Internal energy, heat and Work 

         (1) Internal energy (E) : Every system having some quantity of matter is associated with a definite amount of the energy. This energy is called as internal energy.

         (i) Characteristics of internal energy

         (a) The internal energy of a system is the extensive property.

         (b) The internal energy is the state property.

         (c) The change in internal energy does not depend on path by which the final state is reached.

         (d) There will be no change in the internal energy in the cyclic process.

         (e) The internal energy of the ideal gas is a function of temperature only.

         (f) Internal energy of the system depends upon the quantity of the substance, its temperature, chemical nature, pressure and volume.

         (g) The unit of E is ergs in CGS or joules in SI

       1 Joule =  ergs.

         (ii) Change in internal energy (ΔE ) : It is neither possible nor necessary to calculate the absolute value of internal energy of a system then,  ΔE = E_f - E_in  ;  is positive if E_f > E_in and negative if E_f < E_in.

         (2) Heat (q) and work (w): The energy of a system may increase or decrease in several ways but two common ways are heat and work.

         Heat is a form of energy. It flows from one system to another because of the difference in temperature between them. Heat flows from higher temperature to lower temperature. Thus, it is regarded as the energy on move.

         Work is said to be performed if the point of application of force is displaced in the direction of the force. It is equal to the force multiplied by the displacement (distance through which the force acts).

         There are three main types of work which we usually come across.  These are electrical work Gravitational work and mechanical work.

         The mechanical work = Force  displacement

          The Electrical work = the potential difference  charge

         Gravitational work = mgh

         (i) Units of heat and work : The heat changes are measured in calories (cal), Kilo calories (kcal), joules (J) or kilo joules (kJ). These are related as, 1 cal = 4.184 J; 1kcal = 4.184kJ

         The S.I. unit of heat is joule (J) or kilojoule. The Joule (J) is equal to Newton - metre (1 J= 1 Nm).

   Work is measured in terms of ergs or joules. The SI unit of work is given by Joule.

                        1 Joule =     10⁷ ergs = 0.2390 cal.

                        1 cal   > 1 joule > 1 erg

         (ii) The sign conventions for the heat and work

         Heat absorbed by system = q positive

         Heat evolved by system = q negative

         Work done on the system    = w positive

         Work done by the system    = w negative.       

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