Joseph Gibbs, age 13, of Lowell, Mass., for his question:
CAN YOU EXPLAIN HEAT?
Heat in physics is the transfer of energy from one part of a substance to another, or from one body to another, by virtue of a difference in temperature. Heat is energy in transit. It always flows from a substance at a higher temperature to the substance at a lower temperature.
Five different temperature scales are in use today: the Celsius scale, known also as the centigrade scale, the Fahrenheit scale, the Kelvin scale, the Rankine scale and the international thermodynamic temperature scale.
The centigrade scale, with a freezing point of zero degrees and a boiling point of 100 degrees, is widely used throughout the world, particularly for scientific work, although it was superseded officially in 1950 by the international temperature scale. In the Fahrenheit scale, used in English speaking countries for purposes other than scientific work and based on the mercury thermometer, the freezing point of water is defined as 32 degrees and the boiling point as 212.
Heat is measured in terms of the calorie, defined as the amount of heat necessary to raise the temperature of one gram of water by one degree Celsius. The word "calorie" comes from the Latin word "calor," which means "heat." A kilogram calorie equals 1,000 calories.
This unit is sometimes called the small or gram calorie to distinguish it from the large calorie, or kilocalorie, which is used in nutrition studies.
In mechanical engineering practice in the United States and Great Britain, heat is measured in British thermal units (BTU). One BTU is the quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit and is equal to 252 calories.
The physical methods by which energy in the form of heat can be transferred between bodies are conduction and radiation. A third method is called convection.
Conduction requires physical contact between the bodies or portions of bodies exchanging heat, but radiation does not require contact or the presence of any matter between the bodies. Convection occurs when a liquid or gas is in contact with a solid body at a different temperature and is always accompanied by the motion of the liquid or gas.
The science dealing with the transfer of heat between bodies is called heat transfer.
A number of physical changes are associated with the change of temperature of a substance. Almost all substances expand in volume when heated and contract when cooled.
The phase of a substance refers to its occurrence as either a solid, liquid or gas and phase changes in pure substances occur at definite temperatures and pressures. The amount of heat required to produce a change of phase is called latent heat, and hence, latent heats of sublimation, melting and vaporization exist.