Laws of thermodynamics
The laws of thermodynamics are of great importance to physics, chemistry and engineering, since they restrict what a device or process can physically achieve. For example, the impossibility of perpetual motion is a consequence of these laws.
The first law of thermodynamics states that energy is conserved. When a system goes from one equilibrium to another, the change of its energy is equal to the work done by external forces, plus the energy transferred by heating () or cooling ():
The internal energy is a state variable, so is independent of the path taken between equilibria. However, the work and heating do depend on the path, so the first law means that the act of transferring energy is path-dependent, but the result has no “memory” of that path.
The second law of thermodynamics states that the total entropy never decreases. An important consequence is that no machine can convert energy into work with 100% efficiency.
It is possible for the local entropy of a system to decrease, but doing so requires work, and therefore the entropy of the surroundings must increase accordingly, such that:
Since the total entropy never decreases, the equilibrium state of a system must be a maximum of its entropy , and therefore can be used as a thermodynamic “potential”.
The only situation where is a reversible process, since then it must be possible to return to the previous equilibrium state by doing the same work in the opposite direction.
According to the first law, if a process is reversible, or if it is only heating/cooling, then (after one reversible cycle) the energy change is simply the heat transfer . An entropy change is then expressed as follows (since by definition):
Confusingly, this equation is sometimes also called the second law of thermodynamics.
The third law of thermodynamics states that the entropy of a system goes to zero when the temperature reaches absolute zero:
From this, the absolute quantity of is defined, otherwise we would only be able to speak of entropy differences .
- H. Gould, J. Tobochnik, Statistical and thermal physics, 2nd edition, Princeton.