NCERT Solution Class 11th Physics Chapter – 12 Thermodynamics Notes

NCERT Solution Class 11th Physics Chapter – 12 Thermodynamics

NCERT Solution Class 11th Physics Chapter – 12 Thermodynamics Notes Closed system, Adiabatic process, Phase diagram discuss Thermodynamics Notes 

NCERT Solution Class 11th Physics Chapter – 12 Thermodynamics

Chapter – 12

Thermodynamics

Notes

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.

W_iso > W_adia during expansion if the initial (V_t) and final (V_f) volumes are the same in both the cases.

Work done during isothermal compression is less than that during adiabatic compression if V_t and V_f 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.

C_P – C_V is constant for all gases.

C_P/C_V is not constant for all gases.

C_P/C_V 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.

PV^γ = 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 + a/V2)(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. W_iso = 2.303 RT log10(V2 /V1)
2. W_adia = Rγ/−1(T₁ – T₂)

Efficiency of heat engine is given by
η = W/Q1=Q1−Q2/Q1

= 1 – Q2/Q1 = 1 – T2/T1

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.

P₁V₁^r = P₂V₂^r for an adiabatic process.

Coefficient of performance of refrigerator is given by
β =  Heat extracted from cold body / Work doneon the refrigerant 
= Q2 /W=Q2/Q1−Q2

In a true camot cycle,
Q2/Q1=T2 /T1

∴ β = T2/ T1−T2

C_P – C_V = r/J

Work done is given by
dW = PdV(J) = PdV/4.2cal.

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

For isochoric process,
ΔQ = n C_u ΔT.

For isobaric process,
ΔQ = n C_P ΔT.