The First Law of Thermodynamics Explained
You can’t create energy from nothing, and you can’t destroy it. Discover how heat, work, and internal energy interact to power the universe.
What is the First Law of Thermodynamics?
At its core, the First Law of Thermodynamics is simply the Law of Conservation of Energy adapted for thermodynamic systems. It states that energy cannot be created or destroyed; it can only be transferred or changed from one form to another.
If you add heat energy to a system (like heating a pot of water or the gas in a car engine), that energy doesn’t just disappear. It does exactly two things:
- Increases the internal energy of the system (makes it hotter or changes its state).
- Causes the system to do work on its surroundings (like expanding and pushing a piston).
The Formula: ΔU = Q – W
Change in Internal Energy
The total kinetic and potential energy of the molecules inside the system. Measured in Joules (J).
Heat Added
The thermal energy transferred into or out of the system. Measured in Joules (J).
Work Done
The mechanical work done BY the system on its surroundings. Measured in Joules (J).
The Sign Convention: Don’t Lose Points Here
The number one reason students get thermodynamics problems wrong is messing up the positive and negative signs. Memorize these rules:
| Variable | Positive (+) Means… | Negative (-) Means… |
|---|---|---|
| Heat (Q) | Heat is added TO the system | Heat is removed FROM the system |
| Work (W) | Work is done BY the system (expansion) | Work is done ON the system (compression) |
| Internal Energy (ΔU) | Temperature increases | Temperature decreases |
Real-Life Examples of the First Law
🚗 The Car Engine
When fuel burns inside an engine cylinder, it adds heat (Q). This heat increases the internal energy of the gas (ΔU), making it expand. The expanding gas pushes down on the piston, doing mechanical work (W) to turn the wheels.
💨 Pumping a Bicycle Tire
When you rapidly compress the air in a hand pump, you are doing work ON the system (negative W). Because the air is compressed quickly, little heat escapes. The result? The internal energy (ΔU) spikes, and the bottom of the pump gets noticeably warm.
Frequently Asked Questions
Can internal energy change if no heat is added?
Yes. This is called an adiabatic process (Q = 0). If the system does work (expands), it uses its own internal energy, causing ΔU to drop and the temperature to decrease.
What is the difference between the First and Second Law of Thermodynamics?
The First Law says energy quantity is conserved (you can’t win). The Second Law introduces entropy and says that energy quality degrades, meaning heat flows naturally from hot to cold (you can’t even break even).