Thermodynamics MCQs | Physics MCQs

This is the definition of specific heat capacity, an intensive property of a substance.

Sublimation is the phase transition of a substance directly from the solid to the gas state, without passing through the liquid state.

The term 'fusion' refers to melting. Latent heat is the energy absorbed or released during a phase change at a constant temperature.

An extensive property is a property that depends on the amount of matter in a sample. Volume, mass, and internal energy are extensive.

An intensive property does not depend on the amount of matter. Density, temperature, and pressure are intensive.

The Carnot engine represents the maximum possible efficiency for any heat engine operating between two given temperatures, due to its ideal, reversible nature.

The first law is a version of the law of conservation of energy, adapted for thermodynamic systems, relating internal energy, heat, and work.

The term 'isochoric' comes from the Greek words 'isos' (equal) and 'khora' (space or volume).

The term 'isobaric' comes from the Greek words 'isos' (equal) and 'baros' (weight or pressure).

Internal energy is a state function. Since a cyclic process returns the system to its initial state, the net change in internal energy is zero.

These are the three fundamental modes of heat transfer.

Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids.

Thermal radiation is the emission of energy as electromagnetic waves. It does not require a medium to travel.

Conduction is the process where heat is transmitted through a substance from a region of higher temperature to a region of lower temperature without any movement of the material itself.

Absolute zero (0 Kelvin) is the lowest possible temperature, where all classical molecular motion ceases.

By physics sign convention for the first law (ΔU = Q - W), work done on a system (compression) is negative, as it increases the internal energy.

By physics sign convention for the first law (ΔU = Q - W), work done by a system (expansion) is positive, as it decreases the internal energy.

By convention, heat absorbed by a system (endothermic) is positive, as it increases the internal energy.

By convention, heat released by a system (exothermic) is negative, as it decreases the internal energy.

The coefficient of performance (COP) is the ratio of the desired output (heat removed from the cold reservoir, Qc) to the required input (work done, W).

The coefficient of performance (COP) for a heat pump is the ratio of the desired output (heat delivered to the hot reservoir, Qh) to the required input (work done, W).

First, convert temperatures to Kelvin: Tₕ = 327 + 273 = 600 K; T꜀ = 27 + 273 = 300 K. The efficiency (η) is calculated using the formula η = 1 - (T꜀ / Tₕ). So, η = 1 - (300 K / 600 K) = 1 - 0.5 = 0.5, which is 50%.

This is the definition of a reversible process, which is an idealization used in thermodynamics.

A state function (like internal energy, pressure, temperature, volume) depends only on the current state of the system, not on the path taken to reach that state.

A path function (like heat and work) is a property that depends on the path taken to get from an initial to a final state.

This is the statement of the third law. At absolute zero, a perfect crystal has a minimum possible entropy, which is defined as zero.

An example is a pot of boiling water without a lid, which exchanges heat (energy) and water vapor (matter) with the air.

An example is a sealed can of soup being heated. It can absorb heat (energy) but no matter can enter or leave.

An example is a perfectly insulated thermos. It is an idealization as perfect isolation is not achievable in practice.

The first law of thermodynamics (ΔU = Q - W) shows that the change in internal energy (ΔU) is the sum of heat added (Q) and work done on the system (-W).

Temperature is directly proportional to the average translational kinetic energy of the particles in a substance.

In an isothermal process (ΔT=0), heat can be added without changing the temperature. Since C = Q/(mΔT), dividing by zero gives an infinite specific heat.

In an adiabatic process, there is no heat exchange (Q=0). Since C = Q/(mΔT), the specific heat is zero.

For a monatomic gas, Cᵥ = (3/2)R and Cₚ = (5/2)R, so γ = (5/2)R / (3/2)R = 5/3 ≈ 1.67.

For a diatomic gas, Cᵥ = (5/2)R and Cₚ = (7/2)R, so γ = (7/2)R / (5/2)R = 7/5 = 1.4.

In an isochoric process, the volume is constant (ΔV=0). Since W = PΔV, no work is done.

A Carnot cycle is a theoretical thermodynamic cycle consisting of four reversible steps: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression.

The efficiency of a Carnot engine depends only on the absolute temperatures of the cold (Tc) and hot (Th) reservoirs.

This equation states that the change in a system's internal energy (ΔU) is equal to the heat added to the system (Q) minus the work done by the system (W).

For a reversible process, the change in entropy (ΔS) is the heat transferred (ΔQ) divided by the absolute temperature (T) at which the transfer occurs.

