NDA (I) 2018 General Ability Test · Question 51 of 150

Correct — A, Any energy transfer that does not involve temperature difference in some way is not heat. This is the definition of heat restated as a test. In physics heat is not a substance a body contains; it is energy on the move, and it moves only because one place is hotter than another. Take the temperature difference away and whatever energy is being shifted has to be called something else — work, usually. So the statement is exactly right: no temperature difference in the picture, no heat.

• (b)Any energy transfer always requires a temperature difference — This turns a statement about heat into a statement about every energy transfer, and that is false. Rub your palms together, stir water with a paddle, compress a gas with a piston, or run a current through a wire — each of these moves energy into a body with no temperature difference driving it. That mode is work, and it is the second of the two ways energy crosses a boundary.
• (c)On heating the length and volume of the object remain exactly the same — Almost every material expands when heated, because the atoms vibrate more strongly and sit further apart on average. That is why railway lines are laid with gaps, why bridges carry expansion joints, and why a metal lid loosens under hot water.
• (d)Whenever there is a temperature difference, heat is the only way of energy transfer — A temperature difference does not shut off the other route. A hot gas expanding against a piston is losing energy as heat to the cooler surroundings and doing work on the piston at the same time — that is the everyday situation inside an engine, and both transfers run together.

Heat is energy transferred between a system and its surroundings because of a temperature difference between them. It is not stored in a body — what a body stores is internal energy, the total of the kinetic and potential energies of its particles. Energy can enter or leave that store in exactly two ways, as heat when a temperature difference drives it, and as work when a force acts through a distance. The first law of thermodynamics is the bookkeeping between the three, written as the change in internal energy equals the heat supplied minus the work done by the system.

The item is testing whether a candidate can hold the line between 'heat' and 'energy'. In ordinary speech the two blur together, and every wrong option here trades on that blur. The safe habit is to ask, for any transfer, what is driving it. If the answer is a temperature difference, it is heat. If the answer is a force pushing something through a distance, it is work. Option (a) says only this and nothing more, which is why it survives; the other three all overreach, either by making the temperature difference necessary for every transfer or by making it sufficient to rule out work.

• Heat is energy in transit driven by a temperature difference; a body stores internal energy, not heat.
• Work is the other mode of energy transfer, and it needs no temperature difference at all.
• The first law of thermodynamics states that the change in internal energy equals the heat supplied to the system minus the work done by it.
• Nearly all solids and liquids expand on heating, which is why expansion gaps are built into rails and bridges.

• Speaking of the 'heat contained in a body' — a body contains internal energy, and heat is only the transfer.
• Assuming that a temperature difference rules out work happening at the same time.
• Forgetting that friction and electrical work raise a body's temperature without any hotter body being involved.

NDA sets this as a four-statement item on the definition of heat, or as a first-law question in which the work term is deliberately made zero.

• practice — not a real PYQ

  Energy transferred to a body by rubbing two surfaces together is best described as

  • (a)heat, because the body gets hotter
  • (b)work, because a force acts through a distance
  • (c)neither heat nor work
  • (d)radiation

  Answer(b) work, because a force acts through a distance — no temperature difference drives the transfer, even though the temperature rises afterwards.

• practice — not a real PYQ

  A gas absorbs 200 J of heat and does 80 J of work on its surroundings. The increase in its internal energy is

  • (a)280 J
  • (b)200 J
  • (c)120 J
  • (d)80 J

  Answer(c) 120 J — by the first law, the change in internal energy is the heat supplied minus the work done by the gas, or 200 minus 80.