3. The stored energy becomes active (kinetic) as the force through which the energy was stored undoes the work, or puts the substance upon which the work was done into its former condition (gravity causing bodies to fall, etc.).
These principles are further illustrated by the
Storing of Energy through Chemical Means.—A good example of storing energy by chemical means is that of decomposing water with electricity. If a current of electricity is passed through acidulated water in a suitable apparatus (Fig. 82), the water separates into its component gases, oxygen and hydrogen. These gases now have power (energy) which they did not possess before they were separated. The hydrogen will burn in the oxygen,[pg 189] giving heat; and if the two gases are mixed in the right proportions and then ignited, they explode with violence. This energy was derived from the electricity. It was stored by decomposing the water.
Fig. 82—Storing energy by chemical means. Apparatus for decomposing water with electricity.
Energy is stored by chemical means by causing it to do work in opposition to the force of chemism, or chemical affinity. Instead of changing the form of bodies or moving them against gravity, it overcomes the force that causes atoms to unite and to hold together after they have united. Since in most cases the atoms on separating from any given combination unite at once to form other combinations, we may say that energy is stored when strong chemical combinations are broken up and weak ones formed. Energy stored by this means becomes active when the atoms of weak combinations unite to form combinations that are strong.[70]
How Plants store the Sun's Energy.—The earth's supply of energy comes from the sun. While much of this, after warming and lighting the earth's surface, is lost by radiation, a portion of it is stored up and retained. The sun's energy is stored both through the force of gravity[71][pg 190] and by chemical means, the latter being the more important of the two methods. Plants supply the means for storing it chemically (Fig. 83). Attention has already been called to the fact (page 112) that growing plants are continually taking carbon dioxide into their leaves from the air. This they decompose, adding the carbon to compounds in their tissues and returning the oxygen to the air. It is found, however, that this process does not occur unless the plants are exposed to sunlight. The sunlight supplies the energy for overcoming the attraction between the atoms of oxygen and the atoms of carbon, while the plant itself serves as the instrument through which the sunlight acts. The energy for decomposing the carbon dioxide then comes from the sun, and through the decomposition of the carbon dioxide the sun's energy is stored—becomes potential. It remains stored until the carbon of the plant again unites with the oxygen of the air, as in combustion.
Fig. 83—Nature's device for storing energy from the sun. See text.
The Sun's Energy in Food and Oxygen.—Food is derived directly or indirectly from plants and sustains the same relation to the oxygen of the air as do the plants themselves. (The elements in the food have an attraction for[pg 191] the oxygen, but are separated chemically from it.) On account of this relation they have potential energy—the energy derived through the plant from the sun. When a person eats the food and breathes the oxygen, this energy becomes the possession of the body. It is then converted into kinetic energy as the needs of the body require.