Heat transfer that does not require a medium is called

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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 — 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 — 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.

Concept: 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.

State PCS 2024 Q12 — Science & Technology PYQ with Answer

State PCS 2024 Q12 — Science & Technology PYQ with Answer