Question

A compass needle just above a wire in which electrons are moving towards east, will point:
A) east
B) west
C) north
D) south

Answer 1

The problem is based on the working principle of the magnetic compass and the attraction and repulsion of a magnetic compass needle with a current carrying wire as a current carrying wire also generates a magnetic field.
The approach to solve this problem is by the right hand thumb rule.

Complete step by step solution:
Step 1: At first we discuss the basic working principle of a magnet. Simplistically, the principle is that two magnetic fields try to align with each other— the earth's field and that of the compass needle, which is a magnet.
From a more deeply technical perspective, the principle is that an object which has a quality called magnetic polar moment$\mu$, which all magnets like the needle have, will feel a torque $\tau$when immersed in a magnetic vector field $B$like the earth's. The torque vector is equal to$\overrightarrow \tau = \overrightarrow \mu \times \overrightarrow B$, which will tend to force the needle to align with the earth's field. The earth's magnetic field runs very closely to directly north/ south.

Step 2: Right hand thumb rule states that if the thumb of the right hand points along the direction of current, then the remaining curled fingers of the same hand give the direction of the magnetic field due to the current.

Step 3: As electrons are moving towards west, the current equivalently moves towards east. Hence from right hand thumb rule, the magnetic field above the wire is towards the south, which is the same direction as that of earth's magnetic field.

Hence the compass points towards the south.

Option (D) is the correct option.

Note : Here the main thing to remember is the right hand rule. Which states that if the thumb of the right hand points along the direction of current, then the remaining curled fingers of the same hand give the direction of the magnetic field due to the current.