The states of a thermodynamic system can be changed by interacting with its surroundings through work and heat. When this change occurs in a system, the system is said to be undergoing a process.
A thermodynamic cycle is a sequence of different processes that begins and ends in the same thermodynamic state.
Some examples of processes:
1. Isothermal process: the temperature is constant T=C
2. Isobaric process: pressure is constant, P=C
3. Constant volume process, v=C
4. Adiabatic process: no heat transfer, Q = 0
5. Reversible process
6. Irreversible process
7. Quasi-static process
thermodynamics Process the entire pv diagram1. An isothermal process occurs at constant temperature. Since the internal energy of a gas is a function only of its temperature, ΔU = 0 for an isothermal process. For the isothermal expansion of an ideal gas we have W = nRT ln (V2/V1). W is positive if V2 > V1. Since ΔU = 0, the heat transferred to the gas is ΔQ = W.
2. An isobaric process is a process that occurs at constant pressure. We then have W = P(V2 – V1). If the pressure of an ideal gas is kept constant, the temperature must increase as the gas expands. (PV/T = constant.) Heat must be added during the expansion process.
We define the enthalpy H of the system by the equation H = U + PV. Enthalpy is, therefore, a physical property of the system. It has the dimensions of energy and the SI unit of enthalpy is Joule. For an isobaric process we write ΔU = ΔQ – ΔW = ΔQ – P(V2 – V1), or, rearranging the terms, ΔH = ΔQ. This expression, frequently used in chemistry, can be considered as the isobaric form of the first law. ΔH = ΔQ only applies to isobaric processes. Chemical reactions (including biological ones) generally occur at constant pressure, and so ΔQ is equal to the change in a physical property of the system.
3. An adiabatic process is a process during which no heat enters or leaves the system. We then have ΔU = -ΔW, that is, ΔW is equal to the change in a physical property of the system. A physical property of the system depends only on the state of the system (P, V, T), and not on how the system was placed in this state.
4. In practice, there are two different ways to prevent heat transfer.
(a) Provide very good thermal insulation of the system.
(b) Complete the process in a very short time interval so that there is no time for appreciable heat transfer. The combustion process inside a car engine is essentially adiabatic for this reason.
5. An isovolumetric or isometric process occurs at constant volume. Then W = 0 and ΔU = ΔQ. All heat added to the system goes to increasing its internal energy.
6. Reversible process: The process in which the system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamic properties of the universe is called reversible process.
7. Irreversible process:
Irreversible process is also called natural process because all processes occurring in nature are irreversible processes.
8. Quasi-static process:
A quasi-static process is an idealized model of a thermodynamic process that happens infinitely slowly. It is important to note that no real process is quasi-static. In practice, such processes can only be approximated by running them infinitely slowly. A quasi-static process often guarantees that the system will pass through a sequence of states that are infinitely close to equilibrium (so that the system remains in quasi-static equilibrium), in which case the process is normally reversible.
