BPSC 70th CCE Prelims General Studies · Question 12 of 143

Correct — B, Radiation. Heat moves in exactly three ways, and the stem's phrase 'does not require a medium' is the definition of one of them. Conduction passes energy from molecule to molecule through a material in contact, with no bulk movement of the material itself, so it needs matter to pass through. Convection carries energy by physically moving heated fluid — warm fluid expands, becomes less dense, rises, and cooler fluid flows in behind it — so it needs a liquid or a gas that can flow. Radiation is different in kind from both: it is energy carried by electromagnetic waves, mostly in the infrared band, and electromagnetic waves need nothing to travel through. They cross empty space at the speed of light. The everyday proof is overwhelming and sits directly overhead. Roughly 150 million kilometres of near-vacuum separate the Sun from the Earth, and yet virtually all the energy that drives the Earth's weather, its climate, its winds and ocean currents and every food chain on it crosses that gap in about eight minutes and twenty seconds. Neither conduction nor convection could deliver a single joule across it, because there is essentially nothing in between to conduct or to circulate. Radiation also operates universally rather than only at high temperatures: every object above absolute zero emits it, with the power radiated rising as the fourth power of absolute temperature, the relationship stated by the Stefan-Boltzmann law. That is why a hot object cools even inside a vacuum chamber, and why the Earth loses heat to space at night.

• (a)Reflection — Reflection — not a mode of heat transfer at all, which is what makes it the odd item on this list rather than a near-miss. Reflection is what happens to radiation when it meets a surface and bounces back; it redirects energy instead of transporting it from a hotter body to a cooler one. It is genuinely used in heat technology — the silvered walls of a vacuum flask and the mirrored dish of a solar cooker both exploit it — but as a mechanism of transfer it does not exist, and it is here to catch candidates who see the word 'radiation' in their heads and reach for a related optical term.
• (c)Convection — Convection — the mode that demands the most matter of the three, since it works by physically moving the medium itself. It needs a fluid, meaning a liquid or a gas, and cannot occur in a solid or in a vacuum. Almost every large-scale movement of heat on Earth is convective: land and sea breezes, monsoon circulation, the boiling of water in a pan, the circulation of coolant in a car engine. A candidate who reads 'convection' and thinks of open air can slip into believing it works without a medium, but air is the medium.
• (d)Conduction — Conduction — the most tempting wrong answer for anyone who half-remembers that one of the three modes is special, because conduction is the mode people are asked about most often. It needs a medium and, more than that, needs direct contact: energy passes molecule to molecule, and in metals is carried by free electrons, which is why metals conduct far better than wood or air. A steel spoon left in hot tea becomes hot along its length by conduction. In a vacuum there is nothing to conduct through, so it is disqualified for exactly the reason radiation is not.

The three modes of heat transfer are separated by what they need in order to work, and that single question decides most examination items on the topic. Conduction needs a medium and works best in solids, especially metals, because free electrons carry energy quickly; gases are the poorest conductors, which is why still air trapped in wool, foam or a double-glazed window insulates so well. Convection needs a fluid medium that can move in bulk, and is therefore impossible in solids. Radiation needs no medium and works equally well through air and through vacuum. The vacuum flask is the classroom demonstration that ties all three together, because it is a device built to defeat each one in turn: two glass walls with the air pumped out between them remove the matter that conduction and convection would need, the facing surfaces are silvered so that radiation is reflected back into the vessel instead of escaping, and the stopper and the narrow supports are made of poor conductors so that little heat leaks along the solid path that remains. Understand why a thermos has each of its features and you have understood the whole topic.

Read the stem as a definition rather than as a question. 'Heat transfer that does not require a medium' is the standard textbook definition of radiation, so the fastest route is simple recognition. If recognition fails, eliminate. Reflection is not one of the three modes at all and can be struck out on that ground alone. Conduction and convection are the two modes that are defined by their medium — conduction through matter in contact, convection by matter in motion — leaving radiation as the only survivor. The reasoning check that never fails is the Sun. Ask which of the four could carry heat across 150 million kilometres of empty space, and only one answer survives. It is worth carrying the same test in reverse, because BPSC and UPSC both like to ask it that way: heat felt from a fire on your face is radiation, heat felt through the handle of the pan is conduction, and the room warming up overall is convection. One fire, three modes, and the discriminator each time is whether matter had to be present and whether it had to move.

• The three modes: conduction requires a medium and direct contact with no bulk movement, convection requires a fluid medium that physically moves, and radiation requires no medium at all and travels as electromagnetic waves at about 3 x 10^8 metres per second.
• Solar energy crosses roughly 150 million kilometres of near-vacuum to reach the Earth, taking about eight minutes and twenty seconds — a journey only radiation can make.
• Every body above absolute zero radiates. The Stefan-Boltzmann law makes the power radiated proportional to the fourth power of the absolute temperature, so a modest rise in temperature causes a steep rise in emission.
• Wien's displacement law shifts the peak wavelength shorter as a body gets hotter: the Sun radiates mostly in visible and near-infrared light, while the far cooler Earth re-radiates in the far infrared — and it is that outgoing longwave infrared that greenhouse gases absorb.
• A vacuum flask blocks all three modes at once: the evacuated space between its double walls removes the matter needed for conduction and convection, and the silvered surfaces reflect radiation back.
• Metals conduct best because free electrons carry the energy; trapped still air is among the poorest conductors, which is why wool, foam and double glazing insulate.

• Treating reflection as a fourth mode of heat transfer. It is a behaviour of radiation at a surface, not a way of moving heat from a hot body to a cold one.
• Assuming that because convection happens in open air it needs no medium. Air is the medium, and in a vacuum convection stops completely.
• Confusing the two modes that do need matter. Conduction passes heat through a material that stays put; convection carries heat by moving the material itself.

BPSC asks this as a one-line definitional recall straight from the school science syllabus, and builds the option list by adding one word that is not a mode of heat transfer at all — here 'reflection' — alongside the two genuine modes, so a candidate who knows only that there are three modes can still be caught. UPSC almost never asks the definition. It asks the mode to be identified inside a real system instead — the principle on which a car radiator works, or why a cloudy night is warmer than a clear one — so the same three ideas arrive as an application question with a device or a weather observation attached.

• practice — not a real PYQ

  Heat from the Sun reaches the Earth mainly by the process of

  • (a)conduction
  • (b)convection
  • (c)radiation
  • (d)conduction followed by convection

  Answer(c) radiation — space between the Sun and the Earth is very nearly a vacuum, and only radiation crosses a vacuum. Conduction and convection both require matter.

• practice — not a real PYQ

  The vacuum between the two walls of a Thermos flask serves mainly to prevent heat loss by

  • (a)radiation only
  • (b)conduction and convection
  • (c)reflection
  • (d)evaporation

  Answer(b) conduction and convection — both need matter, and the vacuum removes it. Radiation still crosses a vacuum, which is why the flask's walls are separately silvered to reflect it back.