What does the first law of thermodynamics say?

1. The first law of thermodynamics states that, in a closed system, the change in the internal energy is equal to the difference of the supplied heat to the system and the work done by the system on its surroundings.
2. The mathematical representation of the first law of thermodynamics is, $∆U=Q-W$.
3. Here, $∆U$ is the change in internal energy, $Q$ is the heat supplied to the system, and $W$ is the work done by the system.
4. The first law of thermodynamics is similar to the law of conservation of energy.