1. What Is a Thermodynamic Process?
A thermodynamic process is any change that takes a system from one state to another. During a process, one or more state variables like pressure, volume, or temperature change.
The nature of a process depends on which quantities are kept constant and how the system interacts with the surroundings.
2. Quasi-Static Process (Slow Process)
A quasi-static process is one that happens extremely slowly so the system stays in near-equilibrium at every stage. It is an idealised process but helps us calculate work and other quantities accurately.
2.1. Why Quasi-Static is Important
Most thermodynamic equations assume a quasi-static process. This ensures pressure and temperature are well-defined at every instant.
3. Isothermal Process
An isothermal process is one in which temperature remains constant throughout (\( \Delta T = 0 \)). Heat flows in or out to maintain the temperature during expansion or compression.
3.1. Key Features
- Temperature is constant.
- Internal energy remains constant for an ideal gas.
- Work done is higher compared to adiabatic expansion.
3.2. Equation for Isothermal Expansion
\( PV = \text{constant} \)
\( W = nRT \ln \left( \dfrac{V_2}{V_1} \right) \)
4. Adiabatic Process
An adiabatic process is one in which no heat is exchanged with the surroundings (\( \Delta Q = 0 \)). Any change in internal energy comes only from work done.
4.1. Key Features
- No heat enters or leaves the system.
- Temperature changes during the process.
- Expansion cools the gas; compression heats it.
4.2. Equation for Adiabatic Process
\( PV^{\gamma} = \text{constant} \)
Where \( \gamma = \dfrac{C_p}{C_v} \).
5. Isobaric Process
An isobaric process is a process in which pressure remains constant (\( P = \text{constant} \)).
This often occurs when a gas is heated or cooled while the piston can freely move.
5.1. Key Features
- Pressure stays constant.
- Heat supplied does both work and increases internal energy.
5.2. Work Done in Isobaric Process
\( W = P (V_2 - V_1) \)
6. Isochoric (Isovolumetric) Process
An isochoric process is one in which the volume remains constant (\( V = \text{constant} \)).
Since the volume does not change, no work is done (\( W = 0 \)).
6.1. Key Features
- Volume remains fixed.
- All heat added goes into increasing internal energy.
- Often happens in closed, rigid containers.
6.2. Relation in Isochoric Process
\( \Delta Q = \Delta U \)
7. Reversible and Irreversible Processes
Real processes are usually irreversible due to friction, turbulence, or rapid changes. Reversible processes are ideal and happen infinitely slowly.
7.1. Reversible Process
A reversible process can be reversed by an infinitesimal change in a variable. It is quasi-static and involves no losses.
7.2. Irreversible Process
An irreversible process happens quickly, involves heat losses, and cannot be exactly reversed. Most real-life processes fall into this category.
8. Examples of Thermodynamic Processes in Daily Life
- Air in a bicycle pump gets hotter during compression (adiabatic).
- Boiling water at constant pressure (isobaric).
- Gas heating in a rigid container (isochoric).
- Melting ice-water system maintained at constant temperature (isothermal).