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Thermodynamics states a set of four laws which are valid for all systems that fall within the

constraints implied by each. In the various theoretical descriptions of thermodynamics these

laws may be expressed in seemingly differing forms, but the most prominent formulations are

the following:

Zeroth law of thermodynamics:If two systems are each in thermal equilibrium with a third,

they are also in thermal equilibrium with each other.

This statement implies that thermal equilibrium is anequivalence relationon the set

ofthermodynamic systemsunder consideration. Systems are said to be in thermal equilibrium

with each other if spontaneous molecular thermal energy exchanges between them do not lead

to a net exchange of energy. This law is tacitly assumed in every measurement of temperature.

For two bodies known to be at the sametemperature, if one seeks to decide if they will be in

thermal equilibrium when put into thermal contact, it is not necessary to actually bring them into

contact and measure any changes of their observable properties in time.[44]

In traditional

statements, the law provides an empirical definition of temperature and justification for the

construction of practical thermometers. In contrast to absolute thermodynamic temperatures,

empirical temperatures are measured just by the mechanical properties of bodies, such as their

volumes, without reliance on the concepts of energy, entropy or the first, second, or third laws of

thermodynamics.[45][46]

Empirical temperatures lead tocalorimetryfor heat transfer in terms of

the mechanical properties of bodies, without reliance on mechanical concepts of energy.

The zeroth law was not initially labelled as a law, as its basis in thermodynamical equilibrium

was implied in the other laws. By the time the desire arose to label it as a law, the first, second,

and third laws had already been explicitly stated. It was impracticable to renumber them, and so

it was numbered the zeroth law.

First law of thermodynamics:A change in theinternal energyof a closedthermodynamic

systemis equal to the difference between theheatsupplied to the system and the amount

ofworkdone by the system on its surroundings.[47][48][49][50][51][52][53][54][55][56]

The first law of thermodynamics asserts the existence of a state variable for a system, the

internal energy, and tells how it changes in thermodynamic processes. The law allows a given

internal energy of a system to be reached by any combination of heat and work. It is important

that internal energy is a variable of state of the system (seeThermodynamic state) whereas

heat and work are variables that describe processes or changes of the state of systems.

The first law observes that the internal energy of an isolated system obeys the principle

ofconservation of energy, which states that energy can be transformed (changed from one formto another), but cannot be created or destroyed.

[57]

Second law of thermodynamics:Heat cannot spontaneously flow from a colder location to a

hotter location.

The second law of thermodynamics is an expression of the universal principle of dissipation of

kinetic and potential energy observable in nature. The second law is an observation of the fact

that over time, differences in temperature, pressure, and chemical potential tend to even out in

a physical system that is isolated from the outside world.Entropyis a measure of how much this

process has progressed. The entropy of an isolated system which is not in equilibrium will tend

to increase over time, approaching a maximum value at equilibrium.

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In classical thermodynamics, the second law is a basic postulate applicable to any system

involving heat energy transfer; in statistical thermodynamics, the second law is a consequence

of the assumed randomness of molecular chaos. There are many versions of the second law,

but they all have the same effect, which is to explain the phenomenon ofirreversibilityin nature.

Third law of thermodynamics:As a system approaches absolute zero, all processes ceaseand the entropy of the system approaches a minimum value.

The third law of thermodynamics is a statistical law of nature regarding entropy and the

impossibility of reachingabsolute zeroof temperature. This law provides an absolute reference

point for the determination of entropy. The entropy determined relative to this point is the

absolute entropy. Alternate definitions are, "the entropy of all systems and of all states of a

system is smallest at absolute zero," or equivalently "it is impossible to reach the absolute zero

of temperature by any finite number of processes".

Absolute zero, at which all activity (with the exception of that caused by zero point energy)

would stop is 273.15 C (degrees Celsius), or 459.67 F (degrees Fahrenheit) or 0 K (kelvin).

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