Difference Between Isothermal and Adiabatic Process (with Graph)
Physics · Thermodynamics · NEET
In an isothermal process the temperature stays the same (heat can flow in or out, PV = constant). In an adiabatic process no heat flows at all (Q = 0, PV^gamma = constant), so the temperature changes. Memory hook: "Isothermal = same Temperature, Adiabatic = No heat Allowed." On a P-V graph the adiabatic curve is always steeper because gamma is greater than 1.
Both curves start at state A and expand. The adiabatic curve (red, PV^gamma = constant) is steeper and drops below the isothermal curve (blue, PV = constant). Larger area under the isothermal curve means isothermal expansion does more work.
Your doubts, answered
Which curve is steeper on a P-V graph, isothermal or adiabatic?
The adiabatic curve is steeper. For isothermal, PV = constant, so the slope is proportional to -P/V. For adiabatic, PV^gamma = constant, so the slope is proportional to -gamma times P/V. Since gamma is greater than 1 (for example 1.4 for a diatomic gas), the adiabatic slope is gamma times bigger. This is exactly why NEET 2022 asked you to pick the steeper falling curve as the adiabatic one.
Does 'isothermal' mean no heat flows? People confuse it with adiabatic.
No, that is the most common mix-up. Isothermal means constant temperature, so heat DOES flow in or out to keep T fixed. Adiabatic means Q = 0, so no heat flows and the temperature changes instead. Remember: isothermal controls temperature, adiabatic blocks heat.
For the same compression, which needs more work, isothermal or adiabatic?
Adiabatic compression needs more work. When you compress adiabatically, no heat escapes, so the gas heats up and its pressure rises faster. The adiabatic curve climbs above the isothermal curve, so the area under it (the work) is larger. This is the NEET 2016 answer: adiabatic compression requires more work.
For the same expansion, which does more work, isothermal or adiabatic?
Isothermal expansion does more work. Both start from the same point A, but the adiabatic curve falls faster and lies below the isothermal curve. So the area under the isothermal path is bigger. NEET 2023 tested exactly this: W(isothermal) is greater than W(adiabatic).
In adiabatic expansion, does the gas get hotter or colder?
It gets colder. Since Q = 0, the gas does work using its own internal energy, so internal energy and temperature both drop. That is why in the NEET 2023 figure T at point C (end of adiabatic expansion) is less than T at A, not greater.
Why is internal energy change zero in isothermal but not adiabatic?
For an ideal gas internal energy U depends only on temperature. In an isothermal process T is constant, so delta U = 0 (and heat equals work). In an adiabatic process T changes, so delta U is not zero; instead Q = 0, so delta U = -W.
⚠️ The NEET trap ✗ Adiabatic process means the temperature is constant because no heat enters. ✓ No heat enters (Q = 0), but the temperature is NOT constant. With no heat exchange the gas uses its own internal energy, so T falls in expansion and rises in compression. The process with constant temperature is isothermal, not adiabatic. 🧠 Swap the labels and you lose the mark: 'No heat' is adiabatic, 'same temperature' is isothermal.
Real NEET questions
2016
A gas is compressed isothermally to half its initial volume. The same gas is compressed separately through an adiabatic process until its volume is again reduced to half. Then:
A · Compressing the gas isothermally will require more work to be done
B · Compressing the gas through an adiabatic process will require more work to be done ✓
C · Compressing isothermally or adiabatically requires the same work
D · Which case requires more work depends on the atomicity of the gas
Solution: On a P-V diagram the adiabatic curve (PV^gamma = constant) is steeper than the isothermal curve (PV = constant) because gamma is greater than 1. During compression, the adiabatic curve rises above the isothermal curve, since no heat escapes and the pressure increases more rapidly. Work in compression equals the area under the P-V curve. Because the adiabatic curve lies higher over the same volume range, its area is larger, so |W adiabatic| is greater than |W isothermal|. Answer: B.
2022
An ideal gas undergoes four processes from the same initial state (adiabatic, isothermal, isobaric, isochoric). The curve which represents the adiabatic process among 1, 2, 3, 4 is:
A · 1
B · 2 ✓
C · 3
D · 4
Solution: From the common start point, a vertical line is isochoric (constant V) and a horizontal line is isobaric (constant P). The two falling curves are isothermal and adiabatic. Isothermal slope is proportional to -P/V, adiabatic slope is proportional to -gamma times P/V. Since gamma is greater than 1, the adiabatic curve is steeper. The steeper falling curve is curve 2, so the adiabatic process is curve 2. Answer: B.
2023
Reversible expansion of an ideal gas is shown, where AB is isothermal and AC is adiabatic, both starting from state A. Which option is NOT correct?
A · S(isothermal) > S(adiabatic)
B · T(A) = T(B)
C · W(isothermal) > W(adiabatic)
D · T(C) > T(A) ✓
Solution: Check each. (A) Reversible isothermal expansion absorbs heat so delta S is positive, while reversible adiabatic has delta S = 0; so S(isothermal) is greater: correct. (B) AB is isothermal so T is constant, T(A) = T(B): correct. (C) The adiabat AC is steeper and lies below the isotherm AB, so area under AB is larger, W(isothermal) is greater: correct. (D) In adiabatic expansion the gas does work using internal energy, so temperature falls: T(C) is less than T(A), meaning T(C) > T(A) is wrong. Answer: D.
Solved Thermodynamics NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the main difference between isothermal and adiabatic process?
In an isothermal process the temperature is constant and heat can flow (PV = constant, delta U = 0). In an adiabatic process no heat flows (Q = 0), so the temperature changes and PV^gamma = constant.
Which is steeper on a P-V graph?
The adiabatic curve is always steeper because its slope is gamma times the isothermal slope, and gamma is greater than 1.
Is delta U zero in an adiabatic process?
No. In adiabatic, temperature changes, so delta U is not zero; instead Q = 0 gives delta U = -W. Delta U = 0 happens in the isothermal process.
Which process requires more work to compress a gas?
Adiabatic compression requires more work, because the gas heats up and pressure rises faster, giving a larger area under the curve.
Give one real example of each process.
A slow gas process in a very good conductor kept in a bath is nearly isothermal. A fast process in a well-insulated container, or sudden bursting of a tyre, is nearly adiabatic.
