#### What is heat, definition of heat as a form of enery? Properties of heat energy, information about effects of heat.

For many years scientists have wondered about the nature of heat. In the 18th- century some scientists proposed that heat was an invisible, weightless fluid called caloric. According to this theory, caloric soaked into an object when the object was heated and a drained away the when the object was cooled. It could flow from warmer objects to colder ones. This theory explained some of the things that people had observed about heat. For example, the heat from the flame of a burning candle will flow in to a metal bar which is held above the flame.

But this theory did not explain why the masses of substances did not change when they were heated or cooled. In 1798 experiments performed by Thompson showed that heat could not be a fluid, or any kind of matter. 40 years later, James Prescott Joule performed a series of experiments that showed that objects in motion produced heat. In fact, we are all famillar with this effect. For example, when you rub your hands together they warm up. Or, when you hammer a piece of metal, it gets hot.

At the same time scientists found that energy must be used to produce motion. Finally Joule concluded that heat is a form of energy. Other scientist working at this time had suggested that matter is made up of tiny pacticles called atoms and molecules. It was concluded that “heat is related to the motion of molecules or atoms, or heat itself is molecular motion”. Friction also produces heat energy. The work done to overcome the friction between surfaces appears as heat energy.

Heat energy causes the atoms or molecules of a substance to move faster and bump into each other more often. Therefore, the heat given to an object is stored in the motion of the molecules, and the kinetic energy of its molecules increases. On the other hand, when a substance loses heat, molecular motion becomes less and the kinetic energy of the molecules decreases. Since molecules are very small it is not possible to watch the changing of the speed of the molecules when an object is heated orcooled. But a simple experiment may give you an idea about the molecular motion when an object is heated or cooled.

EFFECTS OF HEAT
Heat may cause a change in the:
a- temperature of a body
b~ size of a body
c- physical state of a body
d- chemical constitution of a body
e- electrical state of a body

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