Molecules store chemical energies, which can be released as heat during chemical reactions like when a fuel, for example, methane, cooking gas or coal, burns in the air.  The chemical energy may also be used to do mechanical work, like when a fuel burns in an engine to provide energy through a galvanic cell-like dry cell. So, the various forms of energy are interrelated, and under certain conditions, they can be transformed from one form to another. The study of these energy transformations is called Thermodynamics.

Thermodynamics and its laws are concerned with macroscopic systems consisting of a huge number of molecules. The initial and the final state of a system under consideration are important in order to study the thermodynamics of the changes in that system. Neither the path followed nor the velocity of changes is important for thermodynamic studies.The laws are always applied when a system is in equilibrium or moves from one equilibrium state to another equilibrium state.

What are the processes involved in thermodynamics? Explain it with numerical and examples.

The various processes involved in thermodynamics are:

• Quasi-static process: In this process, the temperature of the surrounding reservoir and the external pressure is only infinitesimally different from the temperature and pressure of the system. 

For example, consider gas in thermal and mechanical equilibrium with its surroundings, and the pressure and temperature of the gas is equal to the pressure and temperature of its surroundings. Now, suppose the external pressure is suddenly reduced like lifting weight on a movable pistol, then ultimately, the pistol will accelerate in the forward direction. 

This process is infinitely slow; it is called quasi-static (meaning nearly static). 

• Isothermal process: – it is the process in which the system’s temperature is kept constant throughout.  For example, it is called an isothermal process when the ice melts naturally without changing its temperature.

For an isothermal process (T fixed), the ideal gas equation gives PV= constant. The pressure of a given mass of gas varies inversely from its volume. This is called Boyle’s law.

Suppose an ideal gas goes isothermally at temperature T from its initial stage P1, V1 to final stage P2, V2. Now, at any intermediate stage with pressure P, volume changes from V to V+∆V

As we know,

∆W = P ∆V

Taking ∆V →0 and summing the quantity ∆W over the entire process

W = V2∫V1 P dV

     = µRT V2∫V1 dV/V (PV = µRT, ideal gas equation)

     = µRT In V2/V1

For an ideal gas, the internal energy only depends on the temperature, and so in an isothermal process, there is no change in internal energy, implying the first law of thermodynamics, heat supplied to the gas is equal to the work done by the gas: Q=W. So, from the above equation, we can see that if V2>V1, W>0 and if V1>V2, W<0 that means in an isothermal expansion, the gas absorbs heat and does work while in isothermal compression work is done by the environment on the gas and heat is released.

Example: the expansion of a gas in a metallic cylinder placed in a large reservoir of fixed temperature.

• Adiabatic process: – in this process, the system is insulated from the surroundings and heat absorbed or released is zero. 

For an adiabatic process of an ideal gas,

PVƴ  = constant, where ƴ is the ratio of specific heat at constant pressure and constant volume.

Ƴ = CP/CV

If an ideal gas changes its state adiabatically from P1V1 to P2V2

P1V1ƴ  = P2V2ƴ

Now work done in an adiabatic change of an ideal gas from state P1V1T1 to P2V2T2.

W = V2∫V1 P dV

     = constant × V2∫V1 dV/Vƴ

     = constant × V-ƴ+1 / 1-ƴ ǀv1v2

= constant / {1-ƴ} × [1/V2ƴ-1 – 1/ V1ƴ-1]

W = 1/ 1-ƴ [P2V2ƴ / V2ƴ-1 – P1V1ƴ/ V1ƴ-1]

     = 1/ 1-ƴ [P2V2 – P1V1] = µR(T1-T2)/ ƴ-1

So, in an adiabatic process, if the work is done by the gas, then W>0, T2<T1, and if work is done on the gas W<0, T2> T1, i.e., the temperature of the gas rises. 

Example: the release of air from a pneumatic tire. 

• Isochoric process: – in this process, V is constant, so no work is done on or by the gas. The heat absorbed by the gas goes entirely to change its internal energy and its temperature. The specific heat of the gas determines the change in temperature for a given amount of heat at constant volume. For example: when the burning of the gasoline-air mixture in an internal combustion engine car is instantaneous (otto cycle).
• Isobaric process: – in this process, P is fixed, so the work done by the gas is 

                                  W = P(V2 – V1) = µR (T2 – T1)

Since temperature changes, so the internal energy also changes. The heat absorbed is partly used to work and partly to increase internal energy. The change in temperature for a given amount of heat is determined by the specific heat of the gas at constant pressure. 

Example: boiling the water in an open container.

• Cyclic process: – in this process, the system runs to its initial state and since the internal energy is a state variable, ∆U = 0 for a cyclic process. So, the total heat absorbed is equal to the work done by the system. Example: refrigerator or an air conditioner.

Conclusion :

So, to put it simply, thermodynamics is the science of the relationship between heat, work, temperature and energy.  Also, energy can neither be created nor destroyed. It can only be transformed from one form to another, so the study of these energy changes can only be done by following the laws of thermodynamics.