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:
- 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.
- In other words, a perpetual motion of the second kind is impossible without doing anything else.
- 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.
- 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
- Wiso = 2.303 RT log10(\(\frac{\mathrm{V}_{2}}{\mathrm{~V}_{1}}\))
- 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.