Which one of the following statements is NOT correct ?
- (a)In the conduction mode of transference of heat, the molecules of solid pass heat from one molecule to another without moving from their positions
- (b)The amount of heat required to raise the temperature of a substance is called its specific heat capacity
- (c)The process of heat transfer in liquids and gases is through convection mode
- (d)The process of heat transfer from a body at higher temperature to a body at lower temperature without heating the space between them is known as radiation
Correct — B, The amount of heat required to raise the temperature of a substance is called its specific heat capacity. The statement is incomplete, and in physics an incomplete definition is a wrong one. Specific heat capacity is the heat needed to raise the temperature of unit mass of a substance by one degree, so it is measured in joules per kilogram per kelvin. As printed, the statement mentions neither the unit mass nor the one-degree rise, so it would make the quantity depend on how much material you happened to take and by how much you heated it. Drop the words 'per unit mass' and 'by one degree' and what is left is simply the quantity of heat, not a property of the substance at all.
- (a)In the conduction mode of transference of heat, the molecules of solid pass heat from one molecule to another without moving from their positions — This statement is correct, so it cannot be the answer. In conduction the particles vibrate more vigorously and pass energy to their neighbours while staying in place — which is why a metal spoon in hot tea grows warm along its length.
- (c)The process of heat transfer in liquids and gases is through convection mode — Correct as well. Fluids carry heat by bulk movement — warmed fluid expands, becomes less dense and rises while cooler fluid sinks, setting up a convection current, as in a pan of boiling water or in land and sea breezes.
- (d)The process of heat transfer from a body at higher temperature to a body at lower temperature without heating the space between them is known as radiation — Also correct. Radiation travels as electromagnetic waves, needs no medium at all and does not warm the intervening space, which is how the Sun's heat crosses empty space to reach the Earth.
Heat moves in three ways. Conduction passes energy between neighbouring particles that stay put, and dominates in solids, especially metals with free electrons. Convection carries energy in the bulk motion of a fluid, so it belongs to liquids and gases. Radiation carries energy as electromagnetic waves and is the only mode that works through a vacuum. Specific heat capacity is a separate idea altogether — a property of a material, telling you how much heat one kilogram of it needs for a one-degree rise.
Three options describe the modes of heat transfer accurately, and the intruder is a definition with two words missing. That is a standard examiner's move — the sentence sounds like a textbook line, so it slides past a quick reader. The test is to check every definition for its quantifiers: unit mass, unit rise, unit time. Water's high specific heat capacity, about 4200 joules per kilogram per kelvin, is the reason coastal climates are mild and why water is used as a coolant.
- Specific heat capacity is the heat required to raise the temperature of unit mass by one degree, measured in joules per kilogram per kelvin.
- Conduction transfers heat between particles that remain in position and is the dominant mode in solids.
- Convection transfers heat by the bulk movement of a fluid and therefore needs a liquid or a gas.
- Radiation travels as electromagnetic waves, requires no medium, and does not heat the space it crosses.
The first three statements in the question are accurate; the fourth entry here shows what the printed definition of specific heat capacity was missing.
- Accepting a definition that omits 'unit mass' or 'by one degree' — the qualifiers are the definition.
- Assuming radiation warms the air it passes through; it heats the body that absorbs it, not the space in between.
NDA sets a 'which statement is not correct' item on heat almost every session, usually by mutilating one standard definition among three correct sentences.
What is the principle by which a cooling system (Radiator) in a motor car works?
- (a) Conduction only
- (b) Convection
- (c) Radiation only
- (d) Both conduction and radiation
Answer(b) Convection
The same three modes, applied to a machine whose name misleads. A car radiator sheds heat mainly by convection, which is exactly the mode this NDA statement assigns to liquids and gases.
In which of the following phenomena do heat waves travel along a straight line with the speed of light?
- (a) Thermal conduction
- (b) Thermal convection
- (c) Thermal radiation
- (d) Both, thermal conduction and radiation
Answer(c) Thermal radiation
Radiation identified by its signature — straight lines at the speed of light — two years after this paper described it as transfer without heating the space in between. The same property, two wordings.
The Earth's atmosphere is mainly heated by which one of the following?
- (a) Short wave solar radiation
- (b) Reflected solar radiation
- (c) Long wave terrestrial radiation
- (d) Scattered solar radiation
Answer(c) Long wave terrestrial radiation
Heat transfer moved into geography, where the same physics decides how the atmosphere warms. NDA draws on these three modes in both the physics and the geography halves of the paper.
- practice — not a real PYQ
The specific heat capacity of a substance is defined as the heat required to raise the temperature of
- (a)any mass of the substance by any amount
- (b)unit mass of the substance by one degree
- (c)one mole of the substance to its boiling point
- (d)unit volume of the substance by ten degrees
Answer(b) unit mass of the substance by one degree — hence the unit joule per kilogram per kelvin.
- practice — not a real PYQ
Heat from the Sun reaches the Earth mainly by
- (a)conduction
- (b)convection
- (c)radiation
- (d)evaporation
Answer(c) radiation — the only mode that needs no material medium.