Electromagnetism

1 Properties of Magnets2 Electromagnets

Chapter 15.1 Learning Goals

Identify properties of magnetic materials and use interactions between magnets to explain attraction and repulsion.

22.1 Properties of MagnetsIf a material is magnetic, it has the

ability to exert forces on magnets or other magnetic materials nearby.

A permanent magnet is a material that keeps its magnetic properties.

22.1 Properties of MagnetsAll magnets have

two opposite magnetic poles, called the north pole and south pole.

If a magnet is cut in half, each half will have its own north and south poles.

22.1 Properties of MagnetsWhether the two magnets attract or repel depends on which poles face each other.

22.1 Properties of MagnetsMagnetic forces can pass through many materials with no apparent decrease in strength.

22.1 Properties of MagnetsMagnetic forces are used

in many applications because they are relatively easy to create and can be very strong.

Large magnets create forces strong enough to lift a car or a moving train.

22.1 Magnetic fieldsThe force from a magnet gets weaker as it gets farther away.

Separating a pair of magnets by twice the distance reduces the force by 8 times or more.

22.1 Magnetic fieldsA special kind of

diagram is used to map the magnetic field.

The force points away from the north pole and towards the south pole.

22.1 Magnetic fieldsYou can actually see the pattern of the magnetic field lines by sprinkling magnetic iron filings on cardboard with a magnet underneath.

22.1 Magnetic field linesA compass needle is

a magnet that is free to spin.

Because the needle aligns with the local magnetic field, a compass is a great way to “see” magnetic field lines.

22.1 Geographic and magnetic poles

The planet Earth has a magnetic field that comes from the core of the planet itself.

22.1 Geographic and magnetic poles

The names of Earth’s poles were decided long before people understood how a compass needle worked.

The compass needle’s “north” end is actually attracted to Earth’s “south” magnetic pole!

22.1 Declination and “true north”Because Earth’s geographic north

pole (true north) and magnetic south pole are not located at the exact same place, a compass will not point directly to the geographic north pole.

The difference between the direction a compass points and the direction of true north is called magnetic declination.

22.1 Declination and “true north”Magnetic declination is measured

in degrees and is indicated on topographical maps.

22.1 Declination and “true north”Magnetic declination is

measured in degrees and is indicated on topographical maps.

Most good compasses contain an adjustable ring with a degree scale used compensate for declination.

22.1 Earth’s magnetismStudies of earthquake waves reveal that the Earth’s core is made of hot, dense molten metals.

Huge electric currents flowing in the molten iron produce the Earth’s magnetic field.

22.1 Earth’s magnetismThe gauss is a unit used to measure the strength of a magnetic field.

The magnetic field of Earth (.5 G) is weak compared to the field near the ceramic magnets you have in your classroom. (300- 1,000 G).

For this reason you cannot trust a compass to point north if any other magnets are close by.