Question

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

What is the 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) Both the needles will deflect in opposite directions

Answer 1

Hint: A magnetic compass is a device used to find direction and for navigation purpose. This is because it has a needle in it that is made up of a magnet. This needle aligns itself in the direction of the external or the applied magnetic field.

Complete step by step solution:
Step I: Since a compass has two poles- North pole and a South pole. Also it is known that the electric field and magnetic field are produced simultaneously at the same time. Since the current is flowing through the wire, a magnetic field is generated in it. So the North Pole of the magnet will get attracted to the South pole of the field generated in the wire and the South Pole will get attracted to the North Pole of the field of the wire.

Step II: When the wire is placed in a magnetic field, then electric current is induced in it. The magnetic field lines produced around the wire are circular. The points A and B have magnetic field lines in the opposite direction. Therefore the needle will deflect in the opposite direction.

Step III: Since the needles deflect in an opposite direction, therefore option D is the right answer.

Note: It is to be noted that the direction of magnetic field can be known with the Right Hand Thumb Rule. According to this rule, if the current carrying conductor is held in the right hand, then the thumb will point in the direction of current and the fingers will point in the direction of the magnetic field produced.