The following Assertion-Reason Type MCQ which appeared in IIT-JEE 2007 question paper is meant for gauging your grasp of Lenz’s law. Here is the question:
A vertical iron rod has a coil of wire wound over it at the bottom end. The rod goes through a conducting ring as shown in the figure. The ring can float at a certain height above the coil
because
Statement-2
In the above situation, a current is induced in the ring which interacts with the horizontal component of the magnetic field to produce an average force in the upward direction.
(a) Statement-1 is True, Statement-2 is True; Statement-2 is a correct
explanation for Statement-1
(b) Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct
explanation for Statement-1
(c) Statement-1 is True, Statement-2 is False
(d) Statement-1 is False, Statement-2 is True
Statement-1 is true. But statement-2 false[Option (c)].
When the magnetic flux linked with the ring increases because of the increase of current in the coil, the ring will be repelled in accordance with Lenz’s law, so as to oppose the increase of flux through the ring. When the current through the coil decreases, the flux linked with the ring decreases. In this case, the ring will be attracted towards the coil, so as to oppose the decrease of flux through the ring. Since the coil is fed with an alternating current, these actions are repeated successively and the ring can therefore float above the coil.
The ring floats because of Lenz’s law. Basically it is the result of the interaction of the magnetic field produced by the induced current with the magnetic field produced by the coil, but the magnetic field produced by the coil is along the vertical (through the iron rod) and is not horizontal. The vertical field produces the flux and the change of flux through the ring and this is responsible for the floating of the ring.
Statement-2 is therefore false.
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