Metric Heat and Temperature
There are several kinds of energy. Potential energy is stored against a force such as a book on a high shelf where gravity can accelerate it; electrons stored in the components of a battery where charges can accelerate them; or a spring stretched or compressed where the elasticity of the metal can have an effect. The nuclei of large atoms can release energy by splitting and the nuclei of small ones by combining (these processes are called fission and fusion).
Kinetic energy always involves motion. The push of wind, the vibrations of sound, a moving automobile, a spinning top all have kinetic energy because of their motion.
The fundamental unit of energy in the metric system is the joule which is the energy required each second to push an ampere of current through an ohm of resistance. The energy in this case is changed into heat (since energy must be conserved) and in this case the conversion is 100% efficient.
Heat is the energy of molecular motion. All molecules move, so all matter (everything is made of molecules) contains heat energy. A vacuum (empty space) cannot have any heat energy because it doesn't have any molecules. Heat can be transferred from place to place by conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow the radiation to pass. Silver is the best conductor of heat. If you stir a cup of hot coffee or tea with a pure silver spoon, the handle gets hot very fast. Styrofoam is one of the worst conductors, therefore it is used as a heat insulator. Ceiling fans fight convection by blowing the hot air back down to the living space below. Most of the heat we get comes from the sun by radiation. Sunlight amounts to about one kilowatt of heat per square meter of the earth's surface. At low sun angles (extreme latitudes) it is considerably less.
Although the metric unit of energy is the joule, heat is commonly also measured in units called calories (there are about 4.19 joules in a calorie), or in larger units called Calories (note the capital C). A Calorie is 1000 calories, and should always be called a kilocalorie, but it is common practice in food labeling and nutritional references to simply call it a Calorie. The food Calorie is the kilocalorie.
A calorie is the quantity of heat required to raise the temperature of one gram of water, one Celsius degree. (Celsius used to be called centigrade). A Calorie or kilocalorie is the amount of heat required to raise the temperature of one kilogram of water one Celsius degree.
The British system unit of heat is the British Thermal Unit, or BTU. A BTU is the quantity of heat required to raise the temperature of a pound of water one degree Fahrenheit.
Temperature is a measure of the concentration of heat, or the average amount of motion per molecule. Molecules in rapid motion have a higher temperature than those that are moving more slowly. Adding heat to a substance makes the molecules move faster, and therefore increases the temperature. There is an exception to this, however. At a certain temperature, specific to each kind of matter, additional heat will change the substance from a solid to a liquid, or from a liquid to a gas, without any increase in temperature. In the case of water, adding heat to ice at 0 degrees Celsius produces water at 0 degrees Celsius. The heat is all used in separating the molecules, and the molecules are not moving any faster as a result. At 100 degrees Celsius, hot water can be changed into steam. It requires a lot of heat but the steam has the same temperature as the water from which it was formed. It takes about 80 calories to melt a gram of ice, and about 540 calories to evaporate or boil a gram of hot water.
Temperatures are commonly measured with thermometers, which may be of many types. The mercury-filled glass thermometer, commonly used for taking human body temperatures (98.6 degrees Fahrenheit is the same temperature as 37 degrees Celsius), is useless below -40 degrees, because mercury freezes at -40. (Minus 40 is the same temperature on both Fahrenheit and Celsius scales.)