the transfer of heat and the performance of work may both cause the same effect in a system. heat and work are different forms of the same entity, called energy , which is conserved. energy which enters a system as heat may leave the system as work ,or energy which enters the system as work may leave as heat.
let us consider a closed system which consists of a known mass of water contained in an adiabatic vessel having a thermometer and a paddle wheel, as shown in fig. 4.1. let a certain amount of work w1-2 be done upon the system by the paddle wheel. the quantity of work can be measured by the fall of weight which drives the paddle wheel through a pulley.