22.Energy. -The capacity for doing work, for overcoming resis- tance, for causing change, is called energy. If the position of a body or its composition is such that it can exert force or overcome resistance, its energy is called potential. If the body is actually moving, it is said to have kinetic energy.
When a pendulum bob is pulled aside and held higher than the lowest point of its arc, it has potential energy. If it is allowed to swing, it will change this potential energy into kinetic. The potential energyit had when at its highest point is changed into the same amount of kinetic energy when itreaches the lowest point in its swing.
Fig. 19.
When a gun is loaded the powder has potential energy due to its composition, but when it explodes this is changed into kinetic energy which is imparted to the bullet. The smallest possible piece of nitroglycerine has potential energy on account of the arrangement of its molecules, and this is capable of being readily changed into kinetic energy.
The sun throughout its existence has been sending vast quantities of energy to the earth. This energy has been mostly in the form of heat and light. The ability of the earth to support plant or animal life or to furnish man the power necessary to carry on his industries is due to the energy furnished by the sun. Plants cannot grow without the energy furnished by the sunlight, and animals could not live were it not for the energy furnished them by the plants.
For untold ages plants utilized the sun"s energy and stored it up. It was preserved in the remains of plants in the form of coal. This coal is now being burned to furnish power to carry on man"s industries. The water which the sun has evaporated and carried by cloud and shower to the mountain lake is stored there and has potential energy. It is ready to run down the valleys changing its potential energy into kinetic and doing work. We often think that there are many different sources of energy such as wood, coal, oil, waterpower and others, but when these are traced back, their energy is found to have come from one source, the sun.
Energy may readily be changed into different forms, as when the steam engine transforms the energy in coal into mechanical energy, or when this mechanical energy is changed by the dynamo into electricalTRANSFORMATION OF ENERGY.
energy. The most careful investigations have shown, however, that although its form may change, energy can never be destroyed. Mechanical energy frequently changes into heat, as when two surfaces are rubbed together, but when these two kinds of energy are carefully measured there is found to be no loss. This great truth has been determined by a vast amount of most careful investigation and is called the law of conservation of energy.
23.Heat and Light. -Every one realizes the importance of theheat and light given to the earth by the sun. If plants or animals are where light is entirely excluded, they begin to sicken and die. If they are placed where it is very cold, they freeze and die. Although the sunFig. 20.
gives both heat and light, yet these two are not inseparable. We feel the heat given out by boiling water but there is no light, and we see the light of the moon but there is no heat. We usually say that we feel heat but cannot see it and see light but cannot feel it.
24.eat.
Experiment 17. -Fit a glass flask with a one-holerubber stopper through which passes a glass tube about 20 cm. long. Place this on a ringstand so that the end of the tube extends down intoa bottle nearly filled with water. Gently heat the flask. The air expands and bubbles rise in the water. When the flask cools, the air contracts and water rises in the tube.
Experiment 18. -Fill the flask used in the last experiment with colored water. See that the end of the glass tube passing through the rubber stopper is just even with the bottom of the stopper. Smear the lower part of the stopper with vaseline and insert it in the flask, being careful that the flask and afew centimeters of the tube are filled with colored water andFig. 21.
that there are no air bubbles in the flask. Mark, by slipping over a rubber band, the end of the water column in the tube. Heat the flask. The water expands. Why do water heaters always have a pipe at the top leading to a tank?
Experiment 19. -Pass the ball of a ball-and-ring apparatus through the ring. Notice how closely it fits. Heat the ball in a Bunsen flame for several minutes. See if the ball will now go through the ring. Explain why it does not.
Fig. 22.
Experiment 20. -Heat a metal compound-bar. It bends over on one side. The more the bar is heated the more it bends. The two metals do not expand at the same rate. Why are the ends of steam pipes allowed to be free andFig. 23.
not attached firmly? Why are the ends of the spans of long iron bridges placed on rollers? When iron tires are fitted to wheels they are heated and then placed on the wheels and allowed to cool. Why? Platinum is the onlysubstance that can be used to pass through the glass in an incandescent lamp. Other metals do not expand the same as glass and when they are fused with it and allowed to cool they break it.
When heat was first studied it was thought to be an invisible fluid without weight which worked itself into bodies and caused them to expand in the same way that water affects a sponge or a piece of wood. This fluid was supposed to be driven out by pounding or rubbing. Even the primitive savages knew that fire could be obtained by rubbing two dry sticks together.