The Four Laws of Thermodynamics

Zeroth law - It is the definition of thermodynamic equilibrium. When two systems are put in contact with each other, energy and/or matter will be exchanged between them unless they are in thermodynamic equilibrium. In other word, two systems are in thermodynamic equilibrium with each other if they stay the same after being put in contact.

The original zeroth law is stated as If A and B are in thermodynamic equilibrium, and B and C are in thermodynamic equilibrium, then A and C are also in thermodynamic equilibrium.

Thermodynamic equilibrium includes thermal equilibrium (associated to heat exchange and parameterized by temperature), mechanical equilibrium (associated to work exchange and parameterized generalized forces such as pressure), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential).


1st Law - This is the law of energy conservation. It is stated alternatively in many forms as follows:

The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.
or
The heat flowing into a system equals the increase in internal energy of the system minus the work done by the system.
or
Energy cannot be created, or destroyed, only modified in form.

The second statement can be expressed mathematically in the form of Eq.(1) with negative W representing work done by the system. The adiabatic process in the first statement refers to a system with no heat transfer, i.e., Q = 0.


2nd Law - It can be stated in many ways, the most popular of which is:

It is impossible to obtain a process such that the unique effect is the subtraction of a positive heat from a reservoir and the production of a positive work.
or
A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body.

The first statement is to exclude the un-realistic situations such as to drive a steamship across the ocean by extracting heat from the water, or to run a power plant by extracting heat from the surrounding air. The second statement expresses the impossibility of running refrigeration without work. Another form of the 2nd law states:


The entropy of an isolated system tends to remain constant or to increase. It is in this form that the arrow of time is defined. Figure 01b shows the various ways entropy can be added to a system.


3rd Law: This law explains why it is so hard to cool something to absolute zero:

All processes cease as temperature approaches zero.

This statement is expressed mathematically by Eq.(4), which shows that as the temperature T approaches zero the amount of heat extracted from the system also diminishes to zero. Thus, even using laser cooling would not be able to attain a temperature of absolute zero.