Relationship Between Heat and Work

Energy is the ability to do work or move against a resistance. Conversely, work is the overcoming of resistance through a certain distance by the expenditure of energy.

Work is measured by a standard unit called the foot-pound, which may be defined as the amount of work done in raising one

Pound the distance of one foot, or in overcoming a pressure of one pound through a distance of one foot (Figure 2-1).

The relationship between work and heat is referred to as the mechanical equivalent of heat; one unit of heat is equal to 778.26 ft-lb. This relationship (i. e., the mechanical equivalent of heat) was first established by experiments conducted in the nine­teenth century. In 1843 Dr. James Prescott Joule (1818-1889) of Manchester, England, determined by numerous experiments that when 772 ft-lb of energy had been expended on 1 lb of water, the temperature of water had risen 1°F and the relationship between heat and mechanical work was found (Figure 2-2). The value 772 ft-lb is known as Joule’s equivalent. More recent experiments give higher figures and the value 778 (1 Btu = 778.26 ft-lb). (See the preceding section.)

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers