Thermodynamics - Important Questions (WBCHSE Class 11)

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Complete thermodynamics question bank — all laws, processes, derivations. You fill answers.

2 marks · definitions

2 marks1

Define a thermodynamic system. Name its types (open, closed, isolated).

Answer:

In thermodynamics, a system is a specific portion of the universe that is chosen for study. Everything outside the system is called the surroundings, and the two are separated by a boundary (which can be real or imaginary, fixed or movable).

The classification of these systems depends on whether they can exchange matter and energy (in the forms of heat or work) with their surroundings.

Types of Thermodynamic Systems

1. Open System
An open system can exchange both energy and matter with its surroundings.
Example: A pot of boiling water without a lid. Heat (energy) enters from the stove, and steam (matter) escapes into the air.

2. Closed System
A closed system can exchange energy, but not matter, with its surroundings. The amount of matter inside remains constant.
Example: A sealed cylinder with a piston. Heat can be transferred through the walls, and work can be done by moving the piston, but no gas enters or leaves.

3. Isolated System
An isolated system can exchange neither energy nor matter with its surroundings. It is completely cut off from the rest of the universe.
Example: A perfectly insulated thermos flask (in an ideal sense). In reality, a truly isolated system is difficult to achieve, though the entire Universe is often considered one.

System Type	Mass Exchange?	Energy Exchange?
Open	Yes	Yes
Closed	No	Yes
Isolated	No	No

2 marks2

What are intensive and extensive properties? Give one example of each.

Answer:

Intensive Properties
An intensive property is a physical property of a system that does not depend on the system size or the amount of material present. It is an "intrinsic" characteristic.
Logic: If you have a bucket of water at 25°C and pour it into two smaller cups, the water in each cup is still 25°C.
Example: Temperature. Others include pressure, density, and boiling point.

Extensive Properties
An extensive property is a physical property that does depend on the amount of matter in the system. These properties are additive.
Logic: If you have two identical gold bars, the total mass is the sum of the two. If you take one away, the mass changes.
Example: Mass. Others include volume, total energy, and enthalpy.

Quick Comparison Table

Property Type	Depends on Amount?	Additive?	Examples
Intensive	No	No	Temperature, Density, Color
Extensive	Yes	Yes	Mass, Volume, Heat Capacity

2 marks3

What are state variables? Give two examples.

Answer:

In thermodynamics, state variables (also known as state functions or state parameters) are physical quantities that describe the current condition or "state" of a system.

Examples: Temperature (T), Pressure (P), Volume (V), Internal Energy (U), Enthalpy (H), Entropy (S), Gibbs Free Energy (G), etc.

Key Characteristics:

They depend only on the current state of the system, not on how the system reached that state.
The change in a state variable between two states is independent of the path taken.
They are exact differentials in mathematical form.

State Variable	Symbol	Type
Temperature	T	Intensive
Pressure	P	Intensive
Volume	V	Extensive
Internal Energy	U	Extensive
Enthalpy	H	Extensive
Entropy	S	Extensive

Important Note: State variables are different from path variables (like heat and work), which depend on the process taken to reach a state.

2 marks4

Define thermodynamic equilibrium.

Answer:

A system is in thermodynamic equilibrium when its macroscopic properties (like temperature, pressure, and concentration) remain constant over time and do not change even if the system is left to itself.

For a system to be in true thermodynamic equilibrium, it must simultaneously satisfy three specific conditions:

1. Thermal Equilibrium
There is no flow of heat within the system or between the system and its surroundings. This means the temperature is uniform throughout the entire system.
Condition: ΔT = 0

2. Mechanical Equilibrium
There are no unbalanced forces acting within the system or between the system and its surroundings. The pressure remains constant and uniform at all points.
Condition: ΔP = 0

3. Chemical Equilibrium
The chemical composition of the system remains constant over time. This means no net chemical reactions are occurring, and there is no transfer of matter from one part of the system to another.
Condition: Chemical potential is uniform throughout the system.

Type of Equilibrium	Condition	Description
Thermal	ΔT = 0	Uniform temperature, no heat flow
Mechanical	ΔP = 0	Uniform pressure, no unbalanced forces
Chemical	μ = constant	Uniform chemical potential, no composition change

Important Note: All three conditions must be satisfied simultaneously for a system to be in complete thermodynamic equilibrium. If any one condition is not met, the system is not in true thermodynamic equilibrium.

2 marks5

State Zeroth Law of Thermodynamics.

Answer:

Zeroth Law of Thermodynamics

"If two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other."

2 marks6

Define isothermal process.

