The electrical device used for converting mechanical energy into electrical energy is called

Correct — D, generator. An electric generator converts mechanical energy into electrical energy through electromagnetic induction. A coil is rotated in a magnetic field, or a magnet is moved past a coil, so that the magnetic flux linked with the coil keeps changing; a changing flux induces an electromotive force, and a current flows if the circuit is closed. The mechanical energy may come from a bicycle pedal driving a dynamo, from steam turning a turbine in a thermal station, or from falling water in a hydroelectric plant, but the conversion is the same in each case.

• (a)voltmeter — A measuring instrument, not a converter. It reads potential difference and is connected across a component in parallel, with a very high resistance so that it draws almost no current.
• (b)ammeter — Also a measuring instrument. It reads current, is connected in series in the circuit, and has a very low resistance so that it disturbs the current as little as possible.
• (c)motor — The exact reverse conversion. A motor turns electrical energy into mechanical energy, using the force experienced by a current-carrying conductor placed in a magnetic field.

Michael Faraday's discovery of electromagnetic induction in 1831 is the basis of the generator. Whenever the magnetic flux through a circuit changes, an electromotive force is induced in it, proportional to the rate at which the flux changes. A generator arranges that change mechanically, by rotation, and so converts the energy of motion into electrical energy.

The four options are best sorted by what each one does to energy. Two of them measure and convert nothing. The remaining two are the same machine run in opposite directions — the motor consumes electrical energy and delivers motion, the generator consumes motion and delivers electrical energy — and their construction is so similar that many machines can serve as either. The distinction between the two kinds of generator is worth knowing as well. An alternating-current generator takes its output through slip rings, so the current reverses every half turn; a direct-current generator replaces those with a split-ring commutator, which flips the connections at the moment the induced emf would otherwise reverse, keeping the output in one direction. Almost all bulk electricity in India is produced as alternating current for this reason of transmission convenience.

• A generator converts mechanical energy into electrical energy by electromagnetic induction; a motor does the reverse.
• Faraday discovered electromagnetic induction in 1831, and the induced emf is proportional to the rate of change of magnetic flux.
• An alternating-current generator uses slip rings; a direct-current generator uses a split-ring commutator.
• A motor works on the force experienced by a current-carrying conductor in a magnetic field.
• A voltmeter is connected in parallel and has high resistance; an ammeter is connected in series and has low resistance.

• Reversing the motor and the generator, since the two machines look so alike.
• Treating a measuring instrument as an energy converter.
• Assuming a generator creates energy rather than converting it from a mechanical source.

Asked as a straight definition-to-device item, with the reverse conversion and two measuring instruments supplied as the wrong options.

• practice — not a real PYQ

  An electric motor converts

  • (a)electrical energy into mechanical energy
  • (b)mechanical energy into electrical energy
  • (c)chemical energy into electrical energy
  • (d)electrical energy into chemical energy

  Answer(a) electrical energy into mechanical energy — the reverse of what a generator does.

• practice — not a real PYQ

  In a direct-current generator, the device used to keep the output in one direction is

  • (a)a pair of slip rings
  • (b)a split-ring commutator
  • (c)a transformer
  • (d)a rheostat

  Answer(b) a split-ring commutator — it reverses the connections just as the induced emf would otherwise change sign.