Thermodynamics is the branch of physics that deals with the concepts of heat and temperature, and the inter-conversion of heat and other forms of energy. It is a macroscopic science, which involves the macroscopic variables, such as pressure, temperature, volume etc., while describing the state of any object. The state of a system is in thermodynamic equilibrium if the macroscopic variables characterizing the system don’t change with time. If two systems in thermal contact with each other cease to have a net exchange of energy, then they are said to be in thermal equilibrium. The zeroth law of thermodynamics states that two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other. The thermodynamic variable that remains the same for all the three systems is temperature.
The first law of thermodynamics states that some amount of the energy supplied to the system goes in increasing the internal energy of the system and the rest goes in work on the environment.
An equilibrium state of a thermodynamic equilibrium is described by state variables, such as pressure, volume, temperature, mass etc. The value of state variable depends only on the particular state but not on the path used to arrive at that state. Non-equilibrium states of a system are difficult to deal with. So, it is convenient to think of an idealized process in which the system is in equilibrium in each stage. Such a process is infinitely slow and thus named quasi-static. There are different thermodynamic processed depending upon the macroscopic variable that remains constant – isothermal (temperature), isobaric (pressure), isochoric (volume), adiabatic (heat absorbed or released) and cyclic (internal energy).
Heat engine is a device in which a system undergoes a cyclic process so as to convert heat into work. A refrigerator is the reverse of heat engine. It is the second law of thermodynamics that give the fundamental limitation to the efficiency of a heat engine and the coefficient of performance of a refrigerator. According to this law, no process is possible, the sole result of which is the absorption of heat from a reservoir and complete conversion of heat into work.
The heat engine, for which the efficiency is the highest, is based on idealized reversible processes. Such an engine is called Carnot engine. A reversible process is the one which can be reversed so that both the system and the surroundings return to their original states, with no other change elsewhere in the universe. However, all spontaneous processes of nature are irreversible. This is why, no engine operating between two temperatures can have efficiency greater than that of the Carnot engine operating between the same temperatures.