Answer:

An isothermal process is a thermodynamic process in which the temperature of the system remains constant throughout the process ($\Delta T = 0$).

Note:

(1) Work done in isothermal process (for ideal gas):

$W = nRT \ln \left( \frac{V_2}{V_1} \right) = 2.303 \, nRT \log_{10} \left( \frac{V_2}{V_1} \right)$

(2) PV diagram: In an isothermal process, the curve is a rectangular hyperbola ($PV = \text{constant}$).

2 marks7

Define adiabatic process.

Answer:

An adiabatic process is a thermodynamic process in which no heat exchange takes place between the system and its surroundings ($Q = 0$).

Special Notes:

(1) For an ideal gas, adiabatic process follows:

$PV^\gamma = \text{constant}$

$TV^{\gamma-1} = \text{constant}$

$T^\gamma P^{1-\gamma} = \text{constant}$

where $\gamma = \frac{C_p}{C_v}$ (ratio of specific heats).

(2) Work done in adiabatic process (for ideal gas):

$W = \frac{P_1V_1 - P_2V_2}{\gamma - 1} = \frac{nR(T_1 - T_2)}{\gamma - 1}$

Examples:

Rapid compression in a diesel engine (causes ignition)
Expansion of gas in a nozzle
Sound wave propagation in air
Cloud formation due to rising air expansion

2 marks8

Define isobaric and isochoric processes.

Answer:

1. Isobaric Process
An isobaric process is a thermodynamic process in which the pressure ($P$) of the system remains constant throughout the change ($\Delta P = 0$).

2. Isochoric Process
An isochoric process (also known as an isometric or isovolumetric process) is a thermodynamic process in which the volume ($V$) of the system remains constant ($\Delta V = 0$).

Process	Constant Variable	Work Done (W)
Isobaric	Pressure ($P$)	$P\Delta V$
Isochoric	Volume ($V$)	Zero ($0$)

2 marks9

Differentiate between heat and temperature.

Answer:

Aspect	Heat	Temperature
Definition	Heat is the total energy of molecular motion in a substance.	Temperature is the measure of the average kinetic energy of molecules.
Symbol	$Q$	$T$
Unit (SI)	Joule (J)	Kelvin (K) or °C
Property Type	Extensive property (depends on amount of substance)	Intensive property (independent of amount)
Flow	Heat flows from hotter to colder body	Temperature determines direction of heat flow
Measurement	Measured by calorimeter	Measured by thermometer
Effect	Heat can change temperature, state, or do work	Temperature indicates degree of hotness or coldness

Example: A cup of coffee at 80°C has higher temperature than a swimming pool at 30°C, but the swimming pool contains more heat (more total thermal energy) due to its larger mass.

2 marks10

What is internal energy?

Answer:

Internal energy (U) is the total energy stored within a system at the molecular level. It is the sum of the microscopic kinetic energies and potential energies of all the atoms or molecules that make up the system.

Note:

It is an extensive property (depends on amount of substance)
It is a state function (depends only on initial and final states)
For an ideal gas, internal energy depends only on temperature
Change in internal energy: $\Delta U = Q - W$ (First Law of Thermodynamics)

2 marks11

What is the work done in a cyclic process?

Answer:

In a cyclic process, a system starts at an initial state, undergoes a series of changes, and eventually returns to its original state. Because the system returns to its starting point, the net change in internal energy (ΔU = 0).

From the First Law of Thermodynamics:

$$\Delta U = Q - W$$

Since ΔU = 0, we get:

$$Q = W$$

Therefore, in a cyclic process:

The net work done by the system equals the net heat absorbed by the system.
On a PV diagram, the work done is represented by the area enclosed by the cycle.

$$\text{Work} = \oint P \, dV = \text{Area enclosed in PV diagram}$$

2 marks12

What does the area under a PV diagram represent?

Answer:

In a P−V diagram (Pressure-Volume diagram), the area under the curve represents the total work done by or on the system during a thermodynamic process.

$$\text{Work} = \int_{V_i}^{V_f} P \, dV$$

The area under the curve equals the magnitude of work. The sign depends on the direction:

Expansion (volume increases) → Work done by the system (positive)
Compression (volume decreases) → Work done on the system (negative)

2 marks13

State the First Law of Thermodynamics.

Answer:

It states that energy can neither be created nor destroyed; it can only be converted from one form to another or transferred between a system and its surroundings.

Mathematical Expression
The law is expressed by the following equation:

$$\Delta Q = \Delta U + W$$

Where:

ΔQ: The net heat added to the system.
ΔU: The change in the internal energy of the system.
W: The work done by the system on its surroundings.

