Physics Topics such as mechanics, thermodynamics, and electromagnetism are fundamental to many other scientific fields.
List the Important Properties of Magnetic Materials
The two most important properties of magnets are :
- A freely suspended magnet always points in the north-south direction.
- Like magnetic poles repel each other; unlike magnetic poles attract each other.
We will now study these two properties of magnets in detail, one by one.
1. A Freely Suspended Magnet Always Points In North-South Direction
If a bar magnet is freely suspended, it will swing until its one end points towards the north pole of earth and the other towards the south pole of the earth. In other words, a freely suspended magnet comes to rest always in the north-south direction. Figure shows a bar magnet suspended from a thread which is tied to a wooden stand. We call it a freely suspended magnet because it is free to move in any direction.
The thread does not prevent it from swinging in any direction. We can see from Figure that the freely suspended bar magnet is pointing in the north and south directions. Let us now turn the magnet by hand so that it points in another direction and then leave it. We will see that after a while the magnet comes back to its earlier position and again points in the same north-south direction.
The end of magnet which points towards the north direction is called north-seeking end or north pole of the magnet. In Figure, the north pole of magnet has been marked N. The end of magnet which points towards the south direction is called south-seeking end or south pole of the magnet. In Figure, the south pole of magnet has been marked S. We usually call them north and south poles of the magnet or just N and S poles.
If, however, a wooden scale, a plastic scale, an iron bar, a copper plate or an aluminium rod are freely suspended by using thread, they do not always come to rest pointing along the same direction (north-south direction). It is only a freely suspended magnet (or a freely pivoted magnet) which always comes to rest along the same direction (north-south direction) even after being disturbed again and again.
Before we study how to find directions by using freely suspended (or freely pivoted) magnets, we should know what are the various directions on the earth. The four major directions are : North, South, East and West. These are denoted by the letters N, S, E and W respectively.
We can have four more directions as : North-East (NE), North-West (NW), South-East (SE) and South-West (SW). North-East is midway between North and East directions; North-West is midway between North and West directions; South-East is midway between South and East directions whereas South-West is midway between South and West directions.
The sun always rises in the morning from the East direction. So, if we stand facing the morning sun rising from the East direction with both the arms extended on sides, then our left arm will point towards North and the right arm will point towards South. This fact was used in olden days to mark the North pointing end of a natural magnet so that directions could be found even when the sun was not there.
The property of a freely suspended magnet to align itself always along the North-South of the earth is used to find directions at unknown places. For centuries, travellers on land and at sea have been using this property of magnets to find directions. In olden days, travellers to far off places used to carry natural magnets (which were strips of magnetite) and thread with them. They used to find directions along their way by suspending the natural magnet with a thread.
Later on, when artificial magnets were available, an instrument (or device) called ‘compass’ was developed by scientists to find the directions more conveniently. In a compass, the magnet is not suspended from a thread. The tiny magnet in a compass is free to turn (or rotate) on a pivot or pin. It is called freely pivoted magnet. Just like a freely suspended magnet, a freely pivoted magnet also always comes to rest along the north-south direction. We will now describe the compass in detail.
The compass is an instrument to find the directions at a place. The compass is based on the property of a freely pivoted magnet to align itself along the north-south direction. The compass has a tiny magnet in the form of a needle which can rotate freely on a pivot (or pin). The pivoted magnetic needle of the compass is enclosed in a small brass box having a glass top (see Figure).
Thus, the compass needle is a tiny magnet which can rotate freely on a pivot. The compass has a circular dial with various directions marked on it. The needle of compass always comes to rest along the north-south direction. The north pole of compass needle is painted with a different colour to distinguish it from the south pole.
Thus, the differently painted end of compass needle is its north pole and it always shows the north direction of the earth. In order to find the directions at a place, the compass is kept on a flat surface at that place. The needle of compass will soon come to rest (or become stationary) along the north-south direction.
The compass is then rotated until the painted end of needle is towards N (North) mark on the dial and the other end of needle is towards S (South) mark on the dial (see Figure). We can now find any direction by looking at the dial of the compass.
A compass helps the people to find their way. For example, compass is used by soldiers in the jungle to find direction for their movement. The compass is also used by sailors while at sea so as to take their ships in the right direction. We will now discuss briefly why a freely suspended magnet (or a freely pivoted magnet) always points in the same direction : north-south direction. Actually, our planet earth also shows magnetism.
The earth behaves as if a giant bar magnet is buried at its centre. It is the force exerted by the earth’s imaginary magnet which acts on a freely suspended bar magnet (or freely pivoted magnetic needle of a compass) and makes them always point in the north-south direction.
Attraction and Repulsion Between Magnets
If we hold one bar magnet in our left hand and another similar bar magnet in our right hand, and bring the ends of these magnets close to one another, then our hands can feel the force of attraction or repulsion acting between the poles of the magnets which are facing each other.
