Heat
Heat is the energy that flows between two objects because of a difference in temperature. Heat always flows from a body at a higher temperature to one at a lower temperature.
Scientists use the term heat differently than do nonscientists. The average person may think of heat as the amount of energy contained in a body. The correct term for that property, however, is thermal energy.
Thermal energy and temperature
According to the kinetic theory of matter, all matter is composed of particles that are constantly in motion. Temperature is a measure of the motion of those particles. The more rapidly particles are in motion, the higher the temperature; the less rapidly they are moving, the lower the temperature.
In theory, it would be possible to reduce the motion of the particles in an object to zero. In that case, the object would contain no thermal energy. The temperature at which all particle motion ends is called absolute zero. Scientists have come within a few millionths of a degree of absolute zero but have never actually reached that point.
Heat Transfer
Another way to think of heat is as a transfer of thermal energy from one place to another. This process occurs in one of three ways: conduction, convection, and radiation.
Conduction. Conduction is the process of heat transfer. Rapidly moving molecules in a hot material collide with slower moving molecules in a cool material. The fast-moving molecules slow down and the slow moving molecules increase their speed. Conduction occurs, then, when two bodies of different temperatures are in contact with each other.
Convection. Convection is the process by which large masses of a fluid (a liquid or gas) move, carrying thermal energy. When water in a container is heated, for example, it expands. Cooler water around it pushes the lighter water upward. As the warm water rises, it begins to cool and starts to move downward in the liquid again. Eventually, a circular motion is produced within the liquid, forcing heat to be transferred throughout the liquid.
Radiation. Finally, thermal energy can be transferred by radiation. Hot bodies emit electromagnetic radiation that corresponds to their temperature. This radiation passes through space until it comes into contact with a body with less thermal energy. The cooler body then absorbs this radiation and becomes warmer.
Transfer of thermal energy. The transfer of thermal energy from one place to another occurs in one of three ways: conduction, convection, and radiation. In conduction, rapidly moving molecules in a hot material collide with slower moving molecules in a cool material. The fast-moving molecules slow down and the slow-moving molecules increase their speed. Conduction occurs, then, when two bodies of different temperatures are in contact with each other.
Convection is the process by which large masses of a fluid (a liquid or gas) move, carrying thermal energy. When water in a container is heated, for example, it expands. Cooler water around it pushes the lighter water upward. As the warm water rises, it begins to cool and starts to move downward in the liquid again. Eventually, a circular motion is produced within the liquid, forcing heat to be transferred throughout the liquid.
Finally, thermal energy can be transferred by radiation. Hot bodies emit electromagnetic radiation that corresponds to their temperature. This radiation passes through space until it comes into contact with a body with less thermal energy. The cooler body then absorbs this radiation and becomes warmer.
Heat units
Since heat is a form of energy, the units used to measure heat are the same as those used to measure energy. In the metric system, one of the earliest units used to measure heat was the calorie. The calorie is defined as the amount of heat energy needed to raise the temperature of one gram of water one degree Celsius. To be precise, the temperature change is specified as an increase from 14.5°C to 15.5°C.
In the International System of Units (the SI system), the unit of energy is the joule. A calorie is defined as 4.184 joules.
Specific heat
Materials differ from each other with regard to how easily they can be warmed. One could add a joule of heat to a gram of water, a gram of iron, a gram of mercury, and a gram of ethyl alcohol and notice very different results. The temperature of the mercury would rise the most, and the temperature of the water would rise the least.
The specific heat capacity (or just specific heat) of a material is defined as the amount of heat required to raise the temperature of one gram of the material one degree Celsius. It takes 4.18 joules to raise the temperature of 1 gram of water 1 degree Celsius (at a temperature of 25°C). In comparison, it takes only 0.14 joule to raise the temperature of the same amount of mercury by one degree Celsius and 0.45 joule to raise the temperature of the same amount of iron by one degree Celsius. It takes 2.46 joules to raise the temperature of the same amount of ethyl alcohol by one degree Celsius.
[ See also Energy ; Temperature ; Thermodynamics ]