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Exam level8 past questions

Thermodynamic Processes and P-V Diagrams

Study different thermodynamic processes (isothermal, isobaric, isochoric, adiabatic, polytropic, cyclic), their definitions, characteristics, and graphical representation on P-V diagrams.

Why this shows up in the exam

NEET questions frequently require you to identify, analyze, and compare processes using P-V diagrams and process definitions.

How NEET tests this

Numerical · 6 QsStatement analysis · 3 QsDirect recall · 3 Qs

Learn the idea

Thermodynamic processes are distinguished by which variable (P, V, T, or Q) stays constant; the key insight is that each has a simple algebraic relation (PV=const, P=const, V=const, PV^γ=const) that lets you jump from one state to another on a P‑V diagram.

🧠 Memory hook: I A P C – Isothermal, Adiabatic, Pressure‑constant, Constant‑volume – each letter reminds you which variable stays fixed and the corresponding formula.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Isothermal: temperature constant → PV = constant
  • Isobaric: pressure constant → work = PΔV
  • Isochoric: volume constant → work = 0
  • Adiabatic: no heat exchange → PV^γ = constant (γ = Cₚ/Cᵥ)
  • Cyclic process: net work = area enclosed by the path on a P‑V diagram
  • Polytropic: PVⁿ = constant, n is the polytropic index

How to approach it

  1. 1Read the statement or diagram and label each segment as isothermal, isobaric, isochoric or adiabatic
  2. 2Write the appropriate relation for that segment (PV=const, P=const, V=const or PV^γ=const)
  3. 3Use the given initial state to find the unknown pressure or volume at the end of the segment
  4. 4If work is asked, integrate P dV or use geometric area; for a cycle, take the enclosed area
  5. 5Combine successive relations to reach the final required quantity

Worked example — watch it click

A monoatomic gas at a pressure P, having a volume V expands isothermally to a volume 2V and then adiabatically to a volume 16V. The final pressure of the gas is (take γ=5/3):

  • A)64 P
  • B)32 P
  • ✅P/64
  • D)16 P

The concept behind this problem

The example forces you to apply two different relations in succession – first PV=const for the isothermal stretch, then PV^γ=const for the adiabatic stretch – exactly what the definition of those processes demands.

Step by step

  1. 1Isothermal: P₁V₁ = P₂V₂ gives P(V) = P₂(2V), so P₂ = P/2.
  2. 2Adiabatic: P₂V₂^γ = P₃V₃^γ gives (P/2)(2V)^(5/3) = P₃(16V)^(5/3).
  3. 3Solving: P₃ = (P/2)(2/16)^(5/3) = (P/2)(1/8)^(5/3) = (P/2)(1/32) = P/64.

Watch out

Students often forget to update the pressure after the isothermal step and keep using the original P, which gives a wrong final pressure.

Common slip-ups that cost marks

  • •Treating any expansion as adiabatic without confirming Q=0 – irreversible adiabatic still has Q=0 but the relation PV^γ=const holds only for reversible adiabatic
  • •Mixing up the sign of work – work done by the system is positive (area under the curve)
  • •Assuming work in an isochoric step – it is always zero

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 8NEET 1999

A thermodynamic system is taken from state A to B along ACB and is brought back to A along BDA as shown in the P-V diagram. The net work done during the complete cycle is given by the area:

Diagram for this question

Push further

More challenging

8 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 8

An ideal gas is taken through a cyclic process involving an isothermal expansion, followed by an isochoric cooling, and finally an adiabatic compression back to its initial state. Which of the following statements is true regarding this cycle?

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