Heat is one of the most important concepts of thermodynamics. Heat concept is very closely related to work concept. Heat and work are both forms of transfer of energy. The difference between heat and work is that they are different forms of energy transfer. Heat is associated with direct contact between bodies or with the radiant transfer of energy while heat involves a rise in temperature of a body, which is determined by the energy of microparticles contained in a body.
British scientist James Joule conducted an experiment with an aim of determining the relationship between the amount of work spent to bring about the production of heat and determine the amount of heat liberated. The apparatus consisted of a calorimeter inside a box to avoid heat loss to the surrounding. It also consisted a spindle carrying brass paddles which acted like churner and were capable of turning between fixed vanes. The spindle was attached to a drum, which was in turn rotated by falling weights, attached to two pulleys. Two vertical scales were fixed to note vertical distances by which the weights fall.
A known mass amount of water in a calorimeter was churned but not allowed to rotate due to firm vanes. The potential energy of falling weights was converted to kinetic energy of paddles. As a result of friction derived from paddles, KE is converted to heat and the temperature of water in calorimeter rises, which is measured by use of a thermometer. The process was repeated several times such that the temperature of the water would be accurately measurable. He found that in all cases, 4186 joules of work (w) produced the same amount of heat (H) which would raise one kilogram of water by one degree centigrade.
Symbolically, the relation can be written as w/H=J. He, therefore, concluded that heat is a form of energy.