Heat and Thermodynamics

81. For an ideal gas, which one of the following statements does not hol

For an ideal gas, which one of the following statements does not hold true?

[amp_mcq option1=”The speed of all gas molecules is same.” option2=”The kinetic energies of all gas molecules are not same.” option3=”The potential energy of the gas molecules is zero.” option4=”There is no interactive force between the molecules.” correct=”option1″]

This question was previously asked in
UPSC CDS-1 – 2019
Download PDFAttempt Online
An ideal gas is a theoretical model with specific assumptions. Let’s evaluate each statement:
A) The speed of all gas molecules is same. This is false for an ideal gas. At any given temperature, the speeds of the molecules in an ideal gas follow a distribution (like the Maxwell-Boltzmann distribution), meaning molecules have a wide range of speeds.
B) The kinetic energies of all gas molecules are not same. This is true. Since the speeds are not the same (from point A), and kinetic energy is proportional to speed squared ($KE = 1/2 mv^2$), the kinetic energies of individual molecules are also not the same. The average kinetic energy, however, is directly proportional to the absolute temperature.
C) The potential energy of the gas molecules is zero. This is true for an ideal gas. A key assumption of the ideal gas model is that there are no intermolecular forces between the molecules. Potential energy due to interparticle interactions is therefore considered zero.
D) There is no interactive force between the molecules. This is true. This is another fundamental assumption of the ideal gas model, simplifying calculations by ignoring attractions and repulsions.
The question asks which statement does *not* hold true for an ideal gas. Statement A is the one that is false for an ideal gas.
– Ideal gas molecules have a distribution of speeds and kinetic energies.
– Ideal gas molecules have no intermolecular forces.
– Potential energy due to intermolecular forces is zero in an ideal gas.
Real gases deviate from ideal gas behavior, especially at high pressures and low temperatures, where intermolecular forces and the volume of the molecules themselves become significant. The ideal gas law (PV=nRT) is derived based on these ideal gas assumptions.

82. Which of the following is/are state function/functions? 1. q+w 2.

Which of the following is/are state function/functions?

  • 1. q+w
  • 2. q
  • 3. w
  • 4. H-TS

Select the correct answer using the code given below.

[amp_mcq option1=”1 and 4 only” option2=”1, 2 and 4″ option3=”2, 3 and 4″ option4=”1 only” correct=”option1″]

This question was previously asked in
UPSC CDS-1 – 2019
Download PDFAttempt Online
A state function is a property whose value depends only on the state of the system, not on the path taken to reach that state.
1. q+w: According to the first law of thermodynamics, $\Delta U = q+w$, where U is internal energy. Internal energy (U) is a state function, so the change in internal energy ($\Delta U$) is also a state function. Thus, q+w represents $\Delta U$ and is a state function.
2. q: Heat (q) is a path-dependent quantity; the amount of heat transferred depends on the process followed.
3. w: Work (w) is a path-dependent quantity; the amount of work done depends on the process followed.
4. H-TS: This expression is the definition of Gibbs Free Energy (G). Gibbs Free Energy (G) is a state function, as it is defined in terms of state functions (Enthalpy H, Temperature T, and Entropy S).
Therefore, q+w and H-TS are state functions.
– State functions are independent of the path.
– Internal energy ($\Delta U = q+w$) is a state function.
– Gibbs Free Energy ($G = H-TS$) is a state function.
– Heat (q) and Work (w) are path functions.
Examples of other state functions include pressure (P), volume (V), temperature (T), enthalpy (H), entropy (S), and internal energy (U). Examples of path functions include heat (q) and work (w).

83. Which of the following represents a relation for ‘heat lost = heat

Which of the following represents a relation for ‘heat lost = heat gained’?

[amp_mcq option1=”Principle of thermal equilibrium” option2=”Principle of colors” option3=”Principle of calorimetry” option4=”Principle of vaporization” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2018
Download PDFAttempt Online
The principle of ‘heat lost = heat gained’ is the fundamental basis of calorimetry. Calorimetry is the science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state due to chemical reactions, physical changes, or phase transitions under specified constraints. In an isolated system, when substances at different temperatures come into contact, heat flows from the hotter substances to the colder substances until thermal equilibrium is reached. The total amount of heat energy lost by the hotter substances equals the total amount of heat energy gained by the colder substances.
The principle of conservation of energy applied to heat transfer between objects in thermal contact is known as the principle of calorimetry.
Thermal equilibrium is the state achieved when objects in contact have reached the same temperature and there is no net heat flow. Vaporization is a phase transition. The principle of colors relates to optics, not heat transfer.

