By going through these CBSE Class 11 Physics Notes Chapter 12 Thermodynamics, students can recall all the concepts quickly.

## Thermodynamics Notes Class 11 Physics Chapter 12

→ A thermodynamic system is a collection of a large no. of atoms or molecules confined within the boundaries of a closed surface so that it has definite values of P, V, and T.

→ Work is done during expansion or contraction of the system and is given by dW = PdV where dV = change in volume at constant pressure P.

→ The temperature of the system decreases during expansion and increases during contraction.

→ The slope of the adiabatic curve is steeper than that of the isothermal curve.

→ Wiso > Wadia during expansion if the initial (Vt) and final (Vf) volumes are the same in both the cases.

→ Work done during isothermal compression is less than that during adiabatic compression if Vt and Vf are the same in both cases.

→ Δp = 0 in isobaric process and ΔV = 0 for an isochoric process.

→ Heat engines are devices that convert heat into work.

→ The refrigerator is regarded as a heat engine in the reverse direction.

→ 1 litre = 10-3 m3.

→ SI and G.G.S. unit of heat capacity is JK-1 and cal/°C respectively.

→ η of Carrot heat engine is independent of the nature of the working substance.

→ CP – CV is constant for all gases.

→ CP/CV is not constant for all gases.

→ CP/CV has different values for mono, di, and triatomic gases.

→ U for a system is the unique function of the state of the system i.e. U is a unique function of P, V, T.

→ The refrigerator absorbs heat from the cold reservoir and rejects the heat to the hot reservoir..

→ The liquid used as a working substance in the refrigerator is called refrigerant.

→ The most commonly used refrigerants are pheon (dichlorodifluoromethane), SO2 and ammonia.

→ Freon or SO2 are used in household refrigerators.

→ NH3 is used for large-scale refrigeration.

→ U for real gas depends on T and V i.e. U = f (T, V).

→ U for ideal gas depends only on T i.e. U = f (T).

→ For isothermal process, dU = 0 and dQ = dW.

→ For an adiabatic process, dQ = 0 and dU = – dW.

→ PVγ = constant for an adiabatic process.

→ Open system: The system which can exchange energy with the surroundings is called an open system.

→ Closed system: The system which cannot exchange energy with its surroundings is called a closed system.

→ The first law of thermodynamics: According to this law, the total energy of an isolated system remains the same. However, it can change the form, Mathematically,
ΔQ = ΔU + ΔW

where ΔQ = amount of heat supplied,
ΔU = change in the internal energy and
ΔW = the amount of work done by the system
ΔW = ΔQ – ΔU.

→ Zeroth law of thermodynamics: If two given bodies are in thermal equilibrium with a third body individually, then the given bodies will also be in thermal equilibrium with each other.

→ The second law of thermodynamics:

1. It is impossible to get a continuous supply of work from a body by cooling it to a temperature lower than that of its surroundings.
2. In other words, a perpetual motion of the second kind is impossible without doing anything else.
3. It is impossible to make heat flow from a body at a lower temperature to a body at a higher temperature without doing any work.
4. It is impossible to construct a device that can without other effect lift one object by extracting internal energy from another.

→ Isothermal process: The variation of P with V at T remaining constant is called the isothermal process.

→ Isobaric process: A process in which volume (V) and temperature (T) vary but the pressure (P) remains constant is known as the isobaric process.

→ Isochoric process: A process in which volume remains constant but P and T can change is known as the isochoric process.

→ Adiabatic process: A process in which the total heat content of the system (Q) remains conserved when it undergoes various changes is called an adiabatic process.

→ Indicator diagram: The graph between (P) and volume (V) of a thermodynamic system undergoing certain changes is called a P-V diagram or an indicator diagram as it is drawn with the help of a device called an indicator.

→ Non-cyclic process: A process in which a system after undergoing certain changes does not return to its initial state is called a non-cyclic process.

→ Cyclic process: A process in which a system after undergoing certain changes returns to its initial state is called a cyclic process.

→ External combustion engine: An engine in which fuel is burnt in a separate unit than the main engine is called an external combustion engine.

