That energy is also something which cannot be created or destroyed is not so generally recognized. Transformations of energy from one form to another are constantly occurring before our very eyes; and yet we seldom stop to think what the conservation of energy means in any given case. Energy itself is often defined as that which has the capacity for doing work, and work is done when force or resistance is overcome. A hod carrier does work when, overcoming the force of gravity upon his body and his hod of brick, he climbs to the top of a ladder; and the work done is a measure of the energy expended. Energy stored up in his body has been transferred to the brick in their elevated position, and if they are allowed to fall to the ground their energy is turned into heat, developed by their impact upon the ground. Again, work is done by a windmill in pumping water up into an elevated reservoir, and the so-called ‘potential’ energy which the water possesses in its elevated position has all been transferred to the water from the wind which drove the mill. If the water be allowed to flow down to the ground again through a water motor the latter could drive machinery and so do work; and the work it could do plus the heat produced by friction would exactly equal the work done in pumping the water up to its elevated position. Thus is the energy conserved, and not destroyed. More or less of it is dissipated by friction, and lost, so far as useful effect may go. But it all remains in existence, somewhere.
Again, coal is burned under the boiler of a steam engine. Heat is produced, steam is generated, the engine does work. The coal possessed a store of energy, potentially. That is, the coal had the capacity of uniting with the oxygen of the air and setting free a store of energy. This energy, potential or latent in the coal, becomes kinetic and evident in the heat of the boiler and the work of the engine. Moreover, the work done by the engine added to the heat given off by the boiler and engine is exactly equal to the total store of energy possessed by the coal. And if from a store of energy, either in the body of a man or horse, or in a pile of wood or coal, a certain portion is expended in doing work, the amount remaining is exactly the difference between that expended and the original amount. In short, energy can be measured, stored up and expended, just as truly as merchandise or money.
Thus the conservation of energy means that energy cannot be created or destroyed; but it may be transferred from one body to another or transformed from one form to another. Heat may be converted into work and work into heat. The chemical energy of a zinc rod may be expended to generate an electric current, and the latter passing through a coil of wire or the filament of a lamp gives up its energy to produce heat and light. The last form of this energy is equal in quantity to the first.
Niagara represents a vast store of energy. Millions of tons of water falling 160 feet could do a vast amount of mechanical work if properly applied through water wheels. More than 50,000 horse power of useful work is actually derived from Niagara’s waters, but this is only a small fraction of the total. The energy is, however, given up in falling, even though no useful work is done. In fact, the water is slightly heated by the impact, and the amount of heat produced is exactly equivalent to the mechanical energy lost by the water.
A cannon ball receives a large amount of kinetic energy from the exploded powder as it leaves the muzzle of a great gun. If it be suddenly stopped by a rigid target its mechanical or mass energy is at once converted into heat; that is, into the vibratory motion of the molecules. Ball and target are highly heated. Indeed, lead bullets are often melted by the heat of impact. Meteors flying through space come into our atmosphere and their speed is checked by its resistance. Part or all of their kinetic energy is thus converted into heat. Both air and meteor are heated; heated to so high a temperature that the meteor becomes brilliantly luminous, and we call it a shooting star. The idea of heat due to frictional resistance is common enough. The exact equivalence between the mechanical energy lost and the heat produced is the thing to be especially noticed here.
Let us now take as a final example a locomotive engine. It takes on a store of fuel and water and, directed by its engineer, sets out for a day’s duty. The coal to be burned possesses a definite amount of energy. Let us say every pound has one unit of energy, and suppose 5,000 pounds of coal are taken. What becomes of these 5,000 units of energy, appearing as heat when the coal is burned?
1. A large amount of heat is required to keep the boiler and engine hot, due to the loss of heat to the atmosphere. The engine cylinders, as well as fire box and boiler, must be kept very hot; other parts of the engine become more or less heated. All parts therefore continually give off heat, and a large part of the heat produced by the burning coal is thus expended.
2. A second portion is expended in doing work. If our locomotive hauls a 500-ton train up a one-per cent. grade for 100 miles it would be doing 2,640,000 foot-tons of work in addition to that required to overcome the friction of the rails and the resistance of the atmosphere. This would require nearly 500 units of energy which would come from the heat of the coal. The work is done through the agency of steam, but the energy of the steam comes from the burning coal. A small amount of work is also done in pumping water from the tank on the tender into the boiler and in pumping air into the reservoir for the use of the air brakes. This may be called the internal work of the engine. A second portion of the heat is therefore expended in internal and external work.
3. The steam after expanding in the cylinders of the engine escapes into the atmosphere. Although it has been cooled somewhat by expansion, it is still hot, and carries a large amount of heat away with it. Moreover, the smoke and hot air which pass out through the smokestack carry away a large quantity of heat. Hot ashes likewise carry away heat. Hence a third portion of heat is lost through smoke and steam and ashes. And this is the largest portion of the total quantity of heat generated by the burning coal.
When coal is burned, oxygen of the air unites chemically with the carbon and hydrogen of the coal to form carbonic acid, or carbon dioxid, as it is technically called, and water vapor. The incombustible mineral matter of the coal remains as ashes. Hence smoke contains carbonic acid gas and water vapor in addition to fine particles of unburned coal carried away in the draft of air.