Question

If the key in the arrangement is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are:

(A) Concentric circles
(B) Elliptical in shape
(C) Straight lines parallel to each other
(D) Concentric circles near the point O but of elliptical shapes as we go away from it

Answer 1

Hint
When current passes through a long straight conductor, a magnetic field is produced. But, if the key is taken, the circuit is open, and no electricity passes through the circuit, and the magnetic field associated with the long straight conductor is absent.

Complete step by step answer
The magnetic field due to the long straight current carrying conductor is given by,
$\Rightarrow B = \dfrac{{{\mu _0}I}}{{2\pi R}}$ where ${\mu _0}$ is the permeability of free space, $I$ is the current flowing through the long straight conductor, and $R$ is the distance from the conductor to the point at which the magnetic field is being calculated.
But, here as it is given, the key is open and therefore current is not passing through the long straight conductor. Hence, we can clearly predict that the magnetic field will be zero, due to the current carrying straight conductor. Then, the only magnetic field we can observe will be due to the earth’s magnetic field.
So, when we draw the magnetic field lines due to the earth’s magnetic field over the horizontal plane, we will get straight lines parallel to each other.
Therefore the correct answer is option (C).

Note
The earth’s magnetic field, also called geomagnetic field, is produced due to the electric current produced at the outer core of the earth. The molten iron and nickel in the earth’s outer core are the reason for this electric current. Therefore, the geomagnetic field originates from the earth’s core and extends up to space.