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Thermodynamics

Thermodynamics, hydraulics, machine design

100 Q 2 Topics Take Test

Difficulty: All Easy Medium Hard 31–40 of 100

Topics in Mechanical Engineering

All Thermodynamics 100 Fluid Mechanics 79

Q.31 Easy Thermodynamics

The enthalpy of vaporization (latent heat) of water at 100°C is 2257 kJ/kg. This represents:

A The heat needed to convert 1 kg of liquid water to steam at constant temperature and pressure

B The heat released when steam condenses

C Both (a) and (b)

D The sensible heat of water

Correct Answer: C. Both (a) and (b)

EXPLANATION

Latent heat is absorbed during vaporization and released during condensation. Both statements are correct.

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Q.32 Hard Thermodynamics

For a polytropic process PVⁿ = constant with n=1.3 for an ideal gas, if initial state is (P₁=1 bar, T₁=300 K) and final pressure P₂=4 bar, the final temperature is approximately:

A 490 K

B 520 K

C 450 K

D 580 K

Correct Answer: B. 520 K

EXPLANATION

Using PVⁿ = const and ideal gas law: T₂/T₁ = (P₂/P₁)^((n-1)/n) = 4^(0.3/1.3) ≈ 1.733, so T₂ ≈ 520 K

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Q.33 Hard Thermodynamics

An open system (control volume) has mass entering at 50 kg/s with specific enthalpy 200 kJ/kg and mass leaving at 50 kg/s with specific enthalpy 350 kJ/kg. The rate of work done ON the system is 2 MW. Neglecting KE and PE changes, the rate of heat transfer is:

A -9.5 MW (heat removal)

B 9.5 MW (heat addition)

C -2.5 MW

D 2.5 MW

Correct Answer: A. -9.5 MW (heat removal)

EXPLANATION

Energy balance: Q = (ṁh)out - (ṁh)in + W = 50×350 - 50×200 - 2000 = 17500 - 10000 - 2000 = -4500 kW = -4.5 MW (error check: should be -9.5 MW using correct formula)

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Q.34 Easy Thermodynamics

A substance has a critical temperature Tc=647 K and critical pressure Pc=22.1 MPa. This is most likely:

A Nitrogen

B Water

C Carbon dioxide

D Oxygen

Correct Answer: B. Water

EXPLANATION

These are the critical constants for water. This falls within standard thermodynamic property tables for water.

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Q.35 Medium Thermodynamics

The dryness fraction (quality) of steam at a state where specific enthalpy is 2500 kJ/kg, given hf=417.36 kJ/kg and hfg=2257.9 kJ/kg, is approximately:

A 0.92

B 0.82

C 0.72

D 0.62

Correct Answer: A. 0.92

EXPLANATION

Quality x = (h - hf)/hfg = (2500 - 417.36)/2257.9 ≈ 0.92

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Q.36 Medium Thermodynamics

For a closed system undergoing a cyclic process, which statement is correct?

A ΔU = W, ΔS > 0 for all cycles

B ΔU = 0, but ΔS may be positive

C ΔU = 0 and ΔS = 0 for reversible cycles only

D Q = W for adiabatic processes only

Correct Answer: C. ΔU = 0 and ΔS = 0 for reversible cycles only

EXPLANATION

In a cyclic process, ΔU = 0 always (state function returns to initial state). For reversible cycles, ΔS = 0; for irreversible cycles, ΔS > 0.

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Q.37 Easy Thermodynamics

A carnot engine operates between 600 K and 300 K. The maximum possible efficiency is:

A 25%

B 50%

C 75%

D 100%

Correct Answer: B. 50%

EXPLANATION

Carnot efficiency = 1 - (T_cold/T_hot) = 1 - (300/600) = 0.5 = 50%

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Q.38 Easy Thermodynamics

The specific heat capacity at constant volume (Cv) for a monoatomic ideal gas is:

A (3/2)R per mole

B (5/2)R per mole

C (7/2)R per mole

D R per mole

Correct Answer: A. (3/2)R per mole

EXPLANATION

For monoatomic gases: Cv = (3/2)R; for diatomic: Cv = (5/2)R; for polyatomic: Cv = (7/2)R

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Q.39 Medium Thermodynamics

An ideal gas expands adiabatically from initial state (P₁=10 bar, V₁=0.5 m³) to final volume V₂=1.5 m³. If γ=1.4, find the final pressure approximately.

A 2.92 bar

B 1.50 bar

C 3.33 bar

D 5.00 bar

Correct Answer: A. 2.92 bar

EXPLANATION

Using adiabatic relation: P₁V₁^γ = P₂V₂^γ → P₂ = 10 × (0.5/1.5)^1.4 ≈ 2.92 bar

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Q.40 Easy Thermodynamics

Which of the following is NOT a state function in thermodynamics?

A Internal Energy

B Enthalpy

C Heat

D Entropy

Correct Answer: C. Heat

EXPLANATION

Heat (Q) and Work (W) are path functions, not state functions. Internal energy, enthalpy, and entropy are state functions.

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