Same start, same end, different work
"Same start, same end, different work" is a free, interactive Class 11 Chemistry simulation for NEET and JEE, built on the NCERT section "Pressure-volume work" in Thermodynamics. A piston expands an ideal gas from V1 to V2 at constant T by one irreversible step, several steps, a reversible path or into vacuum; the p–V plot shades the work area and w, q, ΔU are read out with NCERT signs. NEET has asked about this idea 5 times in 10 years of papers. JEE Main asked 51 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Pressure-volume work in Thermodynamics. Change the values and watch what happens.
What it shows
NCERT § "Applications"
A piston expands an ideal gas from V1 to V2 at constant T by one irreversible step, several steps, a reversible path or into vacuum; the p–V plot shades the work area and w, q, ΔU are read out with NCERT signs.
Controls
4 to try
- Segmentedpath one-step / three-step / reversible / free expansion
- Sliderexternal pressure 0–2 bar (one-step)
- Sliderfinal volume 10–40 L
- Toggleexpand / compress
Try this
Take 0.40 mol at 300 K from 10 L to 20 L doing the most work on the surroundings (reversible, w ≈ −6.9 L bar); then make a path with w = 0 (free expansion).
Common mistake
W = +pΔV sign; free expansion does work; reversible and irreversible work are equal.
In NEET
10 years of papers
Asked 5 times in 10 years of NEET.
See the past questions: First Law of Thermodynamics, Thermodynamics (chapter)
In JEE Main
42 shifts analysed
51 questions from Thermodynamics across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT § "Applications"
- For expansion or compression against a constant external pressure, w = −pₑₓ(V_f − V_i) = −pₑₓΔV.
- In expansion ΔV is positive, so w is negative (the system does work); in compression ΔV is negative, so w is positive.
- A reversible process proceeds through a series of equilibrium states, with the external pressure differing from the internal pressure only infinitesimally at every step.
- Reversible isothermal expansion of an ideal gas: w_rev = −2.303 nRT log(V_f/V_i).
- Reversible expansion does more work on the surroundings than an irreversible expansion between the same two states.
- Free expansion is expansion into a vacuum (pₑₓ = 0), so no work is done; for an ideal gas ΔU is also zero because it has no intermolecular attractions.
- For isothermal changes of an ideal gas ΔU = 0, so q = −w: irreversible q = pₑₓ(V_f − V_i), reversible q = 2.303 nRT log(V_f/V_i), free expansion q = 0.
- Work of 1 L bar equals 100 J.
Questions
5 answers
- What does the "Same start, same end, different work" simulation show?
- A piston expands an ideal gas from V1 to V2 at constant T by one irreversible step, several steps, a reversible path or into vacuum; the p–V plot shades the work area and w, q, ΔU are read out with NCERT signs.
- What mistake does the "Same start, same end, different work" simulation fix?
- A common mistake it clears up: W = +pΔV sign; free expansion does work; reversible and irreversible work are equal.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 5 times in 10 years of papers.
- Does JEE Main ask questions from Thermodynamics?
- Yes. JEE Main asked 51 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Pressure-volume work" in Thermodynamics (Class 11 Chemistry).
Embed this simulation
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