CDS (II) 2024 General Knowledge · Question 22 of 120

Correct — B, Boiling water in a pot on a gas stove. Convection is heat carried by the bulk movement of a fluid — the heated material itself moves, taking its energy with it. That is exactly what happens in a pot on a flame. The water at the bottom is warmed first, expands, becomes less dense than the water above it and rises; cooler, denser water sinks to take its place, is warmed in turn, and the cycle sets up a circulating current that carries heat through the whole pot. Anyone who has watched the swirl in a pan of heating water has watched convection. Each of the other three options is a different mode. A brass rod carries heat along its length without any part of the metal moving, which is conduction. Warmth felt from the Sun crosses a hundred and fifty million kilometres of empty space, where there is nothing to conduct or to circulate, so it must be radiation. And a microwave oven does not heat the food from outside at all: its waves are absorbed by the water molecules within the food, so energy is delivered by absorbed radiation rather than carried in by a moving fluid.

• (a)Heating of food in a microwave oven — The oven works by electromagnetic waves that the water in the food absorbs directly, so the heating happens inside the food rather than being carried to it by a fluid. That makes it a radiation process, not convection.
• (c)Feeling the warmth in sun — Sunlight reaches the Earth across empty space, and neither conduction nor convection can operate without a material medium. Heat that crosses a vacuum can only be radiation.
• (d)Heating a brass rod at one end and observing the temperature rise at the other end — This is the textbook demonstration of conduction. Energy is passed along from particle to particle through the solid while the metal itself stays where it is, which is the definition of conduction and the opposite of convection.

Heat moves in three ways. In conduction, energy passes from particle to particle through a material while the material itself stays put, which is how a metal spoon in hot tea grows warm. In convection, the heated fluid itself moves and carries its energy along, which needs a liquid or a gas. In radiation, energy travels as electromagnetic waves and needs no medium at all, which is why it alone can cross the vacuum between the Sun and the Earth.

Every item of this type is settled by one question — does anything material move, and if so, is it the heated substance itself? If nothing moves, it is conduction; if the heated fluid moves in bulk, it is convection; if there is no medium at all, it is radiation. The point to be careful with is that real situations mix the modes, and this item's own answer is a good example: the flame heats the pot by radiation and the pot heats the water touching it by conduction, but the transfer through the body of the water — which is what the item is pointing at — is convection. The same three-way analysis explains a great deal of everyday physics: sea and land breezes, the circulation of the atmosphere, why a room's ceiling is warmer than its floor, and why a vacuum flask has to defeat all three modes with a vacuum gap and a silvered surface.

• Convection is heat transfer by the bulk movement of a heated fluid, and so cannot occur in a solid.
• Conduction passes energy from particle to particle without the material itself moving.
• Radiation needs no medium and is the only mode that can cross a vacuum.
• In a pot of water, the heated water expands, becomes less dense and rises, while cooler water sinks to replace it, setting up a convection current.
• A microwave oven heats food by electromagnetic waves that its water molecules absorb, so the heating begins within the food.

• Calling a boiling pot conduction because a flame touches the vessel; the transfer through the water is convection.
• Treating microwave heating as ordinary cooking from outside; the waves deposit energy within the food.
• Forgetting that convection is impossible in a solid, since nothing in a solid can flow.

Usually as an identify-the-mode item on everyday examples, or as a NOT-correct statement on the definitions; sea breezes and the vacuum flask are the standard applications.

• 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 only radiation can cross it.

• practice — not a real PYQ

  Convection cannot take place in a solid because

  • (a)solids do not absorb heat
  • (b)the particles of a solid cannot move about in bulk
  • (c)solids radiate all the heat they receive
  • (d)solids have no free electrons

  Answer(b) the particles of a solid cannot move about in bulk — convection needs the heated material itself to flow, which only a liquid or a gas can do.