The laws of thermodynamics

In thermodynamics, there are four laws that do not depend on the details of the systems under study or how they interact. Hence these laws are very generally valid, can be applied to systems about which one knows nothing other than the balance of energy and matter transfer. Examples of such systems include Einstein's prediction, around the turn of the 20th century, of spontaneous emission, and ongoing research into the thermodynamics of black holes.

These four laws are:
Zeroth law of thermodynamics, about thermal equilibrium:
If two thermodynamic systems are separately in thermal equilibrium with a third, they are also in thermal equilibrium with each other.
If we grant that all systems are (trivially) in thermal equilibrium with themselves, the Zeroth law implies that thermal equilibrium is an equivalence relation on the set of thermodynamic systems. This law is tacitly assumed in every measurement of temperature. Thus, if we want to know if two bodies are at the same temperature, it is not necessary to bring them into contact and to watch whether their observable properties change with time.

First law of thermodynamics, about the conservation of energy:
The change in the internal energy of a closed thermodynamic system is equal to the sum of the amount of heat energy supplied to the system and the work done on the system.

Second law of thermodynamics, about entropy:
The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value.

Third law of thermodynamics, about the absolute zero of temperature:
As a system asymptotically approaches absolute zero of temperature all processes virtually cease and the entropy of the system asymptotically approaches a minimum value; also stated as: "the entropy of all systems and of all states of a system is zero at absolute zero" or equivalently "it is impossible to reach the absolute zero of temperature by any finite number of processes".

The following has sometimes been called the "Fourth Law of Thermodynamics", about the transfer of heat energy between systems.

Onsager reciprocal relations:
In connected thermodynamic systems which are in equilibrium neither for pressure nor temperature, heat flow between is caused by forces proportional with unit of pressure difference, and equal to the proportional density flow caused per unit of temperature difference.

(source: wikipedia)