Sign Conventions
To use the formula correctly, it is vital to follow these standard sign conventions:

Quantity	Positive (+)	Negative (-)
Heat (Q)	Heat is absorbed by the system	Heat is released by the system
Work (W)	Work is done by the system (Expansion)	Work is done on the system (Compression)
Internal Energy (U)	Temperature increases	Temperature decreases

2 marks14

Define specific heat at constant volume (Cv).

Answer:

Specific heat at constant volume (C_v) is defined as the amount of heat required to raise the temperature of a unit mass (or one mole) of a substance by one degree Celsius (or one Kelvin) while the volume of the system is kept constant.

Mathematically,

$C_v = \frac{1}{n} \left( \frac{dQ}{dT} \right)_V$

Where:

$n$ = number of moles (for molar specific heat)
$dQ$ = heat supplied
$dT$ = change in temperature
$V$ = constant volume

Note:

For an ideal gas:

$C_v = \frac{f}{2}R$

where $f$ = degrees of freedom and $R$ = universal gas constant.

Relation with internal energy:

$dU = nC_v dT$

At constant volume, all heat supplied goes into increasing internal energy (no work done).

2 marks15

Define specific heat at constant pressure (Cp).

Answer:

Specific heat at constant pressure (C_p) is defined as the amount of heat required to raise the temperature of a unit mass (or one mole) of a substance by one degree Celsius (or one Kelvin) while the pressure of the system is kept constant.

Mathematically,

$C_p = \frac{1}{n} \left( \frac{dQ}{dT} \right)_P$

Where:

$n$ = number of moles (for molar specific heat)
$dQ$ = heat supplied
$dT$ = change in temperature
$P$ = constant pressure

Note:

For an ideal gas:

$C_p = C_v + R$

$C_p = \left( \frac{f}{2} + 1 \right) R$

where $f$ = degrees of freedom and $R$ = universal gas constant.

Relation with enthalpy:

$dH = nC_p dT$

At constant pressure, heat supplied equals increase in enthalpy.

Ratio of specific heats:

$\gamma = \frac{C_p}{C_v}$

2 marks16

What is the value of $ C_p - C_v $ for an ideal gas?

Answer:

For an ideal gas, the difference between molar specific heat at constant pressure ($C_p$) and molar specific heat at constant volume ($C_v$) is given by:

$C_p - C_v = R$

where $R$ is the universal gas constant ($R = 8.314 \, \text{J mol}^{-1} \text{K}^{-1}$).

This relation is known as Mayer's formula.

2 marks17

What is a heat engine?

Answer:

A heat engine is a device that converts heat energy into mechanical work by operating in a cyclic process. It absorbs heat from a high-temperature source (reservoir), converts a part of it into useful work, and rejects the remaining heat to a low-temperature sink.

Note:

Efficiency of a heat engine:

$\eta = \frac{W}{Q_H} = 1 - \frac{Q_L}{Q_H}$

Examples of heat engines: Steam engines, internal combustion engines (petrol/diesel engines), gas turbines, etc.

2 marks18

Write the formula for efficiency of a heat engine.

Answer:

The efficiency (η) of a heat engine is defined as the ratio of the net work done (W) by the engine to the total heat absorbed (Q_H) from the high-temperature source.

General Formula
For any heat engine, the efficiency is given by:

$$\eta = \frac{W}{Q_H}$$

Since $W = Q_H - Q_L$ (where Q_L is the heat rejected to the sink), the formula can also be written as:

$$\eta = 1 - \frac{Q_L}{Q_H}$$

2 marks19

What is a refrigerator?

Answer:

A refrigerator is a device that transfers heat from a low-temperature reservoir (cold body) to a high-temperature reservoir (hot body) by doing external work on the system. It operates on the reverse cycle of a heat engine.

Note:

Coefficient of Performance (COP) of a refrigerator:

$COP = \frac{Q_L}{W} = \frac{Q_L}{Q_H - Q_L}$

For a reversible (Carnot) refrigerator: $COP = \frac{T_L}{T_H - T_L}$

2 marks20

What is a Carnot engine?

Answer:

A Carnot engine is a theoretical, idealized heat engine that operates on the Carnot cycle. It is a perfectly reversible engine that provides the maximum possible efficiency that any engine can achieve when operating between two specific temperatures.

Note:

Carnot Efficiency (η)

$η = 1 - \frac{T₂}{T₁}$

where T₁ = Temperature of source (hot reservoir)

T₂ = Temperature of sink (cold reservoir)

(Temperatures in Kelvin)