Revised 24 May 2019

In thermodynamics, a sub-field of physics, the words ‘energy’ and ‘work’ have precise definitions with meanings which differ from colloquial use of these words. 

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. It is meaning­less to talk of ‘work in a sys­tem’ or ‘work of a sys­tem’. 

Work depends on the particular process by which a system is taken from a reference state to a second state. A result of mechanical work on, or by, a system can be a change in kinetic energy, a change in poten­tial energy of the system as in the case of compressing a spring, or dis­sipation into heat as in the case of boring out the centre of a cannon.

Heat is defined as that energy which is transferred bet­ween a system and its surroundings solely by vir­tue of a temperature difference. An early 19th century theory incorrectly regarded heat as a liquid called ‘caloric’. It is incorrect to refer to ‘heat in a body’ because heat is not a substance. Heat is energy in tran­sit. 

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. Calorie with a capital ‘C' is a unit which is used by nutritionists to describe the energy stored in food. One Calorie is equal to one kilocalorie (1,000 calories) with a small ‘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. The units of Calories, calories, and joules are all units of heat energy, and each unit can be converted from one unit to another using a conversion factor. 

One watt (W) is a unit of power as opposed to energy, being the rate of one joule of energy flow per second. One kilowatt (1 kW) being 1,000 watts is a more appropriate unit to describe electricity consumption. One kilowatt hour (kWh) is a unit which describes the electrical energy consumption by a household or firm at the rate of one kilowatt over one hour. This energy consumption can be converted to joules by multiplying the unit of power (watts) by the unit of time (seconds): 

One kilowatt hour = 1,000 watts x 60 minutes x 60 seconds = 3,600,000 joules (3.6 x 106 joules) or 3.6 megajoules (3.6 MJ)

Energy Conversion Table