- 1 What is a Magnetic Field? And How does a Magnetic Field Originate?
- 1.1 Magnetic Field
- 1.2 Magnetic Field Lines
- 1.3 1. To Plot the Magnetic Field Pattern Due to a Bar Magnet by Using Iron Filings
- 1.4 2. To Plot the Magnetic Field Pattern Due to a Bar Magnet by Using a Compass
- 1.5 Properties (or Characteristics) of the Magnetic Field Lines
- 1.6 Magnetic Field of Earth
- 1.7 Magnetic Field
- 1.8 Magnetic Lines of Force or Field Lines
- 1.9 Properties of magnetic lines of force:
The study of Physics Topics involves the exploration of matter, energy, and the forces that govern the universe.
What is a Magnetic Field? And How does a Magnetic Field Originate?
In the previous Chapter we have studied that an electric current can produce heating effect. We will now study that an electric , current can also produce a magnetic effect. The term ‘magnetic effect of electric current’ means that ‘an electric current flowing in a wire produces a magnetic field around it’. In other words, electric current can produce magnetism. This will become more clear from the following activity. Take about one metre long insulated copper wire and wind it round and round closely on a large iron nail (see Figure).
Then connect the ends of the wire to a battery. We will find that the large iron nail can now attract tiny iron nails towards it (as shown in Figure). This has happened because an electric current flowing in the wire has produced a magnetic field which has turned the large iron nail into a magnet. Please note that the current-carrying straight electric wires (like an electric iron connecting cable) do not attract the nearby iron objects towards them because the strength of magnetic field produced by them is quite weak. We will now describe a magnet, poles of a magnet, magnetic field and magnetic field lines briefly. This is necessary to understand the magnetic effect of current.
A magnet is an object which attracts pieces of iron, steel, nickel and cobalt. Magnets come in various shapes and sizes depending on their intended use. One of the most common magnets is the bar magnet. A bar magnet is a long, rectangular bar of uniform cross-section which attracts pieces of iron, steel, nickel and cobalt. We usually use bar magnets for performing practicals in a science laboratory.
A magnet has two poles near its ends : north pole and south pole. The end of a freely suspended magnet (or a freely pivoted magnet) which points towards the north direction is called the north pole of the magnet (see Figure). And the end of a freely suspended magnet (or freely pivoted magnet) which points towards the south direction is called the south pole of the magnet. It has been found by experiments that like magnetic poles repel each other whereas unlike magnetic poles attract each other.
This means that the north pole of a magnet repels the north pole of another magnet and the south pole of a magnet repels the south pole of another magnet; but the north pole of a magnet attracts the south pole of another magnet. These days magnets are used for a variety of purposes. Magnets are used in radio, television, and stereo speakers, in refrigerator doors, on audio and video cassette tapes, on hard discs and floppies for computers, and in children’s toys. Magnets are also used in making electric generators and electric motors. The Magnetic Resonance Imaging (MRI) technique which is used to scan inner human body parts in hospitals also uses magnets for its working.
Just as an electric charge creates an electric field, in the same way, a magnet creates a magnetic field around it. The space surrounding a magnet in which magnetic force is exerted, is called a magnetic field. A compass needle placed near a magnet gets deflected due to the magnetic force exerted by the magnet, and the iron filings also cling to the magnet due to magnetic force.
The magnetic field pattern due to a bar magnet is shown in Figure. The magnetic field has both, magnitude as well as direction.The direction of magnetic field at a point is the direction of the resultant force acting on a hypothetical north pole placed at that point. The north end of the needle of a compass indicates the direction of magnetic field at a point where it is placed.
Magnetic Field Lines
A magnetic field is described by drawing the magnetic field lines. When a small north magnetic pole is placed in the magnetic field created by a magnet, it will experience a force. And if the north pole is free, it will move under the influence of magnetic field. The path traced by a north magnetic pole free to move under the influence of a magnetic field is called a magnetic field line. In other words, the magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move.
The magnetic field lines are also known as magnetic lines of force. The direction of a magnetic field line at any point gives the direction of the magnetic force on a north pole placed at that point. Since the direction of magnetic field line is the direction of force on a north pole, so the magnetic field lines always begin from the N-pole of a magnet and end on the S-pole of the magnet (see Figure). Inside the magnet, however, the direction of magnetic field lines is from the S-pole of the magnet to the N-pole of the magnet.
Thus, the magnetic field lines are closed curves. The magnetic field lines due to a bar magnet are shown in Figure. When a small compass is moved along a magnetic field line, the compass needle always sets itself along the line tangential to it. So, a line drawn from the south pole of the compass needle to its north pole indicates the direction of the magnetic field at that point. We will now describe how the magnetic field lines (or magnetic field) produced by a bar magnet can be plotted on paper.
