Which one of the following heat transfer mechanism does NOT require a medium?

Correct — C, Radiation. Conduction passes energy from particle to particle through direct contact, so it needs matter. Convection moves energy by physically carrying warm fluid from one place to another, so it needs a fluid. Radiation is different in kind: energy leaves a warm body as electromagnetic waves, which travel through vacuum at the speed of light. That is why the Sun's energy crosses about 15 crore kilometres of empty space to reach the Earth, and why a thermos flask is silvered and evacuated — the vacuum stops conduction and convection, and the silvering is there to cut the one mechanism a vacuum cannot stop.

• (a)Conduction — Conduction is the transfer of energy through a material by collisions between neighbouring particles, helped in metals by free electrons. Remove the material and there is nothing to collide.
• (b)Convection — Convection depends on a fluid actually moving. Warmed fluid expands, becomes less dense and rises; cooler fluid sinks to replace it. Neither step is possible in vacuum.
• (d)Collision — Collision is not one of the three modes of heat transfer at all. Particle collisions are the microscopic mechanism inside conduction, not a separate mode, and they too require matter.

Heat moves in three ways. Conduction transfers kinetic energy between adjacent particles without net movement of matter, and works best in metals because free electrons carry energy quickly. Convection transfers energy by bulk movement of a heated fluid, and can be natural, driven by density differences, or forced by a fan or pump. Radiation transfers energy as electromagnetic waves, requires no medium, and depends on the fourth power of absolute temperature by the Stefan-Boltzmann law.

The listing of 'collision' as a fourth option is the item's only real difficulty; it sounds physical enough to distract. It is not a recognised mode. Once the list is reduced to the standard three, the question becomes the single fact that separates radiation from the other two, and the Sun is the memory hook — energy that reaches the Earth across near-vacuum can only have arrived by radiation. The same fact explains why an astronaut in space must radiate away body heat, and why spacecraft carry radiators rather than fans.

• The three modes of heat transfer are conduction, convection and radiation.
• Conduction and convection both require a material medium; radiation does not.
• Radiation travels as electromagnetic waves at the speed of light, which is how solar energy reaches the Earth.
• Radiated power rises as the fourth power of absolute temperature, by the Stefan-Boltzmann law.
• A vacuum flask blocks conduction and convection with its evacuated gap and cuts radiation with a silvered surface.

• Treating 'collision' as a legitimate fourth mode because it sounds mechanical.
• Believing radiation needs air, because most everyday radiant heating happens in air.
• Confusing the microwave oven, which heats by absorbed radiation, with conduction from a hot surface.

A single-fact recall item with a manufactured fourth option. The examiner is checking both the property of radiation and whether a plausible non-term can be rejected.

• practice — not a real PYQ

  Heat from the Sun reaches the Earth mainly by which process?

  • (a)Conduction
  • (b)Convection
  • (c)Radiation
  • (d)Diffusion

  Answer(c) Radiation — the space between the Sun and the Earth is very nearly empty.

• practice — not a real PYQ

  Land and sea breezes along a coast are an example of

  • (a)conduction
  • (b)convection
  • (c)radiation
  • (d)refraction

  Answer(b) convection — unequal heating of land and water sets up a circulation of air.