The first law of thermodynamics arose from the conservation of energy principle. The first law, even though it implies that we cannot create or destroy energy, places no limits on how energy can be transformed from one form to another. Thus, on the basis of the first law, heat can be transformed into work, work into heat, work can be done at the expense of internal energy, and so on. However, if no other laws existed the first law would allow certain phenomena to happen that never happen in reality. For example, consider a heavy body falling on the ground. We will observe that during the impact the body will warm. The opposite phenomenon according to which a body at rest on the ground begins to rise by itself while it is cooling is impossible. Similarly, no engine has yet been built which, for instance, would receive heat from the sea, transform it to work, and then set a ship in motion. Both the above examples are not in disagreement with the first law since the work would be done at the expense of the internal energy of the soil or the sea. The impossibility of these phenomena is due to the second law of thermodynamics, often hailed as the supreme law of nature. We will start our discussion of this law with the following example.

The Carnot cycle

The Carnot cycle is a thermal engine. A thermal engine is one that receives from some source an amount of heat, part of which it transforms into work.