Magnetic fields and Earth

The Earth is a magnet

How does a compass work? The domains in every magnet align themselves in a
north-south direction, which a compass needle indicates according to the Earth's magnetic field. This means that in addition to gravity, the Earth exerts a magnetic force.

One theory points out that the core of the Earth is made of iron, a substance prone to magnetism. Core iron is hot and molten (melted) however, which disturbs the alignment of the domains. The most likely explanation for the Earth's magnetism relates to the movement of substances that creates a current within the Earth. This movement produces an electrical charge that affects magnetic objects.

The Earth rotates on an axis. We label one end of the axis north and the other south. These poles are the geographic poles of the Earth. When we use a compass, the magnetic north and south indicated do not exactly match the geographic north and south. This is because the electrical current does not correspond to the Earth's axis. The degree of deviation (variation) between the geographic poles and the magnetic poles depends on your location. Refer Image 1

Geophysicists, scientists who study the physics of the Earth, have observed that during the Earth's history, the magnetic poles switch, on average, about four or five times every million years. Each reversal takes a few thousand years to complete. The last switch occurred about 800 000 years ago. This fact, with the observation that the strength of the Earth's magnetic field has decreased in the past couple of centuries, leads geophysicists to believe that we are in the middle of a reversal. See animation

Electromagnets

Danish scientist Hans Christian Oersted was the first to observe that a compass needle, when placed near an electrical current, would align itself perpendicular (at an angle of 90°) to the flow of the current. When removed from the current, the compass needle returned to a magnetic north-south alignment.

This observation led to the development of the electromagnet, which is a magnet controlled by the flow of electricity. The stronger the current, the stronger the magnetic force. Using this knowledge, French physicist Andre-Marie Ampére developed the solenoid, an electromagnet with tightly coiled wire that increases the flow of electricity. An iron core further increases the magnetism of an electromagnet. Refer Image 2

The main advantage of an electromagnet over other types of magnets is that the current of electricity controls the presence, strength and direction of the magnetism.

Magnets everywhere

Take a tour of your house and see how many magnetic objects you can find. See if you can identify objects that are permanent magnets, objects that can be made into temporary magnets and appliances that use electromagnets.

Here are a few examples of some objects that use magnetic force: