Solar energy is converted into electric energy through

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UnlockIAS · Accepted Answer

Correct — B, Photo voltaic cells. A photovoltaic cell is a semiconductor device, almost always built on silicon, in which two layers of differently doped material meet at a p–n junction. Sunlight arriving at the cell is a stream of photons; a photon whose energy exceeds the semiconductor's band gap is absorbed and knocks an electron loose, leaving behind a positively charged hole. The built-in electric field across the junction sweeps electrons one way and holes the other, so charge accumulates on the two faces of the cell and, when an external circuit is connected, a current flows. That is a direct conversion of light into electricity — the photovoltaic effect, first observed by Edmond Becquerel in 1839 — with no combustion, no working fluid, no heat engine and no moving parts. The current produced is direct current, which is why a rooftop system needs an inverter before it can feed household appliances or the grid. The word itself decodes the answer: 'photo' for light and 'voltaic' for electricity.

Why the other options are wrong — (a) An electrochemical cell, not a light-driven one. The Leclanché cell, devised by Georges Leclanché in the 1860s, uses a zinc anode, a manganese dioxide cathode and an ammonium chloride electrolyte, and converts chemical energy into electrical energy. It is the direct ancestor of the modern zinc–carbon torch battery. Sunlight plays no part in its operation.  (c) The sealed, paste-electrolyte descendant of the Leclanché cell — the ordinary battery used in a torch, clock or remote control. Its energy comes from a chemical reaction between the electrodes and the electrolyte, and it runs down as the reactants are consumed, whether it sits in sunlight or in a drawer.  (d) The generic name for a galvanic cell, the class that began with Volta's pile of 1800: two dissimilar metals in an electrolyte, generating current from a spontaneous redox reaction. Again the conversion is chemical to electrical. Notice the structure of this option set — three of the four choices are variants of the same electrochemical family, and only one is a light-driven device, so the item is answered by spotting the odd one out.

Concept: There are two quite different ways to make electricity from sunlight, and examiners test the distinction constantly. Photovoltaics converts light directly into electricity in a semiconductor junction: sunlight in, direct current out, with efficiency falling as the cell heats up and output vanishing the moment a cloud passes. Solar thermal, or concentrating solar power, does it indirectly: mirrors or troughs focus sunlight to heat a fluid, the heat raises steam, and the steam drives a conventional turbine and generator — so the electricity is produced by a heat engine exactly as in a coal plant, and the heat can be stored in molten salt to keep generating after sunset. Photovoltaics dominates in practice because modules are modular, need no water and can be mounted on a roof, a canal or a field. The same photovoltaic and photoelectric physics also underlies light-sensing devices such as the photo cell in a smoke detector or an automatic street light, where the point is not power but the electrical signal that light produces.

State PCS 2022 Q67 — Science & Technology PYQ with Answer

State PCS 2022 Q67 — Science & Technology PYQ with Answer