First Law of Thermodynamics
The Intuition: Energy is a Bank Account
Imagine you have a bank account. You can deposit money into it, or withdraw money from it. The total amount of money in your account changes only by the net of what goes in and what comes out. You cannot create money from nothing, nor can you destroy it — it just moves.
Energy works the same way. In any physical or chemical process, energy is never created or destroyed. It is only transferred from one place to another, or converted from one form to another. This is the First Law of Thermodynamics — the law of conservation of energy, applied to systems where heat and work are the currencies.
When you heat a gas in a piston, the gas expands and pushes the piston up. The energy you put in as heat doesn't vanish — part of it stays inside the gas (raising its temperature), and part of it leaves as work done on the piston. The total energy of the universe remains constant.
The Precise Statement
ΔU=Q−W
Where:
- ΔU = change in the internal energy of the system (the energy stored inside — kinetic energy of molecules, potential energy in bonds, etc.)
- Q = heat added to the system (positive if heat flows into the system)
- W = work done by the system on the surroundings (positive if the system expands and pushes against something)
The sign convention is crucial. Many textbooks use Q+W with work done on the system. The version above (Q−W) is the most common in Indian exam syllabi (CBSE, JEE, NEET). Stick to one convention and be consistent.
Common Mistake
Students often forget the sign of work. If a gas expands, it does positive work on the surroundings — so W is positive, and ΔU=Q−W becomes smaller. If a gas is compressed, work is done on the gas — so W is negative, and ΔU=Q−(−∣W∣)=Q+∣W∣, which increases internal energy.
What Each Term Means Physically
Internal energy (U) is the total microscopic energy of the system. For an ideal gas, it depends only on temperature — higher temperature means higher U. For real substances, it also depends on volume and phase.
Heat (Q) is energy transferred due to a temperature difference. If you put a hot pan on a cold stove, heat flows from pan to stove. In thermodynamics, we always ask: who is the system? If the system is the gas, then Q is positive when heat flows into the gas.
Work (W) in thermodynamics is usually pressure-volume work: W=∫PdV. When a gas expands against a piston, it does work on the piston. When you compress a gas, you do work on it.
A Simple Example
Take a cylinder with a movable piston, containing 1 mole of an ideal gas. You supply 500 J of heat to the gas. The gas expands and does 200 J of work on the piston.
- Q=+500 J (heat enters the system)
- W=+200 J (work done by the system)
ΔU=500−200=300 J …