Question

An electric current passes through a straight wire from south to north direction. Magnetic compasses are placed at points A and B as shown in the figure:


What is your observation?

• A. The needle will not deflect
• B. Only one of the needles will deflect
• C. Both the needles will deflect in the same direction
• D. The needles will deflect in the opposite direction

Answer 1

The correct option is D The needles will deflect in the opposite direction

According to the right-hand thumb rule, when an electric current passes through a straight wire and it is held by the right hand with the thumb pointing in the direction of the conventional current and the fingers curling around it, then the fingers gives the direction of the magnetic field around the wire.

In this question, the curling of the fingers around the wire would suggest that at point A, the direction of the magnetic field is coming out of the plane of the paper while at point B, it is into the plane of the paper and since the magnetic field is in opposite directions at both the compass locations, the needles will also deflect in opposite directions.

Hence, option D is the correct answer.