# First Principle of Thermodynamics

Thermodynamics is the branch of physical science concerned with heat and its relation to other forms of energy and work; it is studied in processes in which there is transfer as heat and energy.

It defines macroscopic variables (such as temperature, entropy, and pressure) that describe average properties of material bodies and radiation, and explains how they are related and by what laws they change with time. Thermodynamics does not describe the microscopic constituents of matter, and its laws can be derived from statistical mechanics.

First law of Thermodynamics: A change in the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the amount of work done by the system on its surroundings.

A system is any object or set of objects we want to consider. The rest of the universe, everything that does not belong to the system, known as “environment”. There are several types of systems:

• Closed: no mass enters or leaves, although energy can be exchanged with the environment.
• Open: Mass can enters or leaves.
• Isolated: energy does not pass in any form through its borders.

The first law 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 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 of conservation of energy, which states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.

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The way that is applied to the thermodynamics is:

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Where U is the internal energy of the system (isolated), Q is the amount of heat supplied to the system and W is the work done by the system.

There are several types of processes depending on the magnitude it remains constant, which can be classified into:

• Isothermal: the temperature is to remain constant, so:

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• Adiabatic: it is one that does not allow that the heat flows to the system or from it, so:

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• Isobaric: it is one in which the pressure is kept constant, so:

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• Isovolumetric or isochoric is one in which the volume remains constant, so:

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• Isentropic: it is one in which the entropy remains constant.

FORMULA SUMMARY:

To avoid confusion it has been set criteria signs:

• Heat: absorbed positive and negative detached.
• Work: positive when it is done by the system and negative when it is held against the system although IUPAC states it oppositely.

First law of Thermodynamics

(ΔU = variation of internal energy ; Q = heat ; W = mechanical work)

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Heat-Work Equivalence

(J = 4,1855 J/cal ; A = 0,2389 cal/J)

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Fundamental equation of Calorimetry

(Q = Heat evolved or absorbed ; m = mass ; c = specific heat ; Δt = temperature variation)

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Mayer’s Relation

(Cp = gas heat at constant pressure ; Cv = gas heat at constant volume ; R = 8,3144 J/K.mol = 1,986 cal/K.mol)

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Thermal equilibrium

(c = heat of the solid ; m1 = water mass ; m2 = mass of the solid ; mc = water equivalent of the calorimeter ; t1 = initial water temperature ; t2 = initial solid temperature ; t = equilibrium temperature)

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Relationship between thermometric scales

(C = temperature in degrees Celsius ; F = temperature in degrees Fahrenheit ; T = temperature in degrees Kelvin)

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