Thermodynamic Processes – Types, Key Equations & Real-World Applications

In Thermodynamic Processes, a thermodynamic process refers to the path or series of changes a system undergoes as it moves from one equilibrium state to another. These processes describe how a system’s properties such as pressure, volume, and temperature change over time due to heat exchange, work done, or internal energy changes.

Types of Thermodynamic Processes

• Isothermal Process (Constant Temperature): The temperature (T) remains constant throughout the process. Heat added to the system is used entirely to do work.
  Example: Slow compression or expansion of an ideal gas in a piston at constant temperature.
  Key equation: ΔT = 0; for an ideal gas, PV = constant.
• Adiabatic Process (No Heat Exchange): No heat (Q) is transferred to or from the system. Changes in internal energy come solely from work done on or by the system.
  Example: Rapid compression or expansion of gases where insulation prevents heat transfer.
  Key equation: Q = 0; for an ideal gas, PV^γ = constant (γ = C_p/C_v).
• Isobaric Process (Constant Pressure): Pressure (P) remains constant while volume and temperature may change. Heat added or removed changes the system’s enthalpy.
  Example: Boiling water at atmospheric pressure.
  Key equation: P = constant; at constant pressure, ΔH = Q.
• Isochoric Process (Constant Volume): Volume (V) remains constant, so no boundary work is done.
  Example: Heating gas in a sealed, rigid container.
  Key equation: V = constant, work done W = 0; thus ΔU = Q.

Other Important Thermodynamic Processes

• Polytropic Process: A generalized process where PVⁿ = constant, with n chosen to model heat/work interactions (e.g., n=1 isothermal, n=γ adiabatic for ideal gases).
• Cyclic Process: The system returns to its initial state after a sequence of processes; total change in internal energy over one full cycle is ΔU = 0.

Significance and Applications

Understanding thermodynamic processes is crucial for analyzing engines, refrigerators, compressors, and natural phenomena. For example:

• Isothermal and adiabatic processes help in designing efficient heat engines.
• Isobaric processes are important in phase changes like boiling and melting.
• Isochoric processes are studied in sealed containers and in constant-volume calorimetry.

Note: The first law of thermodynamics (ΔU = Q − W) applies to all these processes, linking internal energy change with heat and work.

MCQ Quiz

1. For an adiabatic process in an ideal gas, which statement is correct?
   a) Q ≠ 0    b) PV = constant    c) PV^γ = constant ✅ Answer: c
2. In an isochoric process, the boundary work W equals:
   a) PV    b) 0 ✅    c) nRT Answer: b
3. Which process keeps pressure constant?
   a) Isothermal    b) Isobaric ✅    c) Adiabatic Answer: b
4. In an isothermal process for an ideal gas, temperature change ΔT is:
   a) Positive    b) Negative    c) Zero ✅ Answer: c
5. Over a complete cycle, the change in internal energy is:
   a) Positive    b) Negative    c) Zero ✅ Answer: c

Tip: For ideal gases, γ = C_p/C_v (≈1.4 for diatomic gases like air).

True / False

1. Isothermal processes have ΔT = 0. — True ✅
2. Adiabatic processes always have Q = 0. — True ✅
3. In an isobaric process, ΔH equals heat at constant pressure. — True ✅
4. Isochoric processes do maximum work. — False ❌ (W = 0)
5. In a cycle, ΔU ≠ 0. — False ❌ (ΔU = 0 over a full cycle)

Frequently Asked Questions (FAQs)

Which thermodynamic process has no heat exchange?

An adiabatic process has no heat exchange with the surroundings, so Q = 0.

Why is work zero in an isochoric process?

In an isochoric process, the volume remains constant (ΔV = 0), hence boundary work W = PΔV = 0.

Which process is best represented by PV^γ = constant?

The equation PV^γ = constant represents an adiabatic process for an ideal gas.

In which process does temperature remain constant?

In an isothermal process, the temperature remains constant, so ΔT = 0.

Why is ΔU zero in a cyclic process?

In a cyclic process, the system returns to its initial state. Since internal energy is a state function, the net change is ΔU = 0.

Chemistry Branch: Physical Chemistry → Thermodynamics → Thermodynamic Processes

• Khan Academy – Thermodynamic Processes