The Laws of Thermodynamics

There are four laws of thermodynamics. But instead of having a first, second, third and fourth law we have a zeroth law to start with!


define fundamental physical quantities (temperature, energy, and entropy) that characterise thermodynamic systems at thermal equilibrium.

describe how these quantities behave under various circumstances, and

forbid certain phenomena (such as perpetual motion).


Zeroth law of thermodynamics

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

This law helps define the notion of temperature. When in thermal equilibrium there is no net heat transfer. heat moves from hot to cold areas so they must be at the same temperature.

First law of thermodynamics

When energy passes, as work, as heat, or with matter, into or out from a system, the system's internal energy changes in accord with the law of conservation of energy.

This means that perpetual motion machines (of the first kind) are impossible.

ΔUsystem = Q - W


Q = heat

W = work done on the system

U = internal energy of the system

Second law of thermodynamics

In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases.

Equivalently, perpetual motion machines (of the second kind) are impossible.

The second law of thermodynamics indicates that natural processes lead towards spatial homogeneity of matter and energy, and especially of temperature. This movement implies that processes are irreversible.

The existence of a quantity called the entropy of a thermodynamic system is implied - choas or complexity - the 'mixing up' of materials - a movement away from order, separation into a single kind and regimentation.

When two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium with itself but not with each other, are then allowed to interact, they will eventually reach a mutual thermodynamic equilibrium.

The sum of the entropies of the initially isolated systems is always less than or equal to the total entropy of the final combination - it is rather like haveing the contents of two drawerers merged into one big drawer - more confusion - more dificult to locate things.... less order!.

Equality can only occur if the two original systems have all their respective intensive variables (temperature, pressure etc.) equal; then the final system also has the same values.

This statement of the second law is founded on the assumption, that in classical thermodynamics, the entropy of a system is defined only when it has reached internal thermodynamic equilibrium (thermodynamic equilibrium with itself).

The second law is applicable to a wide variety of processes, reversible and irreversible. All natural processes are irreversible. Reversible processes do not occur in nature. A good example of irreversibility is in the transfer of heat by conduction or radiation. It was known long before the discovery of the notion of entropy that when two bodies initially of different temperatures come into thermal connection, the heat always flows from the hotter body to the colder one.

Entropy is given the symbol 'S'.


Third law of thermodynamics

The entropy of a system approaches a constant value as the temperature approaches absolute zero.

With the exception of non-crystalline solids (glasses) the entropy of a system at absolute zero is typically close to zero, and is equal to the logarithm of the product of the quantum ground states.