NDA (II) 2022 General Ability Test · Question 100 of 150

Correct — C, Faraday's laws of electromagnetic induction. A generator converts mechanical energy into electrical energy by rotating a coil in a magnetic field. As the coil turns, the magnetic flux linked with it changes continuously, and Faraday's laws say that a changing flux induces an emf whose size equals the rate at which the flux changes. In the DC version a split-ring commutator swaps the coil's two connections to the external circuit twice in every rotation, so the current delivered outside always flows the same way.

• (a)Ohm's law — Ohm's law, V = IR, relates the current in a conductor to the potential difference across it. It describes how an emf that already exists drives a current; it says nothing about where that emf comes from.
• (b)Joule's law of heating — Joule's law gives the heat produced by a current, H = I²Rt. It is the working principle of electric heaters, irons and filament bulbs — devices that consume electrical energy rather than generate it.
• (d)None of the above — This would only be right if option (c) were wrong, and it is not. Electromagnetic induction is precisely the principle every generator, DC or AC, runs on.

Michael Faraday's laws of electromagnetic induction state that an emf is induced in a circuit whenever the magnetic flux through it changes, and that the induced emf is proportional to the rate of that change. A generator is that law turned into a machine: mechanical work spins a coil in a magnetic field and comes out as electrical energy. A motor is the same machine run the other way, using the force experienced by a current-carrying conductor placed in a magnetic field.

All three named laws in the options are real laws of electricity, which is what gives the item its bite — the question is which law creates the emf, not which law describes what happens to a current afterwards. Ohm's law and Joule's law both begin with a current that already exists. Only induction explains its origin. It also helps to know that a DC generator and an AC generator differ only in how the output is collected, not in principle: the DC machine uses a split-ring commutator and the AC machine uses two slip rings, but both are Faraday's law in hardware.

• By Faraday's first law, a changing magnetic flux through a circuit induces an emf in that circuit.
• By Faraday's second law, the induced emf equals the rate of change of magnetic flux linkage.
• A generator converts mechanical energy into electrical energy; an electric motor does the reverse.
• A DC generator uses a split-ring commutator, while an AC generator uses slip rings.
• Lenz's law fixes the direction of the induced current — it always opposes the change that produced it.

• Picking Ohm's law simply because it is the most familiar electrical law.
• Assuming DC and AC generators work on different principles — only the collecting arrangement differs.
• Confusing the generator, which runs on induction, with the motor, which runs on the force on a current-carrying conductor.

As direct recall of the principle, or through the paired distinction — which rule applies to a motor and which to a generator.

• practice — not a real PYQ

  The split-ring commutator in a DC generator serves to

  • (a)increase the magnitude of the induced emf
  • (b)keep the current in the external circuit flowing in one direction
  • (c)reduce friction between the coil and the magnets
  • (d)convert electrical energy into mechanical energy

  Answer(b) keep the current in the external circuit flowing in one direction — it reverses the coil's connections twice per rotation.

• practice — not a real PYQ

  Which one of the following devices converts mechanical energy into electrical energy?

  • (a)Electric motor
  • (b)Transformer
  • (c)Generator
  • (d)Ammeter

  Answer(c) Generator — it uses electromagnetic induction; the motor does the reverse conversion.