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Exam level7 past questions

Laws of Thermodynamics

Understand the zeroth and first laws of thermodynamics, including their statements, implications, and applications to physical systems.

Why this shows up in the exam

NEET tests your ability to recall, interpret, and apply these fundamental laws to solve conceptual and numerical problems.

How NEET tests this

Numerical · 4 QsStatement analysis · 2 QsMulti-concept

Learn the idea

Zeroth law tells us that temperature is the property that becomes equal when two bodies are in thermal equilibrium; First law states that for a closed system the change in internal energy equals heat added to the system minus work done by the system.

🧠 Memory hook: Zeroth = “Zero” temperature difference; First = “First” check – Q in minus W out gives ΔU

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

Formulas & facts to keep ready

  • Zeroth law: If A is in thermal equilibrium with B and B with C, then A is in thermal equilibrium with C; temperature is the indicator of equilibrium (NCERT)
  • First law for a closed system: ΔU = Q – W (Q positive when added, W positive when done by the system)
  • Heat is energy transfer due to temperature difference; work is energy transfer due to macroscopic forces
  • For an ideal gas the internal energy depends only on its temperature
  • In a cyclic process ΔU = 0, therefore ΣQ = ΣW

How to approach it

  1. 1Read the statement carefully and note what quantities are given (Q, W, temperature, etc.)
  2. 2Choose the appropriate law – zeroth for temperature equality, first for energy balance
  3. 3Write the equation using NCERT sign convention (ΔU = Q – W or dU/dt = dQ/dt – dW/dt) and substitute the numbers
  4. 4If the process is cyclic set ΔU = 0 and relate total heat and work

Worked example — watch it click

An electric heater supplies heat to a system at a rate 100 W. If the system performs work at a rate of 75 J/s, then the rate at which the internal energy increases is:

  • A)75 W
  • ✅25 W
  • C)100 W
  • D)125 W

The concept behind this problem

The example asks for the rate of increase of internal energy, so you must apply the first law in its rate form and use the NCERT sign convention for heat and work.

Step by step

  1. 1By the first law of thermodynamics: dU/dt = dQ/dt - dW/dt.
  2. 2Heat supplied dQ/dt = 100 W, work done dW/dt = 75 J/s = 75 W.
  3. 3Therefore, rate of increase of internal energy dU/dt = 100 - 75 = 25 W.

Watch out

Students often add the work term instead of subtracting it, giving 175 W instead of the correct 25 W.

Common slip-ups that cost marks

  • •Mixing up the sign of work – work done by the system is subtracted, not added
  • •Thinking the zeroth law talks about heat content; it only guarantees equal temperature
  • •Ignoring the work term and concluding ΔU = Q when work is also present

🌟 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 7

Assertion A: The zeroth law said that, when two systems A and B are in thermal equilibrium, there must be a physical quantity that has the same value for both. Reason R: The physical quantity which is the same for both systems is temperature.

Push further

More challenging

15 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 15

During an isothermal compression of an ideal gas, 30 J of work is done ON the gas. What is the change in the internal energy of the gas?

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