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Heat is often confused with temperature, but the two are different concepts. Let us consider an example. If a red-hot piece of iron is dropped into a beaker of water, the temperature of water will increase and that of the piece of iron will decrease, until a thermal equilibrium is reached when both of them will have the same temperature. Hence, "something" is transferred from iron to water. This "something" that flows from a body at a higher temperature to one at a lower temperature, when placed in contact (or exposed to each other) is called heat.

Until the nineteenth century, scientists believed that heat was some kind of a fluid called caloric which flows like any actual fluid (liquid or gas). According to the caloric theory, every object has a certain amount of caloric in it. When caloric is added to an object, its temperature increases; and when caloric escapes from it, its temperature decreases. However, nobody could detect this caloric; so it was assumed to be odourless, tasteless, and invisible. Although caloric was such a mysterious substance, yet the caloric theory could successfully explain many aspects of heat.

One of the main troubles with the caloric theory was that it could not account for the heat generated by friction. By rubbing our hands together or by rubbing two pieces of metal together, we can generate heat indefinitely. If you rub the two metal pieces dipped in water in a beaker and if you have the patience and energy to continue rubbing, you may actually succeed in boiling water in the beaker. The amount of heat generated by friction seems limitless. This does not agree with the caloric theory which maintains that heat is a substance and, therefore, every object can contain only a fixed amount of it.

Figure - Joule’s paddle wheel experiment

It was Count Rumford (1753-1814) who rejected the idea that heat was a substance and said that heat could only be a kind of motion. It was an Englishman, James Prescott Joule (1818-1889), who performed the crucial experiment that settled the question. Joule was convinced that heat is a form of energy. His famous paddle wheel experiment is illustrated in the figure.

The slowly-falling weight does work on the paddle wheel, causing it to turn against the resistance (or friction) offered by the water. The potential energy lost by the falling weight is first converted into kinetic energy of the paddle wheel which, in turn, produces heat by friction with the water. By measuring the rise in the temperature of water, Joule could calculate how much heat was produced by a particular amount of mechanical work and came to the conclusion that a fixed amount of mechanical work (or energy) always produced a fixed amount of heat. More accurate experiments have shown that 4.18 J of mechanical work produces 1 cal of heat.

Objectives of this Lesson

  • Introduction of Measurement of Temperature
  • Absolute Temperature and Absolute Scale
  • Thermal Expansion
  • Specific Heat Capacities
  • Latent Heat Capacities

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