Since ΔQ = TΔS for a reversible process, the heat transferred is the integral of TdS, which is the area under the curve on a Temperature-Entropy (T-S) diagram.

This type of machine would create energy from nothing, violating the first law (conservation of energy).

This type of machine would convert heat completely into work with 100% efficiency, which violates the second law.

This statement describes the natural direction of heat flow and is the principle behind refrigeration.

This statement means it's impossible to create a device operating in a cycle that extracts heat from a single reservoir and converts it entirely into work.

This is the definition of heat capacity (an extensive property), which is different from specific heat capacity (an intensive property).

Evaporation is a surface phenomenon where a liquid turns into a gas at any temperature below the boiling point.

Latent heat is the energy absorbed or released during a phase transition (like melting or boiling) that occurs at a constant temperature.

The term 'vaporization' refers to boiling. It's the energy needed to overcome intermolecular forces in the liquid state.

Above the critical temperature, a substance exists as a supercritical fluid, which has properties of both a gas and a liquid.

This process, also known as throttling, is a constant enthalpy process and is fundamental to refrigeration and gas liquefaction.

An ideal gas has no intermolecular forces, so its temperature does not change upon free expansion (throttling).

Below the inversion temperature, a gas cools upon expansion (positive coefficient). Above it, the gas heats up (negative coefficient).

This is a key concept in the kinetic theory of gases that relates to properties like viscosity and thermal conductivity.

According to the kinetic theory of gases, pressure arises from the collisions of molecules with the container walls.

The root mean square speed is given by v_rms = √(3RT/M), so it is proportional to √T.

The Maxwell-Boltzmann distribution is a probability distribution for the speeds of particles in a gas at a certain temperature.

This law states that the total energy of a system is shared equally among all its degrees of freedom, where k is the Boltzmann constant.

A monatomic gas (like Helium or Neon) has three translational degrees of freedom (movement in x, y, and z directions).

A diatomic gas (like O₂ or N₂) has three translational and two rotational degrees of freedom at ordinary temperatures.

The van der Waals equation provides a more realistic model for the behavior of real gases by correcting for molecular size and attraction.

This is the Stefan-Boltzmann law, expressed as P = σAT⁴, which describes the power radiated from a black body.

This law (λ_max = b/T) explains why hotter objects glow with shorter wavelengths (e.g., from red hot to white hot).

A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

Emissivity is a measure of how well a surface radiates energy. A perfect black body is a perfect emitter, so its emissivity is 1.

Thermal conductivity quantifies how effectively a material transfers heat through conduction.

This is described by Fourier's law of heat conduction, which states that the rate of heat flow is proportional to the area and the temperature gradient.

According to the First Law of Thermodynamics, ΔU = Q - W. Here, Q is positive (+2000 J) because heat is absorbed, and W is positive (+500 J) because work is done by the system. Therefore, ΔU = 2000 J - 500 J = 1500 J.

The unit for thermal conductivity is watts per meter-Kelvin.

This law is an approximation that applies when the temperature difference between the object and its surroundings is small.

The Kelvin scale is the standard thermodynamic temperature scale, as it is based on an absolute zero point.

This fixed point is used to define the Kelvin scale. It is also equivalent to 0.01 °C.

Efficiency measures how effectively a device converts the heat it receives into useful work.

According to the second law of thermodynamics, the entropy of the universe (an isolated system) always increases in any spontaneous process.

A reversible process is defined as one that can be reversed by an infinitesimal change in conditions, ensuring the system remains in equilibrium throughout.

All real-world processes are irreversible due to factors like friction, heat loss, and other dissipative effects that increase the total entropy.

Enthalpy (H) is a thermodynamic property defined as the sum of the internal energy (U) and the product of pressure (P) and volume (V).

This is a key property of enthalpy, making it very useful for analyzing processes that occur at constant pressure, such as many chemical reactions.

Gibbs free energy (G) is defined as enthalpy (H) minus the product of temperature (T) and entropy (S).

A negative change in Gibbs free energy (ΔG < 0) indicates that a process is spontaneous under these conditions.

For an isobaric (constant pressure) process, work done is W = PΔV. First, convert pressure: 150 kPa = 150,000 Pa. The change in volume is ΔV = 0.5 m³ - 0.2 m³ = 0.3 m³. So, W = 150,000 Pa * 0.3 m³ = 45,000 J, which is equal to 45 kJ.

Helmholtz free energy (A) is defined as internal energy (U) minus the product of temperature (T) and entropy (S). It represents the 'useful' work obtainable from a closed system at constant temperature and volume.