Pole and a South Pole.
We use the idea of a magnetic field to describe the effects of magnetic forces (just
as we use electric fields to describe electrical forces and gravitational fields to
describe gravitational forces.)
Sources of Magnetism
There are three sources of magnetism, or magnetic fields.
1. Some particles are intrinsically magnetic (i.e. act like little bar magnets).
Electrons, protons and neutrons are intrinsically magnetic.
2. Electric currents produce magnetic fields.
3. A changing electric field produces a magnetic field.
• When we use electric currents to produce magnets, we call them electromagnets.
They are important in electric motors.
• The magnetism in iron and permanent magnets is due to the intrinsic magnetism
of electrons. In these materials some of the electrons will orient so that their
magnetic poles align and add to produce larger poles. In materials like copper
and aluminum the electrons do not orient this way and they are not magnetic.
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Magnetic forces on currents
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Magnetic fields produce a force on moving charges or currents. (Note currents
produce magnetic fields and magnetic fields exert forces on currents.) We use
these effects in electric solenoids and electric motors to convert electrical energy
into mechanical energy (motion).
Producing Electric Fields by Magnetism
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We can also use magnets to produce electric fields. Just as a changing electric
field produces a magnetic field, a changing magnetic field will produce an electric
field. Most of our electricity is generated this way, by spinning a magnet inside a
coil of wires. When this happens the two ends of the wire act like the terminals of
a battery. (Note that the as the magnetic spins in the coil below, the polarity of the
ends of the coil will alternate, initially one side will be + and the other -, and then
they will switch as the magnet turns.) This is the usual method for producing
electricity at power plants.