$Assertion :$ A bar magnet is dropped into a long vertical copper tube. Even taking air resistance as negligible, the magnet attains a constant terminal velocity. If the tube is heated, the terminal velocity gets increased.
$Reason :$ The terminal velocity depends on the eddy current produced in the bar magnet.

Both the assertion and reason are true and the reason is the correct explanation of the assertion.

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Both the assertion and reason are true, but reason is not the correct explanation of the assertion

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The assertion is true but reason is false.

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The assertion and reason both are false.

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Solution

The correct option is A Both the assertion and reason are true and the reason is the correct explanation of the assertion.
When the magnet is dropped down through a long copper tube, the magnetic flux linked with the copper tube changes continuously. Due to this change in flux linked with the copper tube, there is induction of an eddy current.

According to Lenz's law, the eddy current generated opposes the change in the magnetic field linked with the copper tube and hence, applies an upward retardation force on the magnet.The other force acting on the magnet is its own weight in the downward direction.

Since the magnet is in the free fall and moves under the influence of the acceleration due to gravity, the velocity of the magnet increases in the downward direction and the magnet is in the accelerated motion. But as the magnet moves downward with increasing velocity, the eddy current generated opposes the change in magnetic flux linked with the copper tube largely.

Hence, the retarding force by the eddy currents increases to such an extent that at a point the weight of the magnet in the downward direction becomes equal to the upward retarding force. Therefore, the velocity of magnet becomes constant.

When the tube is heated, its resistance gets increased due to which eddy currents produced in copper tube becomes weak. Hence, the opposing force also gets reduced and the terminal velocity of the magnet gets increased.

Hence, both assertion and reason are correct and reason is a correct explanation of the assertion.

Thus, $(A)$ is the correct answer.

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