Mechanical Engineering - MCQ Practice Questions

Temperature is an intensive property as it does not depend on the quantity of matter. Mass, volume, and weight are extensive properties.

The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another.

In an isothermal process, temperature is constant. For an ideal gas, internal energy depends only on temperature, so ΔU = 0.

A reversible adiabatic process is also called isentropic, meaning entropy remains constant (ΔS = 0).

In an isochoric (constant volume) process, W = ∫PdV = 0 because volume does not change.

The Carnot efficiency is the maximum theoretical efficiency for any heat engine, given by η = 1 - (T_C/T_H), where temperatures are in Kelvin.

Throttling is an isenthalpic process where enthalpy remains constant (H_initial = H_final) as the fluid expands through a restriction.

Exergy is maximum work available from a system. At dead state (T₀, P₀, same as surroundings), exergy is zero as no useful work can be extracted.

Compressibility factor Z = PV/(nRT). For ideal gas, Z = 1. For real gases, Z ≠ 1 due to molecular interactions.

When n = 0, PV⁰ = P = constant, which is an isobaric process. When n = 1 (isothermal), n = γ (adiabatic), n = ∞ (isochoric).

First Law: ΔE_system = Q - W, where Q is heat added to system and W is work done by system. This is the IUPAC convention.

For an irreversible process in an isolated system, total entropy increases (ΔS_total > 0). Heat transfer between bodies at different temperatures is irreversible.

For a monatomic ideal gas with 3 translational degrees of freedom: Cv = ($\frac{3}{2}$)R. For diatomic: ($\frac{5}{2}$)R; for polyatomic: ($\frac{7}{2}$)R.

The throttling process is isenthalpic (constant enthalpy). The enthalpy remains the same before and after the expansion valve.

For Carnot refrigerator: COP_ref = T_C/(T_H - T_C) = 250/(300-250) = $\frac{250}{50}$ = 5.

Clausius-Clapeyron: dp/dT = L/(T·Δv), where L is latent heat of vaporization and relates pressure-temperature relationship at phase equilibrium.

For a reversible process: ΔS_universe = ΔS_system + ΔS_surroundings = 0. The system's entropy can increase or decrease, but total entropy doesn't change.

For Brayton cycle: η = 1 - 1/r_p^((γ-1)/γ). As pressure ratio r_p increases, efficiency increases. This is why modern turbojets use high pressure ratios.

Maxwell relations come from the Schwarz theorem applied to thermodynamic potentials (U, H, F, G). For example, from dG = -SdT + VdP: (∂S/∂P)_T = -(∂V/∂T)_P.

Otto cycle efficiency: η = 1 - 1/r^(γ-1) = 1 - $\frac{1}{9}$^0.4 = 1 - $\frac{1}{2}$.28 ≈ 0.558 or 55.8% ≈ 58%.