1. To Plot the Magnetic Field Pattern Due to a Bar Magnet by Using Iron Filings
Place a card (thick, stiff paper) over a strong bar magnet (as shown in Figure). Sprinkle a thin layer of iron filings over the card with the help of a sprinkler, and then tap the card gently. The iron filings arrange themselves in a regular pattern as shown in Figure. This arrangement of iron filings gives us a rough picture of the pattern of magnetic field produced by a bar magnet. This happens as follows : The bar magnet exerts a magnetic field all around it. The iron filings experience the force of magnetic field of the bar magnet.
The force of magnetic field of the bar magnet makes the iron filings to arrange themselves in a particular pattern. Actually, under the influence of the magnetic field of the bar magnet, the iron filings behave like tiny magnets and align themselves along the directions of magnetic field lines. Thus, iron filings show the shape of magnetic field produced by a bar magnet by aligning themselves with the magnetic field lines.
There is also another method of obtaining the magnetic field pattern around a bar magnet. This is done by using a compass. A compass is a device used to show magnetic field direction at a point. A compass is also known as a plotting compass.
A compass (or plotting compass) consists of a tiny pivoted magnet usually in the form of a pointer which can turn freely in the horizontal plane. It is enclosed in a non-magnetic metal case having a glass top (see Figure 6). The tiny magnet of the compass is also called ‘magnetic needle’ (or just ‘needle’). The ends of the compass needle point approximately towards the north and south directions.
Actually, the tip of compass needle points towards the north direction whereas its tail points in the south direction. When the compass is placed in a magnetic field (say, in the magnetic field due to a bar magnet), then a force acts on it and it is deflected from its usual north-south position (the axis of needle lines up in the direction of magnetic field).
Thus, a compass needle gets deflected when brought near a bar magnet because the bar magnet exerts a magnetic force on the compass needle, which is itself a tiny pivoted magnet (free to move in the horizontal plane). We can trace the magnetic field lines around a bar magnet by using a plotting compass as decribed below.
2. To Plot the Magnetic Field Pattern Due to a Bar Magnet by Using a Compass
The bar magnet M whose magnetic field pattern is to be traced is placed on a sheet of paper and its boundary is marked with a pencil (see Figure). A plotting compass is now brought near the N-pole of the bar magnet (see position X in Figure). In this position, the N-pole of magnet repels the n-pole of compass needle due to which the tip of the compass needle moves away from the N-pole of the magnet.
On the other hand, the N-pole of magnet attracts the s-pole of compass needle due to which the tail of compass needle comes near the N-pole of the magnet (see position X in Figure). We mark the positions of the tip and the tail of compass needle by pencil dots B and A. That is, we mark the positions of the two poles of the compass needle by pencil dots B and A (tip representing north pole and tail representing south pole).
We now move the compass to position Y so that the tail of compass needle (or south pole) points at dot B (previously occupied by n-pole of compass needle). We mark a dot C at the tip of the compass needle to show the position of its north pole. In this manner we go on step by step till we reach the south pole of the magnet. By doing this we get the various dots A, B, C, D, E, F, G, H, I, J, K and L, all denoting the path in which the north pole of the compass needle moves.
By joining the various dots, we get a smooth curve representing a magnetic field line, which begins on the north pole of the bar magnet and ends on its south pole (see Figure). We can draw a large number of lines of force in the same way by starting from different points near the magnet. Every line is labelled with an arrow to indicate its direction. In this way we will get the complete pattern of the magnetic field around a bar magnet.
The magnetic field pattern around a bar magnet is shown in Figure. This has been traced by using a plotting compass. The magnetic field lines leave the north pole of a magnet and enter its south pole (as shown in Figure). In other words, each magnetic field line is directed from the north pole of a magnet to its south pole. Each field line indicates, at every point on it, the direction of magnetic force that would act on a north pole if it were placed at that point. The strength of magnetic field is indicated by the degree of closeness of the field lines. Where the field lines are closest together, the magnetic field is the strongest.
For example, the field lines are closest together at the two poles of the bar magnet, so the magnetic field is the strongest at the poles. Please note that no two magnetic field lines are found to cross each other. If two field lines crossed each other, it would mean that at the point of intersection, the compass needle would point in two directions at the same time, which is not possible. It should be noted that we have drawn the magnetic lines of force only in one plane around the magnet. Actually, the magnetic field and hence the magnetic lines of force exist in all the planes all round the magnet.
Properties (or Characteristics) of the Magnetic Field Lines
1. The magnetic field lines originate from the north pole of a magnet and end at its south pole.
2. The magnetic field lines come closer to one another near the poles of a magnet but they are widely separated at other places. A north magnetic pole experiences a stronger force when it approaches one of the poles of the magnet. This means that the magnetic field is stronger near the poles. From this we conclude that where magnetic field lines are closer together, it indicates a stronger magnetic field. On the other hand, when magnetic field lines are widely separated, then it indicates a weak magnetic field.
3. The magnetic field lines do not intersect (or cross) one another. This is due to the fact that the resultant force on a north pole at any point can be only in one direction. But if the two magnetic field lines intersect one another, then the resultant force on a north pole placed at the point of intersection will be along two directions, which is not possible.
