Download Lesson 5 Magnetism Notes

Chapter 12 Lesson 5-Magnetism
Vocabulary
1. Magnetism
2. Magnetic field
3. Electromagnet
4. Generator
5. Alternating current
6. Magnetic levitation
Notes
What is magnetism?
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Magnets apply forces to certain metals like iron or nickel.
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Magnets have two poles: north (N) and south (S).
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o
Like poles repel one another
o
Opposite poles attract one another.
Magnets always exist in north-south pairs.
o
If you cut a magnet in half, each half will form a new magnet with two poles.

Compass needles point toward Earth’s North Pole.
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Geographic North Pole and magnetic north pole are in slightly different places.

Atoms act like magnets.

In most materials, the north and south poles of atoms point in random directions.
o

The forces from the random poles cancel each other out.
If the poles of many of the atoms line up in the same direction, a permanent magnet is formed.
o
The forces from the aligned poles of atoms add up and give a magnet its strength.
o
Ex: bar magnets

Iron, nickel, cobalt, and a few other metals are attracted to magnets.

When small pieces of these metals are sprinkled over a magnet, they form lines.

The closer the lines appear, the stronger the magnetic forces are in that area.

The magnetic forces around Earth are similar to those of a bar magnet.
What are electromagnets?
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The moving electrons in electricity generate magnetic fields.
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When the current stops the magnetic field disappears.
o
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The simplest electromagnet is a straight wire.
o
The magnetic field circles around the wire when current is flowing.
o
When you wrap a wire into a loop, you increase the strength of the magnetic field.
o
Many loops together can make a coil.
o
The magnetism from each loop adds up to make the coil a stronger electromagnet.
Placing a rod of iron in a coil will magnetize the iron.
o
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This adds to the strength of the electromagnet’s magnetic field.
An iron rod in an electromagnet’s coil is pulled toward the center of the coil.
o

This means the magnet can turn on and off.
If you try to pull it out, it will spring back.
A voice coil operates audio speakers.
o
Voice coil sits in a permanent magnetic field.
o
Current changes in the coil alter its magnetic field.
o
Causes the forces of the permanent magnet to move it back and forth.
o
Coil is connected to a cone of paper or metal.
o
The coil’s vibrations make the cone move back and forth, creating sound waves in the
air.
o
If several voice coils were placed in a circle, changes in electric current would cause
them to rotate back and forth.

An axle is attached to many coils which are between two permanent magnets.

Forces between the permanent magnets and the coils acting as electromagnets cause the coils
to rotate.

Electric motors are used in many devices, from ceiling fans to cars.
How can magnets produce electricity?

Energy is used to turn the axle of the generator.

As the coil moves through the magnetic field, forces push on its electrons and create an electric
current.

Wires attached to the loop allow the current to flow as the loop rotates.

Whenever a loop moves past the pole of a magnet, the direction of the magnetic forces
changes.
o
This causes changes in the direction of the electric current.

In real generators, there are several coils of wire that spin past many magnets.

U.S. generators produce alternating current that changes directions 120 times each second.
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Devices called transformers use magnetism to lower the voltage used in homes.
What is magnetic levitation?

When two poles of magnets face each other they feel a pushing force.
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If you balance this pushing force against the force of gravity you have magnetic levitation.
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Scientists and engineers have designed trains that use magnetic levitation (maglev for short) to
travel on a track.

Electromagnets in the track and in the train have alternating north and south poles.

By aligning the right type of poles in the track and in the train, electromagnets push the train a
few centimeters above the track.

The train moves forward by switching the poles in the track back and forth.

Maglev trains have no touching parts between the track and the train.
o
This means little to no friction.
o
Still air resistance.
o
Able to provide very efficient means of traveling from city to city.