Lesson 29: Magnets and Magnetic Fields

Download here: Ontario Curriculum Expectations

Watch the video below for a brief overview of magnets and magnetism:

Regardless of whether they are small magnets that you stick on your fridge or huge magnets like the one going through the centre of the Earth, all magnets have a north and a south pole and create a magnetic field.

The Law of Magnetic Poles says that similar magnetic poles will repel each other while opposite magnetic poles will attract each other.

The Domain Theory explains how something becomes magnetized. Magnetism is the result of the direction of spin of the electrons in a material. In most things, there are a similar number of electrons spinning in one direction as those spinning in the opposite direction. As a result, any magnetizing effect is cancelled out. When this is not the case, a magnetized material results. According to the Domain Theory, each region where the electrons have the same spin, and therefore the same magnetic polarity, represents a domain. In the image below, you can see that the domains are all oriented in different directions. As a result, any magnetizing effect is cancelled out.

Each arrow acts like a tiny bar magnet and points
in the direction of a local north pole within each domain.

In the diagrams below, the domains have become aligned. Since they all point in the same direction, there is really only one large domain that creates a strong magnet.

The domains become aligned when exposed to an external magnetic field. A material like iron is called magnetically soft because it is very easy to reorient the direction of the domains. Steel, on the other hand, contains carbon which helps to lock the domains in place. This makes it more difficult to change the direction of the domains. Consequently, steel is considered magnetically hard.

Whenever heat, or the energy of vibration caused by banging, hammering, or dropping an object, are added to a material, it causes the molecules within the material to vibrate more aggressively and this can result in the domains changing their alignment.

A material like iron is very easy to magnetize, but this means that can also be easily demagnetized. Steel, on the other hand, is not easily demagnetized, but it is also much more difficult to magnetize it in the first place. Most common magnets are made out of iron, nickel, cobalt, and their alloys as these materials have the strongest magnetic properties.

Now that you know how a magnet is made, you are ready to investigate the shape and direction of the magnetic fields that are generated around all magnets. Go to the Magnetic Fields multimedia element and complete the activity. When you are finished, return to this page.

Although it can be difficult to conceptualize, it is important to remember that the magnetic field around a magnet extends in all directions around the magnet. In the image below, what is the direction of the magnetic field?

This image shows the magnet field around a bar magnet. Hopefully you said the magnetic field was going left to right, from the north pole to the south pole.