Reversible Process

Card # 29 Reversible process: There are many irreversible process occur in nature. 1st law cannot explain the lack of reversibility. Example: Heat flows naturally from a hot body to cold body but heat will never of itself flow from a cold body to a hot body.

Kelvin Planck Statement It is not possible to construct a heat engine which can absorb heat from a reservoir and convert the entire heat into work.

Clausices Statement It is not possible to transfer heat from a cold body to hot body without the help of some external agency.

Reversible process: Suppose in a process a thermodynamic system goes from an initial state

i to a final state

f. During the process the system absorbs heat Q from the surroundings and performs work W on it. If the process can be turned back such that both the system and the surroundings returns to their original states with no other change anywhere else in the universe then it is called reversible process. If the system and surroundings cannot be turned back to original states then the process is called irreversible process.

Cause of Irreversibility A system will be irreversible 1. If the system is passing through non-equilibrium states Example: from expansion or explosive chemical reaction. 2. Process involves dissipative effects like friction, viscosity, etc, which causes loss of mechanical energy. It is impossible to eliminate them completely.

Internal Energy Internal Energy of a system is a state variable it depends only on the initial and final state. It is the function of Temperature and Volume. Potential Energy Internal Energy U = Kinetic Energy + Of the Molecules of the Molecules If the system is isolated from the surrounding, no heat exchange between systems and surrounding takes place ( Q = 0 ) that is why work done W = 0 . Hence ΔU = 0. Or U = c o n s ta n t . In other words the internal energy of isolated system remains constant.

In case of a cyclic system there will be no change in internal energy (ΔU= 0), heat added will be equal to work done Q = W .