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Thermodynamic processes

A thermodynamic process is the transformation of a system from an initial state to a final state. This transformation is accompanied by changes in the pressure, volume and temperature of the system.

A state is a set of values of physical quantities (e.g. volume, pressure, temperature) that describe the properties of a system.

Thermodynamic processes are represented by pressure-volume (p-V) lines/curves. They are often drawn with arrows indicating the path taken.

Several connected p-V lines/curves are often drawn on the same axes to form a cyclical process.

An example of a cyclic process: A represents an expansion at constant pressure, B represents a decrease in pressure at constant volume, C represents a compression at constant pressure, and D represents an increase in pressure at constant volume.
An example of a cyclic process: A represents an expansion at constant pressure, B represents a decrease in pressure at constant volume, C represents a compression at constant pressure, and D represents an increase in pressure at constant volume.
p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).
p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).

In an isochoric process the volume is kept constant (i.e. $$\Delta V=0$$). This implies that the work done by the gas is zero (i.e. $$W=0$$).

The change in internal energy during an isochoric process is equal to the heat supplied to the system (i.e. $$\Delta U=Q$$).

The p-V graph of an isochoric process is a vertical line.

$$p=$$pressure; $$Q=$$heat supplied; $$T=$$temperature; $$\Delta U=$$change in internal energy; $$V=$$volume; $$W=$$work done

p-V graph showing a different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).
p-V graph showing a different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).

In an isobaric process the pressure is kept constant (i.e. $$\Delta p=0$$). The work done by the gas is given by $$p\Delta V$$.

The change in internal energy during an isobaric process is given by $$\Delta U=Q-p\Delta V$$.

The p-V graph of an isobaric process is a horizontal line.

$$p=$$pressure; $$Q=$$heat supplied; $$T=$$temperature; $$\Delta U=$$change in internal energy; $$V=$$volume; $$W=$$work done

p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).
p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).

In an isothermal process, the temperature is kept constant (i.e. $$\Delta T=0$$). However, there may be heat exchanged between the system and its surroundings (i.e. $$Q$$ is not necessarily $$0$$).

The p-V graph of an isothermal process is a curve. The p-V curve of an isothermal process is gentler than that of an adiabatic process.

$$p=$$pressure; $$Q=$$heat supplied; $$T=$$temperature; $$\Delta U=$$change in internal energy; $$V=$$volume; $$W=$$work done

p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).
p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).

In an adiabatic process there is no transfer of heat between the system and its surroundings (i.e. $$Q=0$$). However, the temperature may not be constant (i.e. $$\Delta T$$ is not necessarily $$0$$).

The change in internal energy during an adiabatic process is equal to the work done on the system (i.e. $$\Delta U=W$$).

The p-V graph of an adiabatic process is a curve. The p-V curve of an adiabatic process is steeper than that of an isothermal process.

$$p=$$pressure; $$Q=$$heat supplied; $$T=$$temperature; $$\Delta U=$$change in internal energy; $$V=$$volume; $$W=$$work done

Process Characteristics
Isochoric $$\Delta V=0$$; $$W=0$$; $$\Delta U=Q$$
Isobaric $$\Delta p=0$$; $$W=p\Delta V$$; $$\Delta U=Q-p\Delta V$$
Isothermal $$\Delta T=0$$
Adiabatic $$Q=0$$; $$\Delta U=W$$

Thermodynamic processes are not necessarily distinct from each other.

It is possible for a thermodynamic process to be both adiabatic and isobaric ($$Q=0$$ and $$\Delta p=0$$), or adiabatic and isothermal ($$Q=0$$ and $$\Delta T=0$$).

$$p=$$pressure; $$Q=$$heat supplied; $$T=$$temperature; $$\Delta U=$$change in internal energy; $$V=$$volume; $$W=$$work done

p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).
p-V graph showing an different thermodynamic processes (1: isothermal, 2: isochoric, 3: isobaric, 4: adiabatic).