 # The First Law of Thermodynamics

We saw that potential energy can turn into kinetic energy. And we saw that kinetic energy can cause radiation. And so on... The three forms of energy can change back and forth from one form to another. However, the total amount of energy in a system remains constant. In other words, if we start with a certain total amount of energy, we will keep that same total amount of energy in our system as long as we don't add any more energy to it or remove any.

Now, what is a system? A system is a part of the world that we choose to look at. A system can be a cup of coffee. It can be a car. Or it can even be a spot that doesn't have any real boundaries, like a small part of an ocean. If we can imagine our system being surrounded by an invisible bubble, then we can define it as a system.

We said above that the energy in a system must balance. This means that the amount of energy that comes into the system must be balanced by an equal amount of energy leaving the system if the amount of energy is to stay the same. Mathematically,

DU + DEk + DEp = Q + Ws