the use of e.m.f

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.