Heat and Temperature

Heat consists of ceaseless movement of the tiny particles, called molecules, of which all substances are made. In the air and in gases the molecules are comparatively far apart and can move freely within the space they occupy. They collide with one another and bump into and bounce off the walls of their container, and the higher the temperature the faster they move. A person in the open air feels hot or cold according to the speed of these molecules as they bombard his skin. If they are moving fast he feels hot and if slow he feels cold.

In liquids and solids, the molecules are more closely crowded together, thus they cannot move so freely and they collide more often. The frequent collisions become extremely rapid vibrations, and the pain felt when touching a hot iron is due to the sudden and violent movement of the molecules of the skin caused by the rapid vibration of the molecules of the iron.

The movement of particles in a liquid can be seen if water containing fine dust particles is put in a glass tank and lit up. When viewed through a microscope, the dust particles can be seen to dart about the tank as they are bombarded by the millions of rapidly moving but invisible molecules of water.

Heat is not the same thing as temperature, although it is a common mistake to think so. A thermometer which is used to measure the temperature, measures hotness but not heat although the boiling water in a kettle is hotter than the water in a hot bath, because its temperature is 212 degrees Fahrenheit as against about 100 degrees Fahrenheit, there is much more heat in the water in the bath. This becomes clear from comparing the number of heat units needed to heat the 2 amounts of water supposing they both start at the same temperature of 60 degrees Fahrenheit. The kettle holds two pounds of water, and to boil that amount we need two times or 304B.Th.Us. The bath holds 150 pounds of water and to heat it to 100 degrees will require 150 times or 6000 B.Th.Us. The same point is explained by the fact that an electric kettle takes roughly twice as long to boil a quart as it does to boil a pint.

The quantity of heat in an object is a measure of the energy of the movement of all its atoms and molecules. It must therefore depend on the number of molecules contained in the object as well as on the temperature, which depends only on the speed at which the molecules move. If the molecules stop moving, the body would contain no heat and its temperature would be the lowest possible. This temperature called absolute zero, is minus 273 degrees centigrade i.e. 273 degrees below zero. Absolute zero has never been reached, although scientists have produced temperatures within one-tenth of a degree of it. Remarkable changes take place in substances at these very low temperatures. For instance, an electric current once started goes on flowing in a conductor almost indefinitely because the resistance of the conductor disappears and no chemical action can take place between substances because their molecules, being almost stopped cannot collide.

At the other end of the scale, there is no limit known to the height to which temperature can rise. In the oxy acetylene flame, and in electric arc welding processes, temperatures from 1800 to 4000 degrees centigrade are reached, and temperature of several million degrees have been reached by nuclear scientists.

Joseph Black, a Scottish scientist of the 18th century, was one of the first to make clear the difference between heat and temperature. He noticed that when ice is melted, the newly formed water is no warmer than the remaining ice, although much heat is needed for melting it. He invented the calorie meter, metal can of known weight and material used for measuring gains and losses of heat, and found that no less gains and losses of heat, and found that no less than 80 calories were needed to melt one gram of ice into water at the same temperature. He called this amount the 'latent heat of fusion'. In the same way he found that 540 calories were needed to change a gram of water at boiling point into steam. A cupful of ice needs 16,000 calories to melt it to water, another 20,000 to heat it to boiling point and finally 1,08,000 calories more to turn it into steam. These large 'latent heat' quantities account for the length of time ice and snow remain after a thaw has started. That is why a kettle does not become red hot when there is water in it.

Heat and Temperature

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