The North pole of a bar magnet faces a coil. The magnet is moved along the axis of the coil. Consider the following statements:
(A) As the North pole moves towards a closed coil, the face of the coil near the magnet behaves as a North pole.
(B) If the coil is an open circuit, no emf is induced across its open ends.
(C) If the induced current flowed the other way, a gentle push would make the approaching magnet keep speeding up without any energy being supplied.
(D) When the North pole is pulled away from a closed coil, the coil repels the magnet.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
By Lenz’s law, the induced current always opposes the change in flux that produces it. So the coil pushes back an approaching pole and pulls back a receding pole. This rule follows from the conservation of energy.
(A) As the North pole moves towards a closed coil, the face of the coil near the magnet behaves as a North pole. — True. The flux through the coil increases. To oppose this increase, the coil shows a North pole to the approaching North pole and repels it.
(B) If the coil is an open circuit, no emf is induced across its open ends. — False. The flux through it still changes, so an emf is induced across the open ends. Only the current is absent, because the circuit is not complete.
(C) If the induced current flowed the other way, a gentle push would make the approaching magnet keep speeding up without any energy being supplied. — True. A South pole of the coil would then face the approaching North pole and attract it. The magnet would keep gaining speed and kinetic energy on its own. That would be a perpetual-motion machine, one that gives energy for nothing. It breaks the law of conservation of energy, so the current never flows this way.
(D) When the North pole is pulled away from a closed coil, the coil repels the magnet. — False. Now the flux decreases. The induced current flows the other way, and a South pole of the coil faces the receding North pole. The force is attractive, and it opposes the motion of the magnet.
NCERT (2026-27):
6.5 Lenz’s Law and Conservation of Energy – “As the North-pole of the bar magnet moves towards the coil, the magnetic flux through the coil increases. … Note that magnetic moment associated with this current has North polarity towards the North-pole of the approaching magnet.”
6.5 Lenz’s Law and Conservation of Energy – “What will happen if an open circuit is used in place of the closed loop in the above example? In this case too, an emf is induced across the open ends of the circuit.”
6.5 Lenz’s Law and Conservation of Energy – “… In that case, the South-pole due to the induced current will face the approaching North-pole of the magnet. The bar magnet will then be attracted towards the coil at an ever increasing acceleration. A gentle push on the magnet will initiate the process and its velocity and kinetic energy will continuously increase without expending any energy.”
6.5 Lenz’s Law and Conservation of Energy – “Similarly, if the North-pole of the magnet is being withdrawn from the coil, the magnetic flux through the coil will decrease. … This would result in an attractive force which opposes the motion of the magnet and the corresponding decrease in flux.”