The scientists, until the nineteenth century, 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 was added to an object, its temperature increased; and when caloric escaped from it, its temperature decreased.
However, nobody could detect this caloric: so it was assumed to be odorless, tasteless, mass less and invisible.
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Although caloric was such a mysterious substance, yet the caloric theory successfully explained many aspects of heat such as the ‘flow of heat’ from a hot to a cold body.
The French scientist Jacques Charles discovered that heat could be transformed into motion. Sir James Prescott Joule, a 19th century physicist later proved that a given amount of mechanical energy always produced the same amount of heat.
This gave rise to the concept of heat as a form of energy and to the development of the kinetic molecular theory. According to this theory, the heat that a body possesses is directly related to the kinetic energy, or energy of motion, of the molecules composing the body. The greater the kinetic energy involved, the hotter the body is. Heat, then, is energy of motion.
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Modern Concept of Heat:
Heat is a form of energy called thermal energy. Thermal energy is the total energy (kinetic energy, vibration energy and rotational energy) of all the individual molecules of which the given object is made.
When an object is heated, its thermal energy increases, i.e., its molecules begin to move more violently and its temperature rises. Thus, temperature is a measurable manifestation of the thermal energy of a body.
Measuring Heat Intensity:
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Temperature is closely related to heat. It indicates the ‘degree of hotness’ or ‘intensity of heat’ of a body. In solids, the molecules attract one another strongly and their positions are fixed. Their motion, therefore, consists of vibration around a fixed point.
The molecules of liquids can move about quite freely from one place to another, but not quite so strongly as the molecules of solids. The molecules of liquids, because they can move, are able to assume the shape of the containing vessel, though the force of gravity keeps them from rising above a certain level in the container.
In gases, the molecules are so far apart that the force of attraction between them is negligibly small. They move about even more freely than the molecules of liquids and occupy the entire volume of the container.
As solids or liquids or gases become hotter, they generally expand. As they become cooler, they generally contract. These effects provide an effective tool for measuring temperature.
When two bodies at different temperatures come in contact with each other, the hotter body becomes cooler and the colder becomes warmer, till they attain the same temperature. We say that the body at higher temperature has lost some heat while that at lower temperature has gained some heat.
Heat flows from a body at a higher temperature to one with a lower temperature. No heat flows when the two bodies are at the same temperature. The state when no heat flows from one body to another is known as the state of thermal equilibrium.
It would be relevant to note here that heat is the internal energy transferred from one body to another due to temperature difference. Thus, heat is energy in transit; after heat has been transferred to a body, it becomes the internal energy (total kinetic and potential energy of the molecules) of the body. Heat and temperature are different things. The SI unit of heat is joule (J).
Thermometers and Thermometric Scales:
The instruments designed to measure temperature are called thermometers.
The Basic Principle of Thermometer:
When the temperature of a solid, liquid, or gas changes, a measurable physical change is imparted to a physical element in the thermometer. In liquid filled and gas thermometers, this physical change takes the form of thermal expansion.
In thermometer, the most widely used scales are the Fahrenheit (°F) and the Celsius (°C).
Specific Heat:
Equal masses of different substances require different amounts of heat to be heated through the same range of temperature.
The specific heat of a substance is the amount of Heat required raising the temperature of one gram of the substance through 1°C.
The unit of specific heat is calorie per gram per degree Celsius (Cal/g/°C) or kilocalorie per kilogram per degree Celsius (K cal/kg/°C).
Effects of Heating
Expansion and Contraction:
All substances, whether solid, liquid or gas, expand on heating and contract on cooling.
The expansion of a solid may be expressed in terms of one, two, or all three of its dimensions.
If the expansion is in one dimension only, then it is called linear expansion.
If the expansion is in two dimensions, then, it is called a real expansion.
If the expansion is in three dimensions (i.e. volume) then, it is called cubical expansion.
Different metals have different coefficients of expansion; one metal will expand more than another when it is subjected to the same degree of heat.
The Coefficient of Linear Expansion a:
It is defined as the change in length of a solid per unit original length per degree rise in temperature. The Coefficient of Areal Expansion B: It is defined as the change in the area of a solid per unit original area per degree rise in temperature. The Coefficient of Cubical Expansion y: It is defined as the change in volume of a solid per unit original volume per degree rise in temperature.