#### What is the usage of heat, heat energy and how do people make advantage of the heat energy? Importance and control of heat.

For many years, scientists studied heat and tried to explain it. According to one theory that they worked out, heat was believed to be some kind of invisible fluid. This fluid was named caloric from a Latin word that means heat. Caloric was thought to flow from one thing into another, much as water soaks into a sponge. When an object was heated, it soaked up a large amount of this invisible fluid. Then as it cooled off, the fluid left it in some way. The space taken up by the caloric made the object expand when heated and contract when cooled. At the time, this seemed like a good theory. It not only explained how materials became heated and cooled but also why heating and cooling caused the expansion and contraction of materials.

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However, scientists discovered other facts that the caloric theory of heat could not explain. For example, if heat were a material, heating and cooling should not only change the volume of an object but also its weight. That is, an object should weigh more after heating, or adding heat, and less after cooling, or removing heat. Yet careful experiments failed to show any gain or loss of weight when materials were heated or cooled. To explain this, caloric was said to be a weightless fluid. But some scientists began to doubt that caloric even existed. For it did not explain why materials became heated by friction. So scientists had to work out a different theory of heat.

Scientists now understand that what we call heat is not an invisible, weightless kind of matter that flows into and out of things. Instead, heat is a form of energy. Though it does seem to flow, it has no weight that can be measured. You know that many facts about matter are explained by the molecular theory, which states that matter itself is made up of very tiny particles called molecules. The molecular theory also helps explain what heat is and how it acts. Scientists now are quite sure that heat is actually the motion of the molecules in matter.

According to the present theory of heat, what we call cold is only a lack of heat, just as darkness is what we have when there is no light. We sometimes say that cold goes into a material when we really should say that heat is going out of it. Scientists explain both heat and cold by talking about heat and what it does. When a material is heated, its molecules gain energy and move faster. Its temperature rises, and it feels warmer. As the molecules move faster, they bump into each other with more force and knock each other farther apart. So the material expands, or it may melt or evaporate. When a material is cooled, just the opposite happens. Its molecules lose some of their energy and move more slowly. Its temperature falls, and it feels cooler. As the molecules move more slowly, they come closer together. So the material contracts, or it may freeze or condense.

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Methods of heating our buildings and keeping things cool have changed as much as theories about heat. Because we have learned how to use and control heat, our lives are much more comfortable and healthy today.