HOW HEAT IS PRODUCED 



121 



an electrical current passes through a substance that 

 is a poor conductor, much of the electrical energy is 

 changed to heat energy. Electricity is a very clean and 

 convenient source of heat and is being used more ex- 

 tensively every day. In the home we have our electric 

 cookers, toasters, coffee percolators, flatirons, and 

 ranges. In some industries electric ovens and furnaces 

 have entirely replaced the older combustion types. 



Heat energy is obtained from friction and compres- 

 sion. We know from our everyday experiences that 

 friction produces heat. When we saw a board, drill a 

 hole, or hammer a nail, the tools become warm. We 



following table shows several of our common fuels 

 classified in this manner. 



FUELS 



Courtesy General Electric Company 



FIG. 186. ELECTRIC RANGE 



rub our hands together in order to warm them when 

 we are outside on a cold day. A match is ignited by 

 the heat produced when it is scratched over a rough 

 surface. In each case mechanical energy is converted 

 into heat energy. 



Heat is also produced by compression. If you have 

 ever pumped up a tire with an air pump you probably 

 have noticed that the piston chamber becomes hot. 

 Most of this heat is caused by the compression of the 

 air. It has been discovered that the temperature in- 

 side the earth rises as we go toward the center at the 

 rate of about one degree centigrade for every hun- 

 dred feet depth. This heat is believed to be due to 

 compression caused by the force of gravity. 



What are the common fuels used to supply heat 

 energy? Not all materials will burn, and because of 

 this fact substances are classified as combustible and 

 non-combustible materials. Fuels are materials that 

 burn readily, producing large quantities of heat at 

 relatively low cost. We are familiar with three general 

 types of fuels classified by the state of matter in which 

 they are commonly used as solid, liquid, or gas. The 



100-1 



90- 



\% Wind 

 2.5% Animal 

 , muscle 

 f 6.3% Water 

 power 



5.5% Firewood 



The heat energy of com- 

 bustion comes from the 

 uniting of elements in the 

 fuel with oxygen from the 

 air. Most of our common 

 fuels contain the elements 

 carbon and hydrogen, 

 which unite with oxygen 

 in the process of burning 

 to form carbon dioxide in 

 the one case and water va- 

 por in the other. 



Exercise. Can you sug- 

 gest a simple method of 

 testing the last statement 

 made in the paragraph 

 above? 



Carbon dioxide and wa- 

 ter vapor are known as 

 products of combustion. 

 Many fuels contains some 

 elements or compounds 

 that do not unite with oxy- 

 gen. During the burning 

 these useless substances 

 are driven off into the air 

 as gases or remain as 

 ashes. 



Exercise. What are the 

 chief energy sources in the 

 United States? Carefully 

 study the data shown in the 

 graph, Figure 187, and find 

 the answers to the following 

 questions. 



iriiat percentage of the 

 energy available in this 

 country is obtained from hard or anthracite coal. 



What percentage from bituminous or soft coal? 



What percentage from natural gas? 



What percentage from oil? 



What percentage from firewood? 



Answer the following questions if you can. 



20- 



10- 



o-i 



FIG. 187. SOURCES OF ENERGY 

 IN THE UNITED STATES 



