A liquid remains hot or cold for a long time in thermos flask because there is no loss or gain of heat by

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Correct — C, Both (a) and (b) — that is, a thermos checks ALL THREE modes of heat transfer: conduction, convection and radiation. A thermos (vacuum or Dewar flask) is a double-walled glass vessel with the air pumped out of the space between the two walls. Conduction and convection both need a material medium — conduction passes energy from molecule to molecule, convection needs a fluid that can physically circulate — so with a vacuum in the gap there is almost nothing left to carry heat either way. Radiation is different: it travels as electromagnetic (mainly infrared) waves and needs no medium at all, so the vacuum cannot stop it. That is why both surfaces facing the vacuum gap are SILVERED: a shiny silvered surface is a poor emitter and an excellent reflector, so radiant heat is bounced back into the hot liquid, or reflected away from a cold one. The last remaining path is the neck, which is closed by a cork or plastic stopper and rests on small insulating supports — both poor conductors. Since option (a) covers conduction and option (b) covers convection and radiation, the option that names all three is (c).

Why the other options are wrong — (a) True but incomplete. The vacuum does stop conduction, but conduction is only one of the three routes heat can take; a flask that blocked conduction alone would still lose heat by radiation. Since option (c) is available, naming conduction alone is a partial answer.  (b) Also true but incomplete. The vacuum stops convection and the silvering stops radiation — but conduction is blocked just as deliberately by the same vacuum, so leaving it out understates what the flask does.  (d) Wrong, because options (a) and (b) together name exactly the three modes the flask is designed to defeat. The correct choice, (c), is on the list.

Concept: Heat moves in three ways. Conduction transfers energy through a material by molecular vibration and, in metals, by free electrons, with no bulk movement of matter — it needs a medium and works best in solids. Convection transfers heat by the bulk movement of a heated fluid that becomes less dense and rises — it needs a liquid or a gas. Radiation transfers heat as electromagnetic waves, chiefly infrared, and is the only mode that crosses a vacuum, which is how the Sun's heat reaches the Earth. A vacuum flask attacks each mode with a different design feature: vacuum for the first two, a reflecting silvered surface for the third.

State PCS 2019 Q15 — Science & Technology PYQ with Answer

State PCS 2019 Q15 — Science & Technology PYQ with Answer