Thus, a magnet can attract another magnet. A magnet can also repel (or push away) another magnet. Now, whether a magnet will attract or repel another magnet depends on which poles of the magnets are facing each other.
(i) When we place two magnets in such a way that the north pole of one magnet faces the north pole of the other magnet, they repel (push away) each other [see Figure (a)]. Similarly, when the two magnets are placed near each other in such a way that south pole of one magnet faces the south pole of the other magnet, even then they repel (push away) each other [see Figure (b)]. Since north-north poles and south-south poles are like poles, we can say that like poles (or similar poles) repel each other.
If we try to place two bar magnets of exactly equal size one above the other such that their north poles are on the same side, then they will just push away each other. This is because their like poles (north poles) being on the same side will repel each other. Similarly, their other like poles (south poles) will also be on the same side at the other end and hence repel each other.
(ii) If we place two magnets near each other in such a way that the north pole of one magnet faces the south pole of the other magnet, they attract each other (and come near to each other) (see Figure).
Since north pole and south pole are unlike poles, we can say that unlike poles attract each other. We will now describe an activity to show that like magnetic poles repel and unlike magnetic poles attract.
2. Like Magnetic Poles Repel; Unlike Magnetic Poles Attract
In order to show that like magnetic poles repel and unlike magnetic poles attract, we have to use two bar magnets. One of the bar magnets is suspended with a thread and the other bar magnet is held in our hand. Let us describe the activity now.
1. Let us bring the north pole of the hand-held magnet near the north pole of the suspended magnet [see Figure (a)]. We will find that the north pole of the suspended magnet moves away from the north pole of our hand-held magnet. This means that the north pole of one magnet repels the north pole of the other magnet.
Similarly, if we bring the south pole of the hand-held magnet near the south pole of the suspended magnet, even then the south pole of suspended magnet moves away showing that the south pole of one magnet repels the south pole of another magnet.
2. We now bring the south pole of the hand-held magnet near the north pole of the suspended magnet [see Figure (b)]. We find that the north pole of the suspended magnet moves towards the south pole of our hand-held magnet. This means that the south pole of one magnet attracts the north pole of another magnet.
Similarly, if we bring the north pole of the hand-held magnet near the south pole of the suspended magnet, even then the south pole of the suspended magnet moves towards the north pole of the hand-held magnet showing that the north pole of one magnet attracts the south pole of another magnet.
The above activity shows that :
N pole brought near to N pole repels
S pole brought near to S pole repels
Thus, like magnetic poles repel each other.
Also, N pole brought near to S pole attracts
S pole brought near to N pole attracts.
Thus, unlike magnetic poles attract each other.
From the above discussion we conclude that if two magnets are brought close together then like magnetic poles will repel each other and unlike magnetic poles will attract each other.
We have already studied that the needle of a compass always aligns itself along the same direction : north-south direction. What will happen if a magnet is brought near a compass ? We know that the needle of compass is itself a tiny magnet (having a N-pole and a S-pole) which is free to rotate on a pivot.
Now, when another magnet is brought near a compass, then this magnet will attract or repel the magnetic needle of compass due to which the compass needle will be disturbed from its usual north-south direction. The compass needle will point in another direction And if we take the magnet away from compass, the compass needle will come back to its original north-south position.
Please note that the various devices and appliances such as mobile phones, music systems, television sets, computers, compact . discs (CDs), cassettes and credit cards, etc., use magnetism for their working. So, we should keep strong magnets away from all these devices so as not to disturb their working.
Activity To Test For A Magnet
Suppose we are given two identical bars of iron, one of which is a magnet and the other is an ordinary bar of iron. We want to find out which of them is a magnet. We can do this by using any object made of iron or steel (such as a paper clip or safety pin) as follows :
1. Take both the identical bars of iron near a paper clip, one by one.
- The iron bar which attracts (or pulls) the paper clip so that the paper clip sticks to it, will be a magnet.
- The iron bar which does not attract (or pull) the paper clip will be an ordinary bar of iron and not a magnet.
Again suppose that we are given a bar magnet whose north pole and south pole have not been marked and we have to find out which of its end is north pole and which end is its south pole. For this purpose, we require another bar magnet whose poles are known and marked as N and S.
The method to find the unmarked poles of a magnet depends on the fact that like poles repel (push away) each other. The method of finding the unmarked poles of a magnet will become clear from the following sample problem.
A bar magnet has no markings to indicate its poles. How would you find out near which end is its north pole located ?
We take another bar magnet whose poles are marked N and S. We bring the N-pole end (North pole end) of this bar magnet near both the ends of the given bar magnet, turn by turn. The end of given bar magnet which will be repelled (pushed away) by the North pole of this magnet will be its North pole (because like poles repel each other).