The behaviour of the needle of the magnetic compass under the influence of a magnet is said to have endlessly fascinated Einstein when he was a child.
Indeed, magnetism as a force of energy, as well as the magnetic field of a magnet, are extremely thought-provoking topics to read and understand, for anyone interested in the sciences generally, and in Physics, specifically.
What is the Magnetic Field?
If a magnetic compass is placed on a table, it is found that its needle rests in geographic north-south direction. But when it is placed near a magnet, the needle swings and then rests in some other direction. As the compass is placed at different positions around a magnet, the direction in which the needle rests, changes such that its one end always points towards the nearer pole of the magnet. This behaviour of the compass needle is due to the influence of the magnet near it. The region in which the compass gets influenced is called the magnetic field of the magnet.
As the distance of point from the magnet increases, the effect of its magnetic field decreases.
Thus, magnetic field of a magnetic can be defined as-
“The space around a magnet in which the needle of a compass rests in a direction other than the geographic north-south direction, is called the magnetic field of the magnet.”
Features of the Magnetic Field-
Magnetic field is a vector quantity. The magnitude of magnetic field at a point is measured by the force which a magnetic pole placed at that point experiences, while the direction of magnetic field is the direction in which the needle of compass rests when it is placed at that point.
When a compass needle is placed at any point in a magnetic field, its needle rests along the magnetic line. An arrow is marked on the magnetic field line from the south pole of the needle to its north pole. The arrow indicates the direction of magnetic field at that point.
Thus we can say that,
“A magnetic field line is a continuous curve in a magnetic field such that tangent at any point of it gives the direction of the magnetic field at that point.”
Properties of Magnetic Field Lines-
Magnetic field lines have the following properties-
1) They are closed and continuous curves.
2) Outside the magnet, they are directed from the north pole towards the south pole of the magnet.
3) The tangent at any point on a field line gives the direction of magnetic field at that point.
4) They never intersect one another. If two field lines intersect, there would be two directions of the magnetic field at that point which is not possible.
5) They are crowded near poles of the magnet and far from the magnet, where the magnetic field is weak.
6) Parallel and equi-distant field lines represent a uniform magnetic field. The earth’s magnetic field in a limited space is uniform.
7) They behave like stretched elastic rubber strings.