Question

What are magnetic field lines? Explain with the properties. Give the direction of the magnetic field for a point.

Answer 1

Hint: We need to understand the definition of the magnetic field and magnetic field lines to describe the different properties of these. The direction and magnitude of these should be dependent on the various parameters involved in the system in which it is considered.

Complete answer:
The magnetic field lines are a characteristic property of a magnetic field. The magnetic field is the force experienced by a magnetic material per unit charge moving with a unit velocity.
i.e.,
\[\begin{align}
  & \text{From Lorentz }\!\!'\!\!\text{ s force,} \\
 & F=qvB \\
 & \Rightarrow B=\dfrac{F}{qv} \\
\end{align}\]
The magnetic field lines are the imaginary lines alone which the magnetic field tends to act at a particular point. It is the direction of the magnetic field at the very point. The magnetic field direction is assumed by the direction pointed by a compass when placed at that spot.
Unlike the electric field lines, the magnetic field lines have two endpoints starting from the north and ending in the south. This property of the magnetic field describes the impossibility of the existence of monopoles in magnetism.
We can discuss the various properties of the magnetic field lines as –
1.The magnetic field lines do not cross each other at any point. It has a unique magnitude and direction at every point in the space. Therefore, they never meet.
2.The magnetic field lines emerge from the north pole to the outside of the material and get into the south pole of the material and complete a full circle inside the material. We cannot split up the field lines, unlike the electric field lines. They exist as dipoles only.

3.The number of magnetic field lines gives the intensity of the field in that region. The intensity is inversely proportional to the square of the distance from the source of the field and the point.

The magnetic field due to the magnetic material placed is as shown above. The field line exits the material from the north pole and enters the material at the south pole.
These are the main properties of the magnetic field lines.

Note: The magnetic fields are almost similar to the electric field except that the magnetic field is always conserved within a given surface. The magnetic field entering the surface and leaving the surface is always a constant as given by Gauss’ law.