Question

A circular loop placed in a plane perpendicular to the plane of paper, carries a current when the key is ON. The current, as seen from points A and B(in the plane of the paper and on the axis of the coil) is anticlockwise and clockwise respectively. The magnetic field lines point from B to A. the N pole of the resultant magnet is on the face close to:

a) A
b) B
c) A if the current is small, and B if the current is large
d) B if the current is small and A if the current is large

Answer 1

In the above question we are asked to determine the poles of the magnetic lines produced with respect to the coil. In the question we are given the information regarding the flow of current in the coil. Hence using the right hand thumb rule we will determine the direction of the field lines produced and accordingly we will be in a position to determine the pole of the magnetic lines with respect to the coil.

Complete step-by-step solution:
To begin with the question let us understand the right hand thumb rule. It states that if there is a current carrying loop or a coil such that if we curl the four fingers of our right hand in the direction of the current in the loop, then the thumb perpendicular to the curl of the hand determines the direction of the magnetic field lines.
Figure below gives a brief description of the above rule.

In the question it is given to us that if we look at the coil from point A, the current in the coil flows anticlockwise and with respect to point B the current flows clockwise where both the points lie on the axis of the loop. Hence from right hand thumb rule, we can imply that the field lines will appear to move from B to A along the axis of the coil and from A to B outside the loop. Since the field lines of a magnet moves from south to north inside the magnet and from north to south outside the magnet, we can imply that that the north pole of the magnetic field lines of the coil lies close to point A
Therefore the correct answer of the above question is option A.

Note: To understand the above scenario more precisely, imagine a magnet instead of a coil. Further compare the field lines of the magnet and the coil and accordingly we get our answer. The above law can also be used to determine the field lines of a straight current carrying conductor.