Question

A short bar magnet passes at a steady speed right through a long solenoid. A galvanometer is connected across the solenoid which graph best represents the variation of the galvanometer deflection $\theta$ with time

(A)

(B)

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(D)

Answer 1

First of all we need to know what a bar magnet is. We find this solution using Lenz’s Law. Moving the bar magnet into the solenoid induces an emf in the solenoid and because the circuit is closed, a current flows and a magnetic field is induced. A galvanometer is an instrument which detects and measures small amounts of current, hence it will detect the amount of current flowing through the bar magnet and also the deflection shows the direction of current.

Complete step by step solution:
Bar Magnet:
A bar magnet is a rectangular piece of an object which shows permanent magnetic properties and is made up of ferromagnetic substances.
When a short bar magnet passes at a steady speed right through a long solenoid then according to Lenz’s Law an magnetic field is induced as the bar magnet gets its own magnetic field due to induced emf. This field produced by the bar magnet is non uniform. As we move the magnet towards the coil a flux induced in this would increase to a certain value and the direction of current will be such that the north pole will be formed at the point of entry to the solenoid. Once the bar magnet enters into the solenoid the current falls down and when the magnet is completely inside the solenoid current becomes zero.
Now when the bar magnet starts leaving the solid then the end becomes north and the direction of current becomes negative again the current increases to a certain extent in the negative direction and then falls to zero when it is completely outside the solenoid.
Therefore the correct option is (A).

Note:
Electromagnet is not always similar to bar magnet, there are also some differences. Remember that bar magnets have a constant magnetic pull and it generates its own magnetic field, you can also try it by sprinkling iron filings on a sheet of paper with a bar magnet in the middle.