Conceptual Physics

Concept of Thermodynamics - Zeroeth Law of Thermodynamics

For a system to be in thermodynamic equilibrium the following conditions must be full filled:-

i) Mechanical equilibrium

ii) Thermal equilibrium

iii) Chemical equilibrium

Mechanical equilibrium:

When there is no unbalanced force between system and its surroundings, the system is said to be in mechanical equilibrium.

Thermal equilibrium:

When the temperature in all parts of system is same as that of surroundings, the system is said to be in thermal equilibrium.

Chemical equilibrium:

If the chemical composition is same throughout the system and surroundings it is said to be in chemical equilibrium.

Zeroeth Law of Thermodynamics:

This law was first enunciated by RH Flower in 1831. According to this law when two systems ‘A’ and ‘B’ are in thermal equilibrium with another system ‘C’ then ‘A’ and ‘B’ will also be in thermal equilibrium.

What does First Law of Thermodynamics infer us?

It is impossible to derive any work without expenditure of an equivalent amount of energy in some other forms.

Heat absorbed by the system should be taken positive. Heat rejected by the system should be taken negative.

For an ideal gas the total kinetic energy (KE) of all its molecules is called internal energy(U). For such a gas the internal energy depends only on Temperature.

What is ENTROPY - Very important Points to be noted

1) The thermal property which remains constant during an adiabatic process is called as entropy.
i.e. dQ/T= constant

2) It is a measure of randomness or disorderliness of molecules.

3) It is independent of the path of thermal cycle.

4) The increase in entropy implies transition from ordered state to disorder state.

5) It is an index of unavailable energy of a system.

6) Entropy could also be termed as thermal inertia since more entropy results in less amount of heat energy being converted into work.

7) The increase in Entropy of a system implies transition of thermal energy from more available energy to less available form for conversion into work.

8) The net change in entropy is zero for any reversible cycle. This statement is called as clausius theorem.

9) Clausius Theorem: - The sum of quantities of heat transfer during the isothermal change divided by absolute temperature of the isothermal in a reversible cycle is zero. Entropy changes linearly in isothermal expansion and remains constant in adiabatic expansion or compression but decreases in isothermal compression.

The shape of Temperature (T) - Entropy (S) diagram (Tephigram) is rectangle.

10) Entropy increases in irreversible process.

11) Definition of second law of thermodynamics in terms of entropy

”Every chemical or physical or natural process in nature takes place in such a manner that total entropy increases or remains constant".

12) The principle of "degradation of energy" states that the available energy tending towards zero.