Question

When an electric current $I$ is passed through a solenoid in the figure shown below, magnetic field at points $A$ and $B$ is equal and opposite to that of the earth. The,

• A. Straight line $AB$ is along north-south.
• B. Straight line $AB$ is along east-west.
• C. If current $I$ is increased, the positions of points $A$ and $B$ will shift away from the solenoid.
• D. If current $I$ is increased, the position of points $A$ and $B$ will shift towards the solenoid.

Answer 1

Answer: (B), (C)

Straight line $AB$ is along east-west.
If current $I$ is increased, the positions of points $A$ and $B$ will shift away from the solenoid.
Options B and C are right.

The magnetic field inside a solenoid is proportional to both the applied current and the number of turns per unit length. There is no dependence on the diameter of the solenoid, and the field strength doesn't depend on the position inside the solenoid, i.e., the field inside is constant.
In this case, magnetic field at points A and B is equal and opposite to that of the earth. This means that the magnetic field is opposite to the north-south direction which the direction of the earth's magnetic field. Also, when the current is increased the strength of the magnetic field is also increased as they are proportional to each other.
Hence, the Straight line AB is along east-west direction and if current is increased, the position of points and will shift towards the solenoid.