UPPSC 2019 Prelims GS Paper-I · Question 15 of 150

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).

• (a)Conduction — 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)Convection and radiation — 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)None of the above — 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.

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.

The commonest confusion is to credit the vacuum with stopping radiation as well. It cannot — radiation needs no medium, and that is precisely why the silvering exists. Pair each feature with the mode it defeats and the question answers itself: vacuum against conduction and convection, silvering against radiation, insulating stopper and supports against the leftover conduction through the neck. Be honest about the limit too: no flask is perfect, so a thermos keeps a liquid hot for hours, not for ever. The design was devised by James Dewar in 1892 for storing liquefied gases before it became a household object.

• Conduction and convection need a material medium; radiation does not and can cross a vacuum
• The vacuum between the double walls of a thermos removes the medium and so blocks conduction and convection
• The silvered inner and outer wall surfaces reflect radiant heat back, because a shiny surface is a poor emitter and a good reflector
• An insulating stopper and small supports at the neck cut the residual conduction path
• The vacuum flask was devised by James Dewar in 1892 for cryogenic work; it slows heat flow but cannot stop it entirely

• Believing the vacuum also stops radiation — it does not; that is what the silvering is for
• Believing the silvering stops conduction — it does not; the vacuum does
• Forgetting that convection is impossible without a fluid, so it cannot occur in a vacuum
• Assuming a thermos stops heat flow completely rather than merely slowing it

A perennial one-liner in both UPPSC and UPSC science sections — 'why does a thermos work', 'which mode of heat transfer operates in a car radiator / a room heater / sunlight', or matching a device to the mode it exploits.

• practice — not a real PYQ

  The vacuum between the double walls of a thermos flask prevents heat loss chiefly by:

  • (a)Radiation only
  • (b)Conduction and convection
  • (c)Radiation and conduction
  • (d)All three modes equally

  Answer(b) Conduction and convection — both need a material medium, which the vacuum removes. Radiation is checked separately by the silvering on the walls.

• practice — not a real PYQ

  Heat from the Sun reaches the Earth mainly by:

  • (a)Conduction
  • (b)Convection
  • (c)Radiation
  • (d)Conduction and convection together

  Answer(c) Radiation — the space between the Sun and the Earth is very nearly a vacuum, and radiation is the only mode of heat transfer that needs no material medium.