Seedha Jawab: In Chemistry, the system is the focus: work done on the system is positive ($w > 0$), work done by the system is negative ($w < 0$), and the First Law is $\Delta U = q + w$. In Physics, the output work done by the gas is the focus: work done by the system is positive ($W > 0$), work done on the system is negative ($W < 0$), and the First Law is $\Delta U = Q - W$. Both frameworks are mathematically identical; mixing them up during mock tests causes an average of 8 to 10 lost marks.
Every NEET dropper has experienced this exact moment of mental paralysis:
On Tuesday morning, you revise Class 11 Physics Heat and Thermodynamics. You solve a problem where an ideal gas expands and pushes a piston. Your Physics teacher tells you: “Work done by the gas during expansion is positive ($W = +P\Delta V$), so internal energy drops: $\Delta U = Q - W$.” You solve 20 questions, get them all right, and feel confident.
On Wednesday evening, you open Class 11 Physical Chemistry Thermodynamics. You encounter the exact same ideal gas expanding against external pressure. Your Chemistry teacher writes: “Work done by the system during expansion is negative ($w = -P\Delta V$), so the First Law is $\Delta U = q + w$.”
Your brain shorts out. Why are the signs completely inverted? Is work positive or negative? Which formula do you apply on Sunday?
The Root of the Clash: Who Does the Accountant Work For?
The physics and chemistry equations look contradictory only because they adopt two different points of view. Think of it like a bank transaction:
┌──────────────────────────────────────┬──────────────────────────────────────┐
│ CHEMISTRY (IUPAC Perspective) │ PHYSICS (Engine Perspective) │
├──────────────────────────────────────┼──────────────────────────────────────┤
│ Focus: The System (The Gas) │ Focus: The Engine (The Surroundings) │
│ │ │
│ • Heat absorbed by system: +q │ • Heat added to system: +Q │
│ • Heat released by system: -q │ • Heat lost by system: -Q │
│ • Work done ON system (compression): │ • Work done BY system (expansion): │
│ w > 0 (system GAINS energy) │ W > 0 (engine DELIVERS work) │
│ • Work done BY system (expansion): │ • Work done ON system (compression): │
│ w < 0 (system LOSES energy) │ W < 0 (surroundings do work) │
│ │ │
│ FIRST LAW: │ FIRST LAW: │
│ ΔU = q + w │ ΔU = Q - W │
│ where w = -P_ext ΔV │ where W = +P_ext ΔV │
└──────────────────────────────────────┴──────────────────────────────────────┘Notice the underlying mathematical truth: both equations yield the exact same change in internal energy ($\Delta U$)!
When a gas expands, it loses internal energy to push the surroundings.
- In Chemistry: $w$ is negative, so $\Delta U = q + (-|w|) = q - |w|$.
- In Physics: $W$ is positive, so $\Delta U = Q - (+|W|) = Q - |W|$.
The physical reality is identical. Only the algebraic bookkeeping changes.
The 3 Golden Rules for Exam Hall Execution
To ensure your hand never hesitates between 2:00 PM and 5:00 PM on test day, lock in these three operational rules:
- Check the Question Paper Section First:
- If the question appears in the Chemistry Section (Questions 46–90): Strictly write $\Delta U = q + w$ and $w = -P_{\text{ext}}\Delta V$. Work done by gas expansion is negative.
- If the question appears in the Physics Section (Questions 1–45): Strictly write $\Delta U = Q - W$ and $W = P\Delta V$. Work done by gas expansion is positive.
- Watch for the Free Expansion Trap:
In an insulated container ($q = 0$) expanding into a vacuum ($P_{\text{ext}} = 0$):
- $w = 0$ (no resistance to push against).
- $q = 0$ (adiabatic).
- Therefore, $\Delta U = 0$ and $\Delta T = 0$ (for ideal gases).
This holds true in both Physics and Chemistry. Free expansion does zero work.
- Isothermal vs Adiabatic Work Magnitudes:
For a given expansion from $V_1$ to $V_2$:
$$|w_{\text{isothermal}}| > |w_{\text{adiabatic}}|$$
Why? Because in an isothermal expansion, heat continuously enters the system to maintain temperature and pressure. In an adiabatic expansion, the system cools as it works, causing pressure to drop faster.
Next Steps for Your Revision Desk
For the master framework and formula matrix, study our flagship guide on Physical Chemistry Formulas for NEET: Ratta vs Concept, and inspect The 5 High-Yield Physical Chemistry Chapters That Deliver 12–14 Questions in NEET. If you struggle with physics numericals alongside chemistry, explore our roadmap on NEET Physics from Zero: How to Cross 140+ Marks and review The Formula Boundary Trap to protect your Sunday mock scores.