Thermodynamics in Physics Questions and Answers

Answer: D

The temperature remains the same.

While ice melts, the heat supplied is used to break the forces holding the particles in the solid, as latent heat of fusion. It does not raise the temperature, so the ice and water stay at 0°C until all the ice has melted.

The temperature neither rises nor falls during the change of state, so the other options are wrong.

Answer: C

The medium is vacuum, because it prevents loss of heat due to conduction and convection.

A thermos flask has double walls with the air pumped out of the space between them. Conduction and convection both need a material medium, so a vacuum stops heat from passing across the gap by either process. The walls are also silvered to reflect radiation, and the stopper reduces loss through the top.

Air would allow convection and some conduction, so options A and D are wrong. Steel is a good conductor of heat and would carry heat away, so option B is wrong.

Answer: D

Evaporation is slowest when both surface area and temperature decrease.

The rate of evaporation increases with the exposed surface area, since more molecules are at the surface ready to escape, and with temperature, since more molecules have enough energy to escape. (It also rises with wind speed and falls with humidity.)

When both factors decrease, both work to slow evaporation, so this case is the slowest. When both increase evaporation is fastest, and when one increases and the other decreases the effects partly cancel.

Answer: D

The correct statement is that the range of a laboratory thermometer is greater than that of a clinical thermometer.

A laboratory thermometer usually reads from about −10 °C to 110 °C, while a clinical thermometer reads only from 35 °C to 42 °C, the range of human body temperature.

A clinical thermometer cannot measure boiling liquids, which are far above 42 °C. 35 °C to 42 °C is the range of a clinical, not a laboratory, thermometer, and 30 °C to 100 °C is not the range of a room thermometer.

Answer: C

Container A will gain as well as lose heat faster than container B.

A black surface is a good absorber of heat radiation and an equally good emitter. In sunlight the black container A absorbs radiation faster and heats up faster; in the shade it radiates heat faster and cools down faster.

A white surface reflects most radiation, so container B both gains and loses heat more slowly. The options that split the roles, or make B faster in either step, contradict the rule that good absorbers are good emitters.

Answer: D

The true statement is that body temperature of Priya is more than that of Sana.

Convert Priya's reading to Fahrenheit:

\[ \begin{aligned} F &= \frac{9}{5}C + 32 \\ &= \frac{9}{5}(38.4) + 32 \\ &= 101.12^\circ\text{F} \end{aligned} \]

Sana's 98.6°F is 37°C, normal body temperature, while Priya at 101.12°F (38.4°C) is higher, and in fact has a fever. So their temperatures are not the same, only Priya has fever, and Sana's is not the higher one.

Answer: A

The part responsible for the greenhouse effect is the Glass sheet.

The glass cover lets the Sun's short-wave radiation pass into the box, but the heat radiated by the hot interior, which is long-wave infrared, cannot easily pass back out through the glass. The trapped heat raises the temperature inside, just as in a greenhouse.

The black coating absorbs sunlight and turns it into heat, but it is the glass that traps that heat. The mirror reflects extra sunlight into the box, increasing the energy that enters. The outer cover of the box provides insulation, which reduces heat loss by conduction, not the greenhouse effect.

Answer: A

Most likely \(T_{1} = T_{2} = T_{3}\).

Left overnight in a closed room, every object exchanges heat with the air until it reaches thermal equilibrium, the same temperature as the room, 30 °C. The plastic bottle, the wooden spoon and the metal spoon therefore all end up at the same temperature.

The metal spoon feels colder to the touch than wood or plastic because metal is a good conductor and draws heat from the hand faster, not because it is at a lower temperature. The options with unequal temperatures confuse how hot an object feels with its actual temperature.

Answer: A

The calorific value is 45000, that is 45000 kJ/kg.

Calorific value is the energy released by completely burning 1 kg of a fuel:

\[ \begin{aligned} \text{CV} &= \frac{1.665 \times 10^{8}\ \text{J}}{3.7\ \text{kg}} \\ &= 4.5 \times 10^{7}\ \text{J/kg} \\ &= 45000\ \text{kJ/kg} \end{aligned} \]

NCERT expresses calorific values in kilojoules per kilogram (kJ/kg), which is the 'standard unit' the key follows; in joules per kilogram the same value is \(4.5 \times 10^{7}\). 22222, 61605 and 25000 do not follow from dividing the energy by the mass.

Answer: D

The group of poor conductors is wool, plastic, wood.

Poor conductors (insulators) do not let heat pass through easily. Wool traps air, plastic and wood are non-metals, and all three are used for handles, clothing and insulation for this reason.

Each of the other groups contains a metal, iron, copper or aluminium, which are good conductors of heat.