Magnetic Lines of Force


Magnetic lines of force is an important topic of the IIT JEE Physics syllabus. Here first we shall define magnetic line of force and then discuss various concepts related to it.

What are magnetic lines of force?

To describe the phenomena related to magnets, lines are used to depict the force existing in the area surrounding the magnet. These lines are called the magnetic lines of force. These lines do not exist actually, but are imaginary lines that are used to illustrate and describe the pattern of the magnetic field. As shown in the figure below, the magnetic lines of force are assumed to originate from the north pole of a magnet, then pass through the surrounding space and then arrive at the South Pole. Then these lines travel inside the magnet from the South Pole to the North Pole and hence complete the loop.

Magnetic lines of force forming a closed loopLines of force are the lines in any such field the tangent of which at any point gives the field direction at that point and its density gives the magnitude of the field. Hence, magnetic lines of force are basically the lines of force which represent the direction of the magnetic field. The imaginary path traced by an isolated (imaginary) unit north pole may also be defined as a line of force.  
Magnetic lines of force are closed curves. Outside the magnet their direction is from north pole to south pole and inside the magnet these are from south to north pole.

Magnetic lines of forceThey don’t have any origin or end and do not interact because if they do so then it would mean two value of magnetic field at a single point, which is not possible. At the poles of the magnet the magnetic field is stronger because the lines of force there are crowded together and away from the poles the magnetic field is week. i.e. magnetic field intensity depends on the number of lines of force. The number of magnetic lines of force passing through unit normal area is defined as magnetic induction whereas the number of lines of force passing through any area is known as magnetic flux.

The lines of force can emerge out of the north pole of magnet at any angle and these can merge into the South Pole at any angle.

Direction of Magnetic lines of force –Physics

The direction of magnetic lines of force at any point gives the direction of the magnetic force on a north pole placed at that point.

Magnetic Lines of force emerge from North Pole. Why?

The direction of magnetic line of force is the direction of force on a North Pole, so the magnetic lines of force always begin on the North Pole of a magnet and end on the South Pole of the magnet. When a small magnetic compass is placed along a lie of force, it sets itself along the line tangential to it. Hence, the line drawn from the South Pole of the compass to its North pole shows the direction of the magnetic field.

Do magnetic lines of force never cross each other? Explain. Does the direction of the magnetic field change at any point along the magnetic lines of force?

It is not so that the magnetic field lines cannot cross but whenever such a situation arises, the field strength has to be zero. A magnetic field line is the path that points in the direction of the magnetic field at every point along it. If one moves with a compass along a magnetic field line, we will notice that the needle always points in the direction the person wants to head in, in order to stay on that magnetic field. If two field lines cross, then it would mean that the magnetic field points in two different directions at one place. But since there is only of the magnetic field at any place at any moment, hence this does not happen.  

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It is vital to master the conceptual questions on magnetic lines of force as this topic is a gateway to many other topics. You can grasp the concept of magnetics by askIITians through free online IIT study material.

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