Question

Which of the following correctly describes the magnetic field near a long straight wire?
(A) The field consists of straight lines perpendicular to the wire
(B) The field consists of a straight line parallel to the wire
(C) The field consists of radial lines originating from wire
(D) The field consists of concentric circles centred on the wire

Answer 1

Hint
You must be aware of the fact that the magnetic field lines are imaginary lines in a region that indicate the presence and strength of the magnetic field in that region. But if the lines are imaginary, then how do we know what shape they have? We get an idea of the shape of the magnetic field lines produced by any magnetic substance by the famous iron filings experiment. Let’s discuss in detail what the iron filing experiment is.

Complete step by step answer
As discussed in the hint, we need to perform the iron filing experiment to find the shape of magnetic field lines. In this experiment, a piece of paper or cardboard is placed horizontally in the magnetic field. Once the cardboard is placed in the field, iron filings are poured over it; iron filings arrange themselves along the magnetic field lines produced by the magnetic substance. When the cardboard is given a gentle tap, the excess iron filings fall off the cardboard and we get the true shape of the magnetic field lines in the region.
When the iron filing experiment is done for a long straight wire, concentric circles are formed by the iron filings. Hence we can say that the field consists of concentric circles centred on the wire or option (D) is the correct answer.
Generally, the cardboard is placed at the centre of the assumed field to get an idea of the complete field structure produced by the magnetic object. But we can also do this experiment for any one particular pole of the magnet as well.

Note
An alternative to the iron filing experiment is the magnetic compass experiment in which a magnetic compass is placed at different points in the field, and the direction of the magnetic needle is plotted on a piece of paper. When this activity is repeated for many different points in the magnetic field, we find the complete structure of the magnetic field lines formed by the magnetic substance.