In order to picture the magnetic field and its strength at different distances from the poles, it is convenie12341 to show the field as lines of force, generally known as magnetic lines of force. These lines have no physical existence and are only a means of representing magnetic field.
Fig. 50(a) shows the magnetic lines of force of a bar magnet. The direction of each line of force is from North Pole to South Pole outside the magnet and from South Pole to North Pole inside the magnet. Near the poles of the magnet where the field is strongest, the lines are dense, but farther away from the poles the lines are less crowded showing lesser field strength.
It practice the lines of force of a bar magnet may be mapped by plotting the successive directions pointed out by small compass needle moving in the field. This is shown in fig. 50(b).
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Although the magnetic influence itself is entirely invisible, its existence and strength can be judged by such lines of force which indicate the direction in which the magnetic influence is acting, and the intensity of the field at any point is given by the density of these lines of force at that point.
The magnetic lines of force have the following properties:
(i) From each point on the North Pole of a bar magnet a line of force is supposed to start and after passing the space it is supposed to end finally on a corresponding point on the South Pole. Inside the magnet the direction of the lines of force is from South Pole to North Pole.
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(ii) A magnetic line of force always forms a closed loop. This is shown in fig. 50(a).
(iii) Two magnetic lines of force never cross each other. If two lines of force cross each other, it means two directions of a force at a point. This is impossible.
(iv) The tendency of a magnetic line of force is to take the path of least reluctance. If a piece of iron is placed in the magnetic field, most of the lines of force will pass through the iron piece.
(v) Magnetic lines of force are unaffected by non-magnetic materials like air, glass, wood, paper etc. But the lines of force concentrate more in a magnetic substance like iron or steel.
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(vi) Magnetic lines of force behave like stretched rubber bands. They tend to contract lengthwise and expand laterally.