84. Which one of the following is the correct relation between the Kelvin

Which one of the following is the correct relation between the Kelvin temperature (T) and the Celsius temperature (t_c)?

[amp_mcq option1=”These are two independent temperature scales” option2=”T = t_c” option3=”T = t_c – 273·15″ option4=”T = t_c + 273·15″ correct=”option4″]

This question was previously asked in
UPSC CDS-1 – 2018
Download PDFAttempt Online
The Kelvin temperature scale is an absolute scale where 0 K is absolute zero. The Celsius scale is a relative scale where 0°C is the freezing point of water. The size of one degree Celsius is equal to the size of one kelvin. The relationship between Kelvin temperature (T) and Celsius temperature (t_c) is given by the formula T = t_c + 273.15. This means that a temperature in Celsius can be converted to Kelvin by adding 273.15.
– The Kelvin scale starts at absolute zero (0 K), where particle motion theoretically stops.
– The Celsius scale is based on the phase change points of water at standard pressure (0°C for freezing, 100°C for boiling).
– The temperature 0°C corresponds to 273.15 K.
– The formula T = t_c + 273.15 correctly shifts the zero point of the Celsius scale to match the absolute zero of the Kelvin scale while maintaining the same interval size.
For most practical purposes and in many physics problems, the value 273.15 is often approximated as 273. The freezing point of water is exactly 273.15 K and the boiling point is 373.15 K. The Kelvin scale is the standard unit of thermodynamic temperature in the International System of Units (SI).

85. A piece of ice, 100 g in mass is kept at 0 °C. The amount of heat it r

A piece of ice, 100 g in mass is kept at 0 °C. The amount of heat it requires to melt at 0 °C is (take latent heat of melting of ice to be 333.6 J/g):

[amp_mcq option1=”750.6 J” option2=”83.4 J” option3=”33360 J” option4=”3.336 J” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2016
Download PDFAttempt Online
The amount of heat required to melt 100 g of ice at 0°C is 33360 J.
– The heat required for a substance to change phase (like melting) at a constant temperature is given by the formula Q = m * L, where Q is the heat energy, m is the mass, and L is the latent heat of fusion (for melting) or vaporization (for boiling).
– Given mass of ice m = 100 g.
– Given latent heat of melting of ice L = 333.6 J/g.
– Heat required Q = 100 g * 333.6 J/g = 33360 J.
Latent heat is the energy absorbed or released during a phase transition at constant temperature and pressure.

86. Two systems are said to be in thermal equilibrium if and only if :

Two systems are said to be in thermal equilibrium if and only if :

[amp_mcq option1=”there can be a heat flow between them even if they are at different temperatures” option2=”there cannot be a heat flow between them even if they are at different temperatures” option3=”there is no heat flow between them” option4=”their temperatures are slightly different” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2016
Download PDFAttempt Online
Two systems are in thermal equilibrium if there is no heat flow between them.
According to the zeroth law of thermodynamics, if two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. Thermal equilibrium is the state where there is no net flow of heat between objects in thermal contact. This condition is met when the objects are at the same temperature.
Heat flow is the transfer of thermal energy from a region of higher temperature to a region of lower temperature. If the temperatures are different and the systems are in thermal contact, heat will flow until thermal equilibrium is reached and their temperatures become equal.

87. After a hot sunny day, people sprinkle water on the roof-top because :

After a hot sunny day, people sprinkle water on the roof-top because :

[amp_mcq option1=”water helps air around the roof-top to absorb the heat instantly” option2=”water has lower specific heat capacity” option3=”water is easily available” option4=”water has large latent heat of vaporisation” correct=”option4″]

This question was previously asked in
UPSC CDS-1 – 2016
Download PDFAttempt Online
The correct option is D) water has large latent heat of vaporisation.
When water is sprinkled on a hot surface like a rooftop, it evaporates. Evaporation is a process that requires energy (heat). This energy is absorbed from the surroundings, including the rooftop itself and the air, as the water changes from liquid to gas phase. The latent heat of vaporisation is the amount of heat energy required to change a substance from a liquid to a gas at a constant temperature. Water has a relatively high latent heat of vaporisation, meaning it absorbs a significant amount of heat from the rooftop during evaporation, leading to a substantial cooling effect.
Other options are less accurate or irrelevant. While water absorbs heat, the primary cooling mechanism here is the energy required for the phase change (latent heat), not simple heat absorption in liquid form (specific heat capacity). A higher specific heat capacity means it takes more energy to raise the temperature of water, which is also a factor in moderating temperature, but the cooling on a hot day is mainly due to the heat removed during evaporation. Water availability is a practical reason for using it, not the scientific principle behind the cooling.
Page 1Page 2Page 3Page 4Page 5