Revision: Thermodynamics >> Thermodynamics Physics Science (English Medium) Class 11 CBSE

The state in which two objects are at the same temperature and there is no net flow of heat between them is called thermal equilibrium.

The temperature at which pure water freezes at 1 atm pressure is called the ice point.

The temperature at which pure water boils and vapourises into steam at 1 atm pressure is called the steam point or boiling point.

Thermometry is the branch of physics dealing with temperature measurement. It relies on the principle that certain physical properties of materials change continuously and predictably with temperature.

A diathermic wall is a partition that freely allows heat to flow between two systems. It is shown as a thin dark line in diagrams. A thin copper sheet is a good example.

An adiabatic wall is an ideal partition that completely prevents heat transfer between two systems. In diagrams, it is shown as a thick, cross-hatched (slanting lines) region.

When two bodies at different temperatures are brought into contact through a diathermic wall, heat flows from the hotter body to the cooler one. This continues until both reach the same temperature, at which point heat flow stops. This state is called thermal equilibrium.

The total energy possessed by any system due to molecular motion and molecular configuration is called internal energy.

The energy transfer due to temperature difference between a system and its surroundings is called heat.

The thermodynamic state variables that depend on the size of the system (e.g., internal energy, volume) are called extensive variables.

The specific values of macroscopic variables that completely describe every equilibrium state of a thermodynamic system are called thermodynamic state variables.

The thermodynamic state variables that do not depend on the size of the system (e.g., pressure, temperature) are called intensive variables.

The heat capacity of a body is the quantity of heat required to raise its temperature by 1°C. It depends upon the mass and the nature of the body.

The quantity of heat needed to raise the temperature of the whole body by 1°C (or 1 K) is called heat capacity.

The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of unit mass of that substance through 1°C (or 1 K).

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Heat capacity of a body when expressed for the unit mass is called the specific heat capacity of the substance of that body.

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The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat of that object.

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The amount of heat per unit mass absorbed or given out by a substance to change its temperature by one unit (one degree), i.e., 1°C or 1 K, is called specific heat capacity.

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The quantity of heat required to raise the temperature of a unit mass of a gas by one degree, whose exact value depends upon the mode of heating the gas and can range from zero to infinity or even be negative, is called the specific heat capacity of a gas.

A procedure by which the initial state of a system changes to the final state is called a thermodynamic process.

An idealised process in which at every stage the system is in equilibrium state (very slow process) is called a quasi-static process.

A device that transforms heat partly into work or mechanical energy (where T_H > T_C​, Q_H > 0, Q_C < 0) is called a heat engine.

Heat engine is a device which takes heat as input and converts this heat into work by undergoing a cyclic process.

A process in which the changes cannot be retraced in the reverse direction (e.g., puncturing an inflated balloon, burning a candle) is called an irreversible process.

A process in which the changes can be retraced in the reverse direction (e.g., melting of ice, freezing of water, condensation of steam) is called a reversible process.

Master Conversion Formula:

\[\frac{T_F-32}{180}=\frac{T_C}{100}\] = \[\frac {T_K−273.15}{100}\]

By substituting equation W = −p_ex . ΔV in the equation ΔU = q + W, we get

ΔU = q − p_ex . ΔV  ...(1)

If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.

The equation (1) becomes ΔU = q_v

Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.

Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]

or

Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]

If system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then systems A and B are in thermal equilibrium with each other.

If two systems A and B are each in thermal equilibrium with a third system C separately, then A and B will also be in thermal equilibrium with each other.

Work Done by a Thermodynamic System:

where p = pressure and ΔV = change in volume.

By substituting equation W = −p_ex . ΔV in the equation ΔU = q + W, we get

ΔU = q − p_ex . ΔV  ...(1)

If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.

The equation (1) becomes ΔU = q_v

Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.

Statement:
The net heat energy supplied to a system is equal to the sum of the change in internal energy of the system and the work done by the system. It is based on the law of conservation of energy.

Formula:

where Q = heat added, ΔU = change in internal energy, W = work done by the system.

The second law of thermodynamics states that the total entropy of a system and its surroundings increases in a spontaneous process.

Mathematically,

ΔS_total = `Delta S_"system" + Delta S_"surroundings" gt 0`

For an equilibrium:

ΔS_total = 0

The second law of thermodynamics is often called a directional law because it defines the natural direction of processes in the universe. It has two statements:

1. Planck's Statement (deals with heat engines)
It is impossible to construct a heat engine that operates in a cyclic process and converts all the heat it absorbs into work without any heat loss.

2. Clausius Statement (deals with refrigerators and heat pumps)
It is impossible to construct a device that operates in a cyclic process and transfers heat from a colder body to a hotter body without the input of external work.

First Law: Energy of system + surroundings remains constant → ΔU = q + W

ΔU: change in internal energy, q: heat, W: work done on system

Sign convention:

• Work by system (−)
• on system (+)
• Heat absorbed (+)
• released (−)

ΔU > 0: energy enters system; ΔU < 0: energy leaves system

• Isothermal: ΔU = 0 → q = −W
• Adiabatic: q = 0 → ΔU = W
• Isochoric: W = 0 → ΔU = q
• Isobaric: ΔU = q + W

• Heat energy absorbed (Q) depends on: mass (m), rise in temperature (Δt), and specific heat capacity (c), i.e., Q ∝ m × Δt × c.
• Heat capacity (C') and specific heat capacity (c) are related by: C′ = m × c.