Magnetism

Permanent Magnets

Some materials can be defined as magnetic whereas others are non-magnetic. A magnetic material is one which is affected by magnetism.

Some materials which are strongly attracted to magnets (magnetic) are:

• Iron and steel
• Other alloys of iron, cobalt or nickel
• Alloys containing a mixture of iron, colbalt and nickel

Some materials which are not attracted to magnets (non-magnetic) are:

• wood
• plastic
• leather

Properties of Magnets

Poles

Poles are the regions on a magnet to which materials are attracted. All magnets have two poles, a north pole and a south pole. Hence, they are called magnetic dipoles.

The two poles of a magnet are either south-seeking or north seeking. A suspended magnet always settles with its poles pointing in the same direction. The north pole of the magnet will always point toward the geographical north pole of the earth hence it is called the north seeking pole or simply the north pole. The south pole of the magnet will always point toward the geographical south pole of the earth hence, it is called the south seeking pole or simply the south pole. This video explains this some more. Because of the property explained above, magnets are used to make the magnetic compass.

Forces between magnets

If two opposite poles of a magnet are brought into close proximity, a force of attraction between the two magnets is observed. Also, if two similarly poles of a magnet are brought into close proximity a force of repulsion is observed. Therefore, "like poles repel and unlike poles attract".

Magnetic Induction

When an unmagnetised iron alloy is brought near to a magnet it is attracted to the magnet. This is a result of temporary magnetism being induced in the material. Magnetic induction always results in attraction, never repulsion. Also, there is always a pair of induced poles.

Permanent and Temporary Induced Magnetism

Refer to page 276 of your text book.

Iron alloys like steel and magnadur are hard to magnetise, hence they are called hard magnetic materials. Materials which are easier to magnetise such as iron and mumetal are called soft magnetic materials. Soft magnetic materials are used to make temporary magnets whereas hard magnetic materials are used to make permanent magnets.

Magnetic Forces

The magnetic field around a magnet is the region in which forces act on other magnets and on magnetic materials by inducing magnetism in them. The direction of a magnetic field at a particular place is the direction of the force it produces on a free magnetic north pole. Remember, field lines always go from north to south.

Magnetic Field Diagrams

You should be able to draw the field lines:

• Around a strong single magnet
• Around and between two strong magnets which are oriented parallel, anti-parallel, and pole to pole with each other just as you did in your SBA. (Page 280 of your text has a few diagrams)

Read up on this entire topic in your text books people. It is important.