In experiment #1, a bar magnet is moved towards a conducting wire loop axially, with the magnet’s north pole facing the loop. In experiment #2, the same process as in experiment #1 is repeated except that the south pole of the magnet faces the loop. Which one of the following statements is true in this context?

Correct — A, The direction of current in the loop will be of opposite nature in both the experiments. Lenz's law fixes the direction: the induced current always flows so as to oppose the change that produced it. When the north pole approaches, the flux through the loop grows in one sense, so the induced current circulates to present a north face to the incoming magnet and push it away. Swap the magnet round and the approaching south pole makes the flux grow in the opposite sense, so the loop must now present a south face — and to do that the current has to reverse. The two experiments therefore give currents of opposite sense. Everything else about them is the same, so the magnitude of the induced current is unchanged.

• (b)The direction of current in the loop will be the same in both the experiments. — This would require the loop to present the same magnetic face to an approaching north pole and to an approaching south pole, which cannot repel both. Reversing the magnet reverses the sign of the changing flux and so reverses the induced current.
• (c)No current will flow in either of the two experiments. — The magnet is moving, so the flux linked with the loop is changing, and by Faraday's law a changing flux induces an emf in a closed conducting loop. Current would fail to flow only if the magnet were held still or the loop were broken.
• (d)More current will flow in the loop in experiment #1. — The magnitude of the induced emf depends on the rate of change of flux — the strength of the magnet, the speed of approach, the area and the number of turns. Which pole leads affects only the sign, not the size.

Faraday's law says the induced emf equals the negative rate of change of magnetic flux, ε = −dΦ/dt. The minus sign is Lenz's law, and it is a statement of energy conservation: if the induced current helped the change along instead of opposing it, a small push would produce ever-growing current and energy from nowhere. Opposition is what makes you do work against a repulsive force as you push the magnet in, and that work is what appears as electrical energy in the loop.

Reason it out with the faces of the loop. In experiment 1 a north pole approaches; to oppose the increase in flux the loop makes the near face a north pole too, so like poles repel and the current, seen from the magnet, runs anticlockwise. In experiment 2 a south pole approaches; the near face must become a south pole, and the current runs clockwise. Same physics, opposite sign. Two related cases are worth practising alongside this one. If the magnet is withdrawn instead of advanced, the flux is falling, so the loop attracts the departing magnet and the current reverses again — meaning that approach with a north pole and withdrawal with a south pole give the same sense of current. And if the loop is cut so that it is no longer a closed circuit, an emf is still induced but no current flows.

• Faraday's law: the induced emf equals minus the rate of change of magnetic flux through the circuit.
• Lenz's law: the induced current opposes the change producing it, and expresses conservation of energy.
• A north pole approaching makes the near face of the loop a north pole; a south pole approaching makes it a south pole — hence opposite currents.
• The magnitude of the induced emf depends on how fast the flux changes, not on which pole leads.
• Withdrawing a magnet reverses the induced current relative to advancing the same pole; a broken loop has an induced emf but no current.

• Confusing Fleming's left-hand rule (motor, force) with the right-hand rule (generator, induced current).
• Believing a stronger effect follows from the north pole simply because it is named first.
• Forgetting that an induced emf exists even when no current flows, as in an open loop.

Asked as a paired-experiment comparison where the only variable changed is which pole faces the loop, so the answer turns purely on the sign fixed by Lenz's law.

• practice — not a real PYQ

  A bar magnet is pulled away from a closed conducting loop along its axis with the north pole facing the loop. The induced current in the loop will

  • (a)flow so as to attract the magnet
  • (b)flow so as to repel the magnet
  • (c)not flow at all
  • (d)flow only while the magnet is stationary

  Answer(a) flow so as to attract the magnet — the flux is falling, so by Lenz's law the loop opposes the fall and tries to hold the magnet back.

• practice — not a real PYQ

  The negative sign in Faraday's law of electromagnetic induction is an expression of

  • (a)Ohm's law
  • (b)conservation of charge
  • (c)conservation of energy
  • (d)Coulomb's law

  Answer(c) conservation of energy — it encodes Lenz's law, without which induction would create energy from nothing.