The first law of thermodynamics says that energy can’t be created or destroyed, it can only change from one form to another. This means that the total amount of energy in a closed system (which means nothing can come in or go out) stays the same, even if the energy changes form.
For example, if you rub your hands together, they start to feel warm because the friction between your hands turns some of the energy from your body’s movement into heat energy. The first law of thermodynamics says that the total amount of energy in your hands and the air around them is the same before and after you rub them together, even though some of the energy has changed from movement energy to heat energy.
The first law of thermodynamics is important because it helps scientists understand how energy moves and changes in different systems, and how to predict those changes.
Mathematically, this can be written as
where \Delta U is the internal energy, \Delta W is the work done on the system, and \Delta Q is the heat added to the system.