Magnetic Field of Earth
A freely suspended magnet always points in the north-south direction even in the absence of any other magnet. This suggests that the earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction : north and south. The shape of the earth’s magnetic field resembles that of an imaginary bar magnet of length one-fifth of earth’s diameter buried at its centre (see Figure).
The south pole of earth’s magnet is in the geographical north because it attracts the north pole of the suspended magnet. Similarly, the north pole of earth’s magnet is in the geographical south because it attracts the south pole of the suspended magnet. Thus, there is a magnetic S-pole near the geographical north, and a magnetic N- pole near the geographical south. The positions of the earth’s magnetic poles are not well defined on the globe, they are spread over an area.
The axis of earth’s magnet and the geographical axis do not coincide with each other. The axis of earth’s magnetic field is inclined at an angle of about 15° with the geographical axis. Due to this a freely suspended magnet (or magnetic needle) makes an angle of about 15° with the geographical axis and points only approximately in the north-south directions at a place.
In other words, a freely suspended magnet does not show exact geographical north and south because the magnetic axis and geographical axis of the earth do not coincide. It is now believed that the earth’s magnetism is due to the magnetic effect of current (which is flowing in the liquid core at the centre of the earth). Thus, earth is a huge electromagnet.
A magnet attracts iron, nickel, cobalt and some metallic alloys. This force of attraction even penetrates wood, glass, paper or other obstructions. It is always observed that the influence of a magnet is felt in its surrounding region. Greater the distance from the magnet, lesser is its influence. Again if a weak magnet is replaced by a stronger one, the range of this influence increases.
Definition: The region surrounding a magnet in which the influence of that magnet is felt, is called the magnetic field of that magnet.
Theoretically, the magnetic field of a magnet extends up to infinity; but in practice, the field is assumed to be extended up to a limited region due to 0 limitation of the experimental arrangement used for identification and 0 presence of other magnetic fields (like earth’s magnetic field) in the environment.
Magnetic Lines of Force or Field Lines
Experiment: A small but powerful bar magnet is kept on a fairly large piece of cork and is let to float on water kept in a large vessel [Fig.(a)]. In the floating condition, the magnet ultimately sets itself at rest along the north-south direction. So, its A’-pole faces the north and the S-pole faces the south. With
the help of a small cork a long but comparatively weak mag-netic needle is set to float vertically on water in such a manner that the n-pole of the needle is just above the water surface but its 5-pole remains deep inside water [Fig.(b)], In this situa-tion, the effect of the bar magnet on the s-pole of the needle becomes negligible due to its depth. Hence, the n-pole of the needle can be treated as an isolated free n -pole with respect to the bar magnet. The n-pole of the needle is brought in contact with the N-pole of the bar magnet at the point A and then released. It is seen that this n-pole moves over the surface of water and follows a curved path ABCD to reach the S-pole of the bar magnet.
Explanation of the experiment: The N-pole of the bar magnet exerts a force of repulsion on the isolated n-pole of the needle but the S-pole exerts a force of attraction on it. The iso-lated free n-pole then starts moving along the resultant of the above two forces. At different points of the magnetic field of the bar magnet, the direction of this resultant force is different. Then, direction of motion of the n-pole will also change. So, when an isolated and free n-pole is allowed to travel in the magnetic field of a magnet, the pole describes a curved path and this path extends from the north pole to the south pole of the magnet.
A magnetic line of force is an imaginary curved line in a magnetic field; the direction of this field at any point is given by the tangent drawn at that point on the line of force passing through that point.
Properties of magnetic lines of force:
i) A magnetic line of force emerges from the north pole of a magnet and terminates at the south pole.
ii) For different initial points adjacent to the north pole of a magnet, different lines of force are obtained in the magnetic field. In Fig., different lines of force at one side of a bar magnet are shown. A number of such lines of force indicate a magnetic field.
iii) Two lines of force never intersect each other. If they intersect, through that point of intersection two tangents can be drawn on the two lines of force and each tangent will indicate the direction of the magnetic field at the point of inter section. But two directions of the magnetic field at a single point is meaningless.
iv) The concept of lines of force is totally imaginary, no such line exists in a magnetic field.
v) At any point in a magnetic field, if a unit area normal to the direction of the lines of force is imagined, the number of lines of force passing through that area is called the number density of the lines of force or magnetic flux at that point. Greater this number density, greater will be the strength of the magnetic field at that point. For example, in Fig. the number density at the point B is less than that at the point A. So the magnetic field at the point B is weaker than that at the point A.
vi) In general, the strength of a magnetic field is different at different points in that field and their directions are also different. Hence, the magnetic lines of force are usually curved lines at different distances. But if the magnetic field is uniform, i.e., its magnitude and direction is the same up to a certain region, then it can be represented by equispaced parallel straight lines [Fig.]. Earth’s magnetic field, very close to the surface of the earth, is such a uniform magnetic field.