Physics of Thermodynamics
Understand how heat, work and internal energy interact through thermodynamic laws, processes and PV diagrams for JEE Main Physics.
The central idea of this chapter
Thermodynamics is energy bookkeeping for a system. Define the system, fix the sign convention, identify the process and then apply the appropriate law.
What should you understand first?
A thermodynamic state is described by pressure, volume and temperature. Heat and work are energy transfers, while internal energy is a state function.
Core concepts
- Zeroth law and equilibrium High
- First law of thermodynamics High
- Heat and work High
- Internal energy High
- PV diagrams Medium
Key processes
- Isochoric W = 0
- Isobaric W = PΔV
- Isothermal ΔU = 0
- Adiabatic Q = 0
- Cyclic ΔU = 0
Thermodynamics formula sheet
Use one sign convention consistently. Here W means work done by the gas, so Q = ΔU + W.
| Topic | Formula / Relation | Meaning or use |
|---|---|---|
| Ideal gas equation | PV = nRT | Relates pressure, volume, temperature and amount |
| First law | Q = ΔU + W | Heat supplied equals increase in internal energy plus work done |
| Work done | W = ∫P dV | Area under the process curve on a PV diagram |
| Internal energy | ΔU = nCᵥΔT | For an ideal gas, internal energy depends only on temperature |
| Isochoric process | V constant | W = 0, therefore Q = ΔU |
| Isobaric process | P constant | W = P(V₂ − V₁) |
| Isothermal process | T constant | For ideal gas, ΔU = 0 and Q = W |
| Adiabatic process | Q = 0, PVᵞ = constant | No heat exchange with surroundings |
| Cyclic process | Initial = final state | ΔU = 0 and net Q = net W |
Choosing the right relation
Thermodynamics questions become manageable when the process and sign convention are identified before substitution.
Key stages
- Define the system Gas / piston
- Identify the process Iso-/Adia-
- Draw the PV path Area = W
- Apply energy conservation Q = ΔU + W
- Check sign convention W by gas
How to read a question
- Volume constant? W = 0
- Pressure constant? W = PΔV
- Temperature constant? ΔU = 0
- No heat exchange? Q = 0
- Closed cycle? ΔU = 0
How to prepare this chapter
Revise the laws, process relations and PV graphs, then practise mixed JEE Main questions.
What to do
- Start with system, surroundings and state variables
- Master the First law and sign convention for work
- Learn isochoric, isobaric, isothermal, adiabatic processes
- Practise PV graph and cyclic process questions
- Finish with chapter PYQs and a timed mock
Common mistakes
- Heat is not a state function; it is path-dependent
- Fix the work sign — expansion positive, compression negative
- Isothermal does not mean Q = 0; for ideal gas ΔU = 0
- Check whether W is done by gas or on gas
- Remember PVᵞ = constant only for adiabatic ideal gas
Ready for Thermodynamics?
Revise the laws, process relations and PV graphs, then practise mixed JEE Main questions.
Thermodynamics FAQ
Short answers to common JEE Main preparation questions.
The first law is conservation of energy applied to a thermodynamic system. With work done by the gas, Q = ΔU + W.
An isothermal process occurs at constant temperature. For an ideal gas, ΔU = 0 and heat supplied equals work done.
An adiabatic process has no heat exchange with surroundings, so Q = 0. For an ideal gas, PVᵞ remains constant.