Question

ASSERTION: Eddy current brakes use the drag force created by eddy currents as a brake to slow or stop moving objects.
REASON: Mechanical energy is converted to resistive heat energy by the principle of energy conservation.
A. Both Assertion and Reason are correct and reason is the correct explanation for assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

Hint: Firstly, recall the concept of eddy currents and think of its various applications. Also, see whether the concept of eddy currents can be applied in brakes. If such an application is possible, understand the core mechanism of that application and thus find an explanation for how the moving object slows down.

Complete answer:
Firstly, let us understand what eddy current is. Induced currents, whose flow pattern resemble swirling eddies in water are produced when bulk pieces of conductors are subjected to changing magnetic flux. These are eddy currents. It is also an example of Lenz’s law.
Lenz’s law, if you may recall, states that, the polarity of induced emf is such that it tends to produce a current that opposes the change in magnetic flux that produced it. Lenz’s law is based on the law of conservation of energy.
Here in case of eddy currents, they flow in a particular way as if they are trying to oppose whatever has caused them. It is due to this property that it is used in brakes of vehicles, engines and other machines.
Let us consider a very simple example to understand the concept of eddy current.
A coin shaped magnet when dropped in a copper pipe takes much longer time than when it is dropped in a plastic pipe. By now we know that eddy currents are the reason for that. As the magnet falls there is a magnetic field moving through the conductor which in turn produces electric currents (eddy currents). By laws of electromagnetism, we know that eddy currents further produce a magnetic field of their own that opposes its cause (Lenz’s Law) that is the falling magnet and hence the magnet is slowed down. In other words, we could say that eddy currents are producing a braking effect on the falling magnet. The kinetic energy of the magnet lost by being slowed down is getting converted into heat energy under law of conservation of energy.
That was a very simplified explanation of how eddy current brakes work. The exact same mechanism happens for eddy current brakes in trains and other vehicles.
From the above explanation, it is clear that Eddy current brakes use the drag force created by eddy currents as a brake to slow or stop moving objects. And also, Mechanical energy is converted to resistive heat energy by the principle of energy conservation and that explains where the kinetic energy lost by slowing down goes.

Hence, the answer to the question is option A.

Note:
Eddy current brakes have both advantages as well as disadvantages. Advantages are that they are quiet, frictionless, wear free and require very little maintenance. They are considered undesirable because they heat up the core and dissipate useless heat energy and thus it is not an energy-efficient brake.