BME Thermodynamics 2 — Questions and Answers
Question 1: Which thermodynamic process occurs at constant entropy?
- Isothermal
- Isobaric
- Isentropic (Correct answer)
- Isochoric
Correct answer: Isentropic
An isentropic process is reversible and adiabatic, meaning entropy remains constant throughout.
Question 2: The Clausius inequality states that for any cycle, the cyclic integral of δQ/T is:
- Always positive
- Always zero
- Less than or equal to zero (Correct answer)
- Greater than or equal to zero
Correct answer: Less than or equal to zero
The Clausius inequality states ∮(δQ/T) ≤ 0, with equality holding only for reversible cycles.
Question 3: In a Brayton cycle, increasing the compressor pressure ratio (up to the optimal value) generally:
- Decreases thermal efficiency
- Has no effect on efficiency
- Increases thermal efficiency (Correct answer)
- Decreases net work output only
Correct answer: Increases thermal efficiency
Higher pressure ratio increases the thermal efficiency of the Brayton cycle because more compression raises the average temperature of heat addition.
Question 4: For an ideal gas undergoing a polytropic process PVⁿ = C, if n equals the ratio of specific heats γ, the process is:
- Isothermal
- Isobaric
- Isentropic (Correct answer)
- Isochoric
Correct answer: Isentropic
When n = γ (= Cp/Cv), the polytropic process becomes isentropic (reversible adiabatic).
Question 5: The coefficient of performance (COP) of a heat pump is related to COP of a refrigerator operating between the same temperatures by:
- COP_HP = COP_R
- COP_HP = COP_R - 1
- COP_HP = COP_R + 1 (Correct answer)
- COP_HP = 1/COP_R
Correct answer: COP_HP = COP_R + 1
COP_HP = COP_R + 1, because the heat pump delivers both the heat removed from the cold reservoir plus the work input.
Question 6: Which statement correctly describes the Joule-Thomson effect for an ideal gas?
- Temperature increases during throttling
- Temperature decreases during throttling
- Temperature remains unchanged during throttling (Correct answer)
- Enthalpy changes during throttling
Correct answer: Temperature remains unchanged during throttling
For an ideal gas, the Joule-Thomson coefficient is zero, so throttling causes no temperature change.
Question 7: The exergy (availability) of a system at the dead state is:
- Maximum
- Equal to internal energy
- Zero (Correct answer)
- Equal to enthalpy
Correct answer: Zero
At the dead state (equilibrium with the environment), a system has zero exergy because no work can be extracted from it.
BME Thermodynamics 2
Which thermodynamic process occurs at constant entropy?