Revised 18 August 2021



  • All forms of energy are inter-convertible and when conversions occur, they do so according to rigorous laws of exchange.

Work & Heat

  • Energy is defined as 'the ability to do work on other bodies’ or as ‘stored work’.

  • Work is said to be done on a system if that system experiences a displacement as the result of a force parallel to and in the same direction as that force.

  • Heat is defined as that energy which is transferred bet­ween a system and its surroundings solely by vir­tue of a temperature difference

  • The unit of heat ener­gy is the cal­orie which is defined as that amount of heat whose absorption by 1 gram (gm) of water at constant atmospheric pressure is accom­panied by a temperature rise from 14.5 to 15.5 °C

  • The mechanical e­quivalent of heat is the joule where 4.185 joules is the amount of mechanical work re­quired to effect the same change in state as that produced by the absorption of one calorie of heat

  • One watt (W) is a unit of power as opposed to energy, being the rate of one joule of energy flow per second.

  • The degree of convertibility of energy - stored work - into applied work is often called availability.

First Law of Thermodynamics - Conservation of Energy 

  • The First Law of Thermodynamics: Energy may be transformed from one form into another, but energy is neither created nor destroyed.

Second Law of Thermodynamics

  • The Second Law of Thermodynamics: All physical processes proceed in such a way that the availability of the energy involved decreases.


  • In a low entro­py system the energy is free in the sense that it is avail­able for producing mech­anical work, whereas in a high entropy sys­tem the energy is said to be bound.

  • The law of entro­py: When all systems taking part in a process are included, the entro­py S of the total system either remains constant or increases.

Entropy as a Measure of Order and Disorder

  • An open system may exchange both energy and matter with the outside, whereas a closed system exchanges only energy and not matter with the outside.

  • Contrary to being a viol­ation to the law of entropy, life forms com­prise sys­tems that hasten the increase of entropy in the uni­verse

  • The Law of Entropy can be stated as: In spontaneous processes, concentrations tend to dis­perse, struc­ture tends to disappear, and order becomes disorder.

  • The First Law of Thermodynamics does not con­tra­dict the laws of mechanics, but the Second Law of Thermodynamics, the law of entro­py, is in direct contradiction with the laws of classical mech­anics in that the law of entropy introduces the element of an irrevocable quali­tative change when systems undergo any process.