Question

What are magnetic field lines? How is the direction of the magnetic field at a point in the magnetic field determined using field lines?

Answer 1

Just like a static charge produces an electric field around it, a moving charge produces a magnetic field around it. Magnetic field around a moving charge can be understood by the nature of magnetic lines in the magnetic field. The direction of the magnetic field at a point in the magnetic field can be determined by using Maxwell’s right hand thumb rule.

Complete step by step answer:
We know that a charge at rest produces electric fields around it. In the same manner, a charge which is moving produces a magnetic field around it to show magnetic properties. Magnetic fields are also present in magnetic materials like bar magnets in addition to moving charges. Magnetic field lines give a pictorial representation of the magnetic field created around a bar magnet or a moving charge. They are nothing but a group of lines drawn around a magnetic material to understand the direction of the flow of the magnetic field. The direction of the magnetic field is represented using arrows. For example, magnetic field lines around a bar magnet are as shown in figure 1, those of a straight conductor carrying current are as shown in figure 2 and those of a solenoid are as shown in figure 3.

Figure 1

Figure 2

Figure 3

From the figures, it is clear that magnetic field lines of a bar magnet are directed from the north pole(N) to the south pole(S) of the bar magnet, magnetic field lines of a current carrying straight conductor in the upward direction are directed in anticlockwise direction and magnetic field lines of a solenoid are directed around the coil, from the negative end to the positive end, as shown.
The direction of magnetic field or magnetic field lines can be easily sorted out using Maxwell’s right hand thumb rule. According to this rule, if we imagine the linear wire conductor to be held in the grip of the right hand so that the thumb points in the direction of the current, then the curvature of the fingers around the conductor will represent the direction of magnetic field lines. This is illustrated in the following figure.

From the figure, it is clear that when we hold our right hand in such a manner that the current is flowing through the grip of our hand with the right thumb pointing in the direction of the current, the direction of our curled fingers will help us determine the direction of magnetic field lines around the linear conductor. The curvature of our fingers will represent the direction of magnetic field lines.

Note:
Maxwell’s right hand thumb rule is seen to be true in figure 1 also. In the figure, it is clear that the current in the straight conductor is flowing from A to B. When we apply the right hand thumb rule, the thumb points upwards and the curvature of the curled fingers points in an anticlockwise direction. This represents the direction of magnetic field lines in the current carrying straight conductor.