The laws of thermodynamics

Remember that scientific laws are not proven from principles, they are just statements shown to be true via repeated experiments.


The zeroth law:

If two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.

The first law:

The energy of an isolated system remains constant.

The second law:

The entropy of an isolated system increases in the course of a spontaneous change.

The third law:

The entropy of a perfect crystal approaches 0 as the temperature approaches 0.


Definitions:

Isolated system: System which matter or energy is not being transferred into. The universe is seen as an isolated system. Smaller things can be modeled as an isolated system, such as a insulated box.

Spontaneous: Happens without interference, such as the transfer of heat from a hot object to a cold object, or some phases changes like the evaporation of water.

Entropy: Often vaguely called a "measure of disorder", or more precisely, the "the number of possible states at the same energy level". Increasing the volume of a gas will increase entropy, since higher volume mean more possible locational arrangements of gas molecules. Increasing temperature will too, since it allows kinetic, vibrational and rotational energy levels to be filled in more possible states. In thermodynamics these possible states are called microstates. Microstates are ways of arranging a system without changing macrostates such as temperature and volume.

Consider a group of atoms in a small separate universe. Being a gas will increase the volume of the atoms, increasing entropy. But exothermic bond formation from being becoming a solid would increase temperature, hence also increasing entropy. How do we know what increase in entropy is higher, and hence what will spontaneously happen?

The answer to this kind of question is given by equations, coming in a later post.

Open system: Energy and matter can transfer between the systems and surroundings

Closed system: Energy but not matter can be transferred between the system and surroundings.

Extra note:

The second law can also be written as "changes which increase entropy happen spontaneously" or "entropy always increases". This has interesting consequences for speculation about the future of the universe in the idea of a "heat death", which is the universe reaching the state of maximum entropy - a dark endless sea of photons.

This the theme of "The Last Question", widely regarded as Isaac Asimov's best short story. It is available here.