Physics MCQs for NEET — Practice Questions with Answers

If the magnetic field is directed away from the reader and the flux is increasing, the induced current must create a magnetic field directed towards the reader (opposite to the original increasing field) to oppose this increase. By the right-hand thumb rule, a counter-clockwise current produces a magnetic field directed towards the reader. (Example 6.4 (i) in NCERT text illustrates this principle in reverse, where flux increases, so current is clockwise to oppose, by creating field towards reader. If field is into the page and increasing, the induced current would aim to create a field coming out of the page, which is counter-clockwise.) Let's re-evaluate: 'The magnetic flux through the rectangular loop abcd increases, due to the motion of the loop into the region of magnetic field, The induced current must flow along the path bcdab so that it opposes the increasing flux.' For a field 'away from the reader' (into the page) that is increasing, the induced current must create a field 'towards the reader' (out of the page). This is achieved by a counter-clockwise current.

If the induced current supported the change, the system would continuously accelerate without external energy input, creating a perpetual motion machine. This violates the law of conservation of energy. (Refer to NCERT text: 'A gentle push on the magnet will initiate the process and its velocity and kinetic energy will continuously increase without expending any energy. If this can happen, one could construct a perpetual-motion machine by a suitable arrangement. This violates the law of conservation of energy and hence can not happen.')

The work done by the person in moving the magnet is converted into electrical energy, which is then dissipated as heat (Joule heating) in the coil due to the induced current. This ensures the conservation of energy. (Refer to NCERT text: 'This energy is dissipated by Joule heating produced by the induced current.')

The mechanical work done in moving the magnet against the repulsive force is converted into electrical energy in the form of induced current, which then gets dissipated as heat due to the resistance of the coil (Joule heating). (Refer to NCERT text: 'Therefore, a person has to do work in moving the magnet. Where does the energy spent by the person go? This energy is dissipated by Joule heating produced by the induced current.')

The negative sign in Faraday's law of induction is precisely the mathematical representation of Lenz's law, indicating that the induced EMF (and thus current) opposes the change in magnetic flux that produced it. (Refer to NCERT text: 'The negative sign in the expression for Faraday’s law indicates this fact.' and 'The negative sign shown in Eq. (6.3) represents this effect.')

According to Faraday's law, an EMF is induced only when there is a change in magnetic flux. If the loop and magnets are stationary, there is no change in flux, hence no induced current, regardless of the magnet's strength. (Refer to NCERT Example 6.5 (a): 'Can we hope to generate current in the loop by using very strong magnets?' The answer implies no, as it's a stationary loop in a fixed field.)

As the magnet falls towards the ring, the magnetic flux through the ring changes. The induced current will create a magnetic field that opposes this change, meaning it will repel the approaching magnet, slowing its fall. This demonstrates the conservation of energy, as the kinetic energy of the magnet is converted into electrical energy in the ring. (Relates to the conceptual question from the prompt example: 'A copper ring is held horizontally and a bar magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet while it is passing through the ring is').

An induced EMF is generated when there is a change in magnetic flux. A rotating conductor in a magnetic field will typically experience a change in the area vector's orientation with respect to the field, leading to a change in magnetic flux and thus an induced EMF. The other options describe situations where magnetic flux remains constant or there is no magnetic flux change relevant to induction. (Refer to Faraday's law: $$\varepsilon = -\frac{d\Phi_B}{dt}$$, implying a change in flux is necessary.)

If the induced current assisted the motion, the system would gain kinetic energy without any external work input, leading to a perpetual motion machine, which violates the law of conservation of energy. (Refer to NCERT text: 'If this can happen, one could construct a perpetual-motion machine by a suitable arrangement. This violates the law of conservation of energy and hence can not happen.')

Even with motional EMF, the principle of Lenz's Law applies. The induced current in the moving rod will create a magnetic force that opposes the motion, thus requiring work to maintain the velocity, ensuring conservation of energy. If the force aided the motion, it would be a perpetual motion scenario. (Conceptual application of Lenz's law and energy conservation to motional EMF).