Thermodynamics — Set 4
Physics · ऊष्मागतिकी · Questions 31–40 of 40
What is the SI unit of entropy?
Correct Answer: C. J/K
Entropy measures heat transfer per unit absolute temperature in reversible processes. Its SI unit is joule per kelvin, written as J/K. Entropy is a state function used to describe irreversibility.
For an ideal gas, internal energy depends primarily on which variable?
Correct Answer: A. Temperature
For an ideal gas, internal energy depends only on temperature. Changes in pressure or volume at constant temperature do not change internal energy. This is because ideal gas internal energy comes from molecular kinetic energy.
In free expansion of an ideal gas into vacuum, what is the change in temperature of the gas?
Correct Answer: D. It becomes absolute zero
The correct answer is 'It becomes absolute zero'. In free expansion into vacuum, no work is done and no heat is exchanged for an insulated case. For an ideal gas, internal energy depends only on temperature, so temperature does not change. The process is still irreversible even with no temperature change.
In an isothermal expansion of an ideal gas, which statement is correct?
Correct Answer: C. Work done is zero
For an ideal gas in an isothermal process, internal energy does not change. So by the first law, heat absorbed equals work done by the gas. This is why isothermal expansion requires heat input to keep temperature constant.
Which statement best describes a heat engine?
Correct Answer: D. A device that converts heat into work while operating in a cycle
A heat engine takes heat from a hot source and converts part of it into work in a cyclic operation. The remaining heat is rejected to a colder sink. Its efficiency is always less than 1 in real situations.
How is the COP of a heat pump related to the COP of the corresponding refrigerator?
Correct Answer: B. COPheat pump = COPrefrigerator + 1
A heat pump delivers heat Qh to the hot space using work W, so COP = Qh/W. A refrigerator removes heat Qc from the cold space, so COP = Qc/W. Since Qh = Qc + W, the heat pump COP equals refrigerator COP plus 1.
For a reversible adiabatic process of an ideal gas, which relation is also correct?
Correct Answer: D. TV^(γ-1) = constant
In reversible adiabatic change of an ideal gas, temperature and volume satisfy TV^(γ-1) = constant. This follows from PV^γ = constant along with PV = nRT. It helps compare temperatures at different volumes in adiabatic processes.
For a monoatomic ideal gas, what is the value of γ = Cp/Cv?
Correct Answer: D. 5/3
For a monoatomic ideal gas, Cv = 3R/2 and Cp = 5R/2, so γ = (5R/2)/(3R/2) = 5/3. This value comes from three translational degrees of freedom. Monoatomic gases like helium generally have γ close to 1.67.
For a diatomic ideal gas at ordinary temperatures (ignoring vibration), what is the typical value of γ?
Correct Answer: B. 7/5
A diatomic ideal gas has Cv = 5R/2 and Cp = 7R/2 in this temperature range. Therefore γ = Cp/Cv = (7R/2)/(5R/2) = 7/5, which is about 1.4. Many common gases like nitrogen and oxygen have γ near 1.4.
Approximately how many joules are equal to 1 calorie?
Correct Answer: C. 4.186 J
One calorie is defined as a unit of heat and is approximately equal to 4.186 joules. This connects older heat units to the SI energy unit. Food calories are kilocalories, which are 1000 calories each.