→ Internal combustion engine: An engine in which the fuel is burnt within the working cylinder of the engine is called an internal combustion engine.

→ Heat engine: A device that uses thermal energy to deliver mechanical energy is called a heat engine.

→ Heat reservoir: A source of heat at constant temperature is called a heat reservoir.

→ Heat sink: A sub-system of the engine in or out of it in which unspent heat is rejected at constant temperature for use is called a heat sink.

→ Working substance: A substance that receives some heat from a source and after converting a part of it into work rejects the remaining heat into the sink. Gas, steam are usual working substances in an engine.

→ Critical pressure: It is the pressure that is necessary to produce liquefaction at the critical temperature.

→ Critical volume: It is the volume of 1 mole of a gas at the critical temperature and critical pressure.

→ Real gas: The real gases are those in which the molecular energy is both kinetic and potential due to attraction between the molecules.

→ Ideal or perfect gas: A gas in which intermolecular attractive force is zero and energy of molecules is only kinetic are called ideal or perfect gases.

→ The internal energy of a perfect gas depends only on its temperature and not on its volume.

→ Phases: The existence of a substance in liquid, vapor, or solid-state are known as three phases of a substance on a given pressure-temperature graph.

→ Phase diagram: The way of showing different phases of substance on a pressure-temperature graph is known as a phase diagram.

→ Reversible process: A process is said to be reversible when the various stages of an operation to which it is subjected can be traversed back in the opposite direction in such a way that the substance passes through exactly the same conditions at every step in the reverse process as in the direct process.

→ Irreversible process: A process in which any one of the conditions stated for the reversible process is not fulfilled is called an irreversible process.

Important Formulae:
→ Equation of state for an ideal gas of μ moles is
PV = μRT

→ Equation of state for a real gas is
(P + $$\frac{\mathrm{a}}{\mathrm{V}^{2}}$$)(V – b) = RT

→ Internal energy of the gas molecules is given by
U = KE. + P.E.

→ First law of thermodynamics is the law of conservation of energy and is mathematically expressed as
dQ = dU + dW
= dU + PdV.

→ Work done during isothermal and adiabatic processes are given by

1. Wiso = 2.303 RT log10($$\frac{\mathrm{V}_{2}}{\mathrm{~V}_{1}}$$)
2. Wadia = $$\frac{R}{\gamma-1}$$(T1 – T2)

→ Efficiency of heat engine is given by
η = $$\frac{\mathrm{W}}{\mathrm{Q}_{1}}=\frac{\mathrm{Q}_{1}-\mathrm{Q}_{2}}{\mathrm{Q}_{1}}$$

= 1 – $$\frac{\mathrm{Q}_{2}}{\mathrm{Q}_{1}}$$ = 1 – $$\frac{\mathrm{T}_{2}}{\mathrm{T}_{1}}$$

where Q1 = heat absorbed from the source at temperature T1
Q2 = heat rejected to the sink at temperature T2.

→ P1V1 = P2V2 for an isothermal process.

→ P1V1r = P2V2r for an adiabatic process.

→ Coefficient of performance of refrigerator is given by
β = $$\frac{\text { Heat extracted from cold body }}{\text { Work doneon the refrigerant }}$$
= $$\frac{\mathrm{Q}_{2}}{\mathrm{~W}}=\frac{\mathrm{Q}_{2}}{\mathrm{Q}_{1}-\mathrm{Q}_{2}}$$

→ In a true camot cycle,
$$\frac{\mathrm{Q}_{2}}{\mathrm{Q}_{1}}=\frac{\mathrm{T}_{2}}{\mathrm{~T}_{1}}$$

∴ β = $$\frac{\mathrm{T}_{2}}{\mathrm{~T}_{1}-\mathrm{T}_{2}}$$

→ CP – CV = $$\frac{\mathrm{r}}{\mathrm{J}}$$

→ Work done is given by
dW = PdV(J) = $$\frac{\mathrm{PdV}}{4.2}$$cal.

→ Internal energy gained or lost by a perfect gas is
ΔU = n CVΔT.

→ For isochoric process,
ΔQ = n Cu ΔT.

→ For isobaric process,
ΔQ = n CP ΔT.