What do the first and second laws of thermodynamics relate to?

Answer 1

The First Law is Conservation of Energy (stated in the language of Thermodynamics). Energy can neither be created nor destroyed.

The Second Law has many formulations; one of them is that entropy increases. Another is that there are irreversible processes in the Universe - irreversible in the sense of energy processes. In other words, useful energy is constantly being converted into unusable energy.

Answer 2

I like this version:

1. You cant' get something for nothing

2. Not only that - you can't even break even.

More formally, the first and second laws of thermodynamics are:

1. "In all cases in which work is produced by the agency of heat, a quantity of heat is consumed which is proportional to the work done; and conversely, by the expenditure of an equal quantity of work an equal quantity of heat is produced."

(Credit to Rudolf Clausius who first stated it this way in 1850)

The implication is that energy is conserved in an isolated system. (A n isolated system is one where no matter enters or leaves and no energy enters or leaves). It also implies that the energy of a closed system remains constant unless heat enters or leaves the system or work is done on or by the system. (A closed system is one where no mass enters or leave the system).

Energy cannot be created or consumed - only the form of the energy can be changed. This should be slightly amended with Einstein's famous equation

E = mc2

which means that energy and mass combined are conserved in an isolated system. Generally the conversion of mass to energy is not of concern - but in cases of nuclear fusion it becomes important.

2. No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature.

(Credit to Rudolf Clausius who stated it this way).

Again, this law has several implications such as that result of any spontaneous process within an isolated system always results in a net increase of the entropy of that isolated system. Since the universe is assumed to be an isolated system this means that all natural processes increase the entropy of the universe. Note that this only applies to the universe as a whole. Many natural processes cause a decrease in entropy locally while increasing the entropy of the surroundings.

Lord Kelvin stated it this way:

No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work.

This version is a formal statement of the earlier, more flippant "can't even break even" statement at the beginning of this answer.

The Second Law of Thermodynamics made it necessary to formulate the "Zeroth"and Third Laws.

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

Third Law: The entropy of a perfect crystal at absolute zero is zero:

S(0 K) = 0 for a perfect crystal

Answer 3

First law tells that heat energy can be converted into equivalent amount of work,but it is silent about the conditions under which this conversion takes place.Tha second law is concerned with the cicumstances in which heat can be converted into work and direction of flow of heat.