The laws of thermodynamics
The first law of thermodynamics says energy is conserved: the change in a system's internal energy equals the heat added minus the work done by it.
Practise this conceptThe first law of thermodynamics is conservation of energy dressed up for gases and engines: a system's internal energy changes by exactly the heat added to it minus the work the system does on its surroundings. Nothing appears from nowhere, and nothing simply vanishes.
Heat and work are two different ways of transferring energy into or out of a system, and the first law is precisely the statement that they're interchangeable in their effect on internal energy — you can raise a gas's internal energy by heating it, by compressing it with a piston, or with some combination of both, and the law tracks exactly how much each contributes.
A classic trap is treating heat and work as though the first law can't relate them — it's actually built entirely around relating them. Compressing a gas quickly with no heat exchanged (an adiabatic process) still raises its internal energy and its temperature, purely through the work being done on it, which is exactly why a bicycle pump gets warm when you use it.
Key ideas
Requires: Ideal gas law, Conservation of energy
Unlocks: Entropy and the second law
Formulas
| Symbol | Name | Unit |
|---|---|---|
| internal energy | J (joule) | |
| heat | J (joule) | |
| work | J (joule) |
Common misconceptions
- Work done on a system always increases its internal energy, but heat added never does.
- Heat and work are completely different quantities that the first law cannot relate to each other.
- A system's internal energy can be created from nothing if enough work is done on it.