Question

In the circuit shown in the figure, a coil is wound around a match box containing a compass needle. When the key is closed, the compass needle placed in the match box deflects. The force which causes this deflection is:

A. magnetic force due to current
B. electric force due to current
C. gravitational force
D. electric force due to magnet

Answer 1

Hint: Inductor is a device which responds to the change in the current. It stores the magnetic energy. When an inductor is connected to the power source initially it won’t allow the flow of current but after some time it will act as a normal conducting wire with no resistance which will be exactly opposite in case of capacitor.

Complete step by step answer:
Normally in case of capacitors initially when power source is connected to the capacitor it acts as
a normal conduction wire and after sometime this will completely not allow the flow of charge but in case of an inductor this will be completely opposite. When an inductor is connected to the power source initially it won’t allow the flow of current but gradually after some time it will allow the flow of current just as the normal conducting wire.

Now in the given question the coil acts as an inductor and the bulb is denoted by ‘B’
The purpose of connecting a bulb is to make it as an L-R circuit because the bulb has resistance and the inductor has an inductance.
The compass needle responds to the magnetic field. Now when the switch is closed initially current changes from zero to some high value that means there is sudden change in current which means that magnetic flux is generated inside the coil.
Due to that magnetic field generated inside the coil the deflection will be caused to the compass needle. It's like the magnetic field exerts the magnetic force on the coil and causes it to deflect.
Hence the magnetic field produced due to current is the reason for the deflection.

So, the correct answer is “Option A”.

Note:
In case of an inductor when the key is closed then there is sudden increase in the flow of current which means that sudden change will be opposed by the inductor. Hence inductor generates emf itself in such a way that it opposes that flow and hence net current flown will be zero and initially inductor doesn’t allow the current.