An AC current can be produced by

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Correct — B, dynamo. Of the three devices named, only one is a source of electrical energy; the other two act on electricity that is already flowing. A dynamo is a generator, and a generator converts mechanical energy — a hand crank, a bicycle wheel, a falling column of water, a steam turbine — into electrical energy through Michael Faraday's law of electromagnetic induction, which he demonstrated on 29 August 1831. Spin a coil of N turns and area A at angular speed ω in a uniform magnetic field B, and the flux threading it is Φ = NBA cos ωt. The induced emf is the rate at which that flux changes, e = −dΦ/dt = NBAω sin ωt, and that is a sine wave. It peaks when the plane of the coil lies along the field, falls to zero twice per rotation when the plane is perpendicular to it, and — this is the whole point of the question — reverses sign every half turn, because the arm of the coil that was cutting upward through the field is now cutting downward through it. Alternating current is therefore not something bolted on to a rotating generator; it is what the geometry produces by itself. What decides whether the machine hands AC or DC to the outside world is only the pair of contacts on the shaft: two continuous slip rings pass the sine wave out unchanged as AC, whereas a split-ring commutator reverses the external connection at the instant the emf reverses and folds the negative half-cycles upward into DC. Every thermal, hydro, nuclear and wind station feeding the Indian grid is a machine of exactly this kind, which is why household supply in India alternates at 50 hertz — the current reverses direction every 1/100 of a second — at a nominal 230 volts. Neither a choke coil nor a transformer can do any part of this: disconnect the supply and their output is zero, while a dynamo needs nothing but a shaft that turns.

Why the other options are wrong — (a) A choke is an inductor — many turns of insulated copper wire, usually wound on a laminated soft-iron core — placed in series in an AC circuit to hold the current down. Its opposition is inductive reactance, X_L = 2πfL, which rises with frequency, and because the current through a nearly pure inductor lags the voltage by close to 90°, the average power it absorbs, VI cos φ, is almost nil. That is exactly why a fluorescent tube uses a choke and not a resistor as its ballast: it limits the current without wasting it as heat, a point UPSC turned into a full question in 2000. Limiting a current is not making one. Put a choke across a steady DC supply, where f = 0, and its reactance vanishes; it behaves as an ordinary length of low-resistance wire.  (c) The genuinely tempting option, because a transformer's secondary winding really does carry an alternating current, so it can look like a producer. It is a converter. Two coils sit on one laminated core and are coupled only by mutual induction: the alternating primary current sets up an alternating flux, and it is the changing of that flux that induces the secondary emf, in the ideal ratio Vs/Vp = Ns/Np. Every link in that chain requires change. Feed a transformer steady direct current and the flux is constant, so no emf appears across the secondary at all and the primary — which has very little resistance — simply overheats; UPSC built an assertion-reason item on precisely this in 1996. A transformer also contributes no energy of its own, since ideally power out equals power in. It changes the voltage of an alternating current; it does not originate one.  (d) This would be right only if none of the three devices could generate an alternating current, and one of them plainly can. The option is a trap for the candidate who correctly works out that a choke coil and a transformer are both passive, and then over-generalises that conclusion across the whole list without pausing to ask what a dynamo actually is. On this paper 'None of the above' is the fourth option on a great many items, so treating it as a hint that the first three are all wrong is an expensive habit.

Concept: Electromagnetic induction is the single principle behind almost all electricity generation on Earth. Faraday's law states that an emf is induced in a circuit whenever the magnetic flux through it changes, with magnitude equal to the rate of change of flux, e = −dΦ/dt; Lenz's law supplies the minus sign, meaning the induced current always opposes the change that produced it, which is the reason a generator becomes harder to turn the moment a load is switched on. Three families of machine are built on this one law and are constantly confused with each other. A generator (dynamo or alternator) turns mechanical energy into electrical energy by moving a conductor relative to a field. A motor is the same machine run backwards, turning electrical energy into mechanical energy. A transformer moves no parts at all: it uses the mutual induction between two stationary coils to change the voltage of an alternating supply, and works only because AC is inherently changing. To these is usually added the choke coil, a pure inductor used not to convert energy but to limit current in an AC circuit. Grasping which of the four is a source and which are merely handlers of an existing supply settles most one-line questions in this chapter.

State PCS 2023 Q26 — Science & Technology PYQ with Answer

State PCS 2023 Q26 — Science & Technology PYQ with Answer