O Level Notes : Physics - Magnetism

We are going to study about magnets! It’s a familiar thing to all of us, after all it has fascinated us all when we were young! We know that it’s something related to attraction, but let’s study it in more detail.

Magnets:

A magnet exists naturally for example magnetite. These magnets attract certain materials like steel, cobalt and nickel. These materials are called magnetic materials. Have you ever tried attracting wood to a magnet? Do they attract? No. Such materials which do not get attracted by magnets are called non-magnetic materials. Just another term and we’ll move on: permanent magnets are those  which retain their magnetism for a long time such as magnetite.

Besides attracting magnetic materials, all magnets have the following properties:

• Magnets have two ends called the poles of the magnet. The poles are where the magnetic effects are the strongest.

                The diagram shows what happens when iron fillings are sprinkled onto the bar magnet, most of them are attracted to the poles of the magnet.

• If you suspend a bar magnet freely, it will always come to rest in a north-south direction, as shown below:

The end of the magnets that points to the northern end of Earth is called the north-seeking pole or simple the north pole. The other end which points to the southern end of the Earth is called the south-seeking pole or the south pole.

• Lastly, if you bring a north pole of one magnet to the north pole of another, they will repel. While it will attract the south pole.

Okay now, how can you tell that a material is a magnet? If it attracts another material, right? Wrong. If it attracts another material, it can either be a magnetic material or a magnet, we can’t be sure. But what if it repels another material? Can it be a magnetic material? Umm..no. It can only be a magnet. So to conclude here: repulsion is the only test to confirm that an object is a magnet.

Magnetic Induction

If you bring a safety pin near a permanent magnet, it is attracted to the magnet. In this case, we say the safety pin has become an induced magnet. This induced magnet is able to attract other safety pins as shown below:

This example shows that magnetic induction is the process of inducing magnetism in magnetic materials.

Thinking a way apart from this, magnetic induction can also occur without any contact with the magnet. Take a look at the diagram below:

The iron rod has become an induced magnet, the north pole has been induced on the rod  and in turn, the other end has become the south pole because the material can’t stay with a single pole, right?

Magnetisation and Demagnetisation:

Consider a bar magnet which you cut into pieces somehow. You will notice that every piece will become a magnet itself with a north and a south pole. The resulting pieces will still be magnets with two poles. The resulting bar, as shown below, is  made up of tiny magnets or magnetic domains.

Now the question is, what is a magnetic domain? Physicists believe that the orbiting motion of electrons in a magnetic material makes each atom an atomic magnet.A group of such atomic magnets pointing in the same direction is called a magnetic domain.

Take a look at the diagram above. The below one is magnetised with each arrow representing an atomic magnet which we just discussed. The base of each arrow in the lower diagram represents south pole while the arrowhead represents north pole.

• Magnetic Saturation: every magnet has a maximum strength. This happens when all the magnetic domains are pointing in the same direction. Such a magnet cannot be any stronger and is said to be magnetically saturated.
• Demagnetisation: this is a process by which the magnetism of magnets is removed. This can be done by heating and hammering. Now why does this happen? It’s simple and logical.

1. When a magnet is strongly heated, the atoms of the magnet will vibrate vigorously and cause the magnetic domains to lose their alignment.
2. And hammering also alter the alignment of the magnetic domains, causing the magnet to lose its magnetism.
3. Another method of demagnetising a magnet is to place it in a solenoid connected to an alternating current supply (a solenoid is a cylindrical coil of insulated copper wires). An alternating current is an electric current which varies in direction many times in one second. The magnet is then slowly withdrawn in an East-West direction. This method is the most effective way of demagnetising.

• Storage of magnets: Magnets are stored in pairs by using soft iron keepers across the ends of the bar magnets. This is done because magnets become weaker after some time due to the ‘free poles’ near the ends of magnets which repel one another if they are placed side by side. Soft iron keepers are shown below.

Ways of making Magnets

Now how can an unmagnetised steel bar be magnetised? There are two ways, let’s learn that.

• Stroking method: the steel bar is stroked many times with the same pole of a permanent magnet from one end to the other in one direction. The stroking magnet is lifted high above the steel bar between several strokes. The steel bar will become permanent magnet due to magnetic induction. This is shown below:

Now how to see which pole is induced at each end of the steel bar? The pole at the end of the permanent magnet (with which you stroke the steel bar) is induced at the end of the steel bar where you start stroking. Get it? Take a look at the diagram an you’ll understand.

• Electrical method using direct current: It’s almost the same as that in demagnetising except that instead of alternating current, direct current is used in this. The steel bar which is to be magnetised is placed inside a solenoid. A large d.c.is passed through the solenoid which magnetises the steel bar in a while. This happens because the electric current produces a strong magnetic field which magnetises the steel bar by aligning its domains. The poles in this case can be determined by right hand grip rule. The diagram below makes it clearer.

The solenoid is held with right hand in such a way that the curl of fingers point to the current direction. The end of the bar to which the thumb points is the North pole.

Magnetic field

A magnetic field is a region in which a magnetic object, placed within the influence of the field, experiences a magnetic force. The magnetic field pattern of different situations is shown below:

• In (a), the field of a single magnet is shown. The nearer the field lines, the stronger is the field.
• In (b), two opposite poles of different magnets are placed together. And this is how the field lines are. Well of course, you can’t see them but if you sprinkle iron fillings and tap them, the field lines will become clear, exactly like the lines we drew.
• Now in ( c), like poles of different magnets are placed together and we know they’ll repel so the field lines will be away from each other. The point marked P is the neutral point where no magnetic force is experienced due to the absence of influence of the filed.

An interesting property of magnetic field is that the field lines tend to pass through magnetic materials like iron.

The field lines will tend to pass through the piece of iron . The area with in the iron experiences no magnetic field.

Lastly, the magnets which retain their magnetism for a long time are called hard magnetic materials while those which do not maintain their magnetism for long are called soft magnetic materials.