Introduction to thermodynamics

Introduction to thermodynamics
A dictionary defines thermodynamics as"the science of the relations between heat and mechanical work",but it might more usefully be defined as the study of the changes in energy accompanying chemical and physical changes,which allows experimentally determined laws to be derived from certain basic principles,and helps to predict changes which have not been observed.It must be remembered that thermodynamics considers only the initial and final states of any system under going a change,and provides no information about the mechanism of the change between these states,or the rate at which the change takes place-this is the subject of reaction kinetics(where the change taking place is a chemical reaction).
The First Law of Thermodynamics.
This is a law based on experience,and is an extension of the principle of conservation of Energy wich states that energy cannot be created or destroyed,provided that ther is no measurable conversion of mass to energy:it can only be converted from one form to another.The consistency of Joules mechanical equivalent of heat is explained by this principle,and it is clear that if it obeyed,no machine can be devised which will do work without the expenditure of an equivalent amount of energy.In order to metal a given mass of ametal,acertain ammount of energy must be supplied-whether by combustion of afuel or the expenditure of electrical energy.
Thermodynamics specifies a "system"as any matter which is being considered which consists of a definite amount of a given substance or substance.The system will have its "surroundings"with which it can exchange energy ,and the system and its surroundings are considered to be isolated they cannot exchange energy with any other system.An example of such asystem would be a steel billet in a reheating furnace-the surroundings of the billet would include not only the structure of the furnace itself,but also the atmosphere inside and outside the furnace,theground on which the furnace stood ,and any other objects which could exchange a measurable amount of energy with the billet , either directly or indirectly.Thus,the First law of thermodynamics states that the total energy of asystem and its surroundings remains constant,evenif it may be changed from one form of energy to another.We can now introduce a quantity U ,which represents the total energy of asystem-whether it be kinetic,electrical,rotational,vibrational,or any other form of energy except the energy due to its position in space,which is assumed to be consta.U is called the internal or intrrinsic energy of the system and is not usually known quantitatively because of the difficulty of measuring all the different forms of energy possesed by actual systems.This is not important because we are concerned with changes in energy ,which can be measured.If the state of a system changes from A to B,for instance as aresult of achange in tempreature,then we say that the change in intrrnal energy is the internal energy in the final state,UB minus internal energy in the initial state UA.