NDA (II) 2024 General Ability Test · Question 94 of 141

Correct — C, 'The transfer of energy due to temperature difference.' Heat is energy in transit: it flows from a body at higher temperature to one at lower temperature purely because of the temperature gap between them, and stops once they reach thermal equilibrium (equal temperature). It is not the energy a body 'contains' (that is internal/thermal energy) but the energy crossing a boundary because of a temperature difference.

• (a)The transformation of energy from one form to another — That defines energy conversion in general (e.g. electrical to light); heat is one specific mode of energy transfer, not any conversion of energy.
• (b)The conversion of energy into mass and vice-versa due to temperature difference — That describes mass-energy equivalence (E = mc^2), a nuclear/relativistic idea; it has nothing to do with the everyday flow of heat down a temperature gradient.
• (d)The change in volume of a substance with temperature — That is thermal expansion — an effect that heat can cause, not the definition of heat itself.

Heat and temperature are distinct. Heat is the energy transferred between bodies because of a temperature difference; temperature measures how hot a body is (the average kinetic energy of its particles). Heat always flows spontaneously from higher to lower temperature and ceases at thermal equilibrium. It moves by conduction, convection and radiation.

The options are designed to sound 'energy/temperature-flavoured' — energy conversion in general (a), E = mc^2 (b) and thermal expansion (d). Only (c) captures the defining feature of heat: energy transfer driven by a temperature difference.

• Heat is energy in transit; it flows spontaneously from higher to lower temperature and stops at thermal equilibrium.
• Temperature measures the average kinetic energy of a substance's particles; heat is the energy exchanged because of a temperature difference.
• The SI unit of heat is the joule (J); the older unit is the calorie (1 cal is about 4.186 J).
• Heat is transferred by three modes: conduction, convection and radiation.

• Confusing heat (energy in transit) with temperature (a measure of hotness).
• Calling thermal expansion or general energy conversion the 'definition' of heat.

Asked as a 'best defines' item that separates heat from temperature, or by matching the three modes of heat transfer.

No directly related past PYQ was found.

• practice — not a real PYQ

  The SI unit of heat is the

  • (a)newton
  • (b)joule
  • (c)pascal
  • (d)watt

  Answer(b) joule — heat is a form of energy, measured in joules.

• practice — not a real PYQ

  Heat transfer that takes place without any movement of the medium's particles (and can occur through a vacuum) is

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

  Answer(c) radiation — energy travels as electromagnetic waves, needing no medium.