19. HEAT AND ENERGY

by E. N. da C. Andrade

It was the great service of Joule to prove the exact equivalence of heat and mechanical work: that wherever mechanical work apparently disappears, not only is heat generated, but an amount of heat exactly proportional to the energy that seems lost. The work can be done in various ways: by rubbing pieces of metal together, or by stirring water, or--what is important for us - electrically, by pushing electric charges through a wire by electromotive force, or, in more usual words, by passing an electric current through a resistance.

The work done in stirring water in the ordinary way - say in stirring a cup of coffee - is, of course, very small, and consequently no appreciable amount of heat is generated: stirring the hot coffee actually makes it cooler, because fresh layers of hot liquid are brought into contact with the cold air and further evaporation is encouraged. If, however, complicated paddles are used, in conjunction with vanes protruding from the side of the vessel, it can be made quite difficult to move the paddles and, corresponding to the greater work done, larger quantities of heat are generated, which can be accurately measured. It is interesting to note that when J. R. Mayer, who, round about 1842, was one of the earliest to realise the equivalence of heat and work, was explaining his idea, his friend Jolly objected: "But in that case water ought to get warmer if you shake it," which appeared to him absurd. Mayer left without saying a word, but entered his friend's room some weeks later with the words: "So it does," assuming that his friend, like himself,

had been thinking of nothing else since. He had turned to experiment: he had asked Nature to decide. The celebrated Rowland in a most accurate and laborious research on the quantity of heat produced by a measured amount of energy used in stirring water, often raised the temperature of water by more than 20°C, and Reynolds and Moorby raised water to boiling-point by churning it.

Heat, then, is a form of energy, and not only is work turned into heat at all places where there is friction, including the "friction" of electricity passing through a wire (a process clearly visible in the electric lamp or electric radiator), but heat can be turned into work. In every steam-engine there is an actual disappearance of heat corresponding to the energy represented by the work done. If all the heat generated by the furnace is measured, and allowances made for the heat given to the condenser water, not as much heat is gained by the surroundings as if the same amount of fuel were burnt without driving an engine.

(from An Approach to Modern Physics, G. Bell and Sons, Ltd., London)