The concept of temperature has two related, but different, interpretations. On a general level, temperature is associated with the sense of hot and cold. If you put your finger in a pan of hot water, heat energy flows from the water to your finger; you say that the water is at a higher temperature than that of your finger. If you put your finger in a glass of ice water, heat energy flows as heat away from your finger.
The direction of heat energy flow is the basis of one definition of temperature. Temperature is the property of objects—or more generally of systems—that determines the direction of heat energy flow when the objects are put in direct contact with each other. Energy flows as heat from objects at a higher temperature to ones at a lower temperature. When heat energy ceases to flow, the objects are at the same temperature and are said to be in thermal equilibrium.
The second definition of temperature is more rigorous. It deals with the factors that are responsible for an object's being warm or hot on the one hand or cool or cold on the other. This definition is based on the behavior of the particles (atoms, molecules, ions, etc.) of which matter is made. On this level, temperature can be defined as the total kinetic energy of the particles of which a material is made.
Kinetic energy is the motion of particles. Particles that are rotating rapidly on their axes, vibrating back and forth rapidly, or traveling rapidly through space have a large amount of kinetic energy. Particles that are moving slowly have relatively little kinetic energy.
Absolute temperature scale: A temperature scale that has the lowest possible temperature—at which all molecular motion ceases—set at zero.
Absolute zero: The lowest possible temperature at which all molecular motion ceases. It is equal to −459°F (−273°C).
Kinetic energy: Energy of an object or system due to its motion.
Pyrometer: A device for obtaining temperature by measuring the amount of radiation produced by an object.
Resistance thermometer: A device for obtaining temperature by measuring the resistance of a substance to the flow of an electrical current.
Thermometer: A device for obtaining temperature by measuring a temperature-dependent property (such as the height of a liquid in a sealed tube) and relating this to temperature.
From this perspective, a glass of warm water has a high temperature because the molecules of the water are moving rapidly. The molecules of water in a glass of cool water, by comparison, are moving more slowly.
Thermometers are devices that register the temperature of a substance relative to some agreed upon standard. For example, a thermometer that reads 32°F (0°C) is measuring a temperature equal to that of ice in contact with pure water.
Thermometers use changes in certain physical or electrical properties to detect temperature variations. The most common kind of thermometer consists of a liquid—usually mercury or alcohol—sealed in a narrow tube. When the thermometer is placed in contact with a substance, heat travels into or out of the thermometer. If heat leaves the thermometer, the sealedin liquid is cooled and it contracts (takes up less space); the level of the liquid in the thermometer falls. If heat enters the thermometer, the liquid is warmed and it expands; the level of the liquid in the thermometer rises.
A resistance thermometer is based on the fact that all things resist the flow of an electric current to some degree. Furthermore, such resistance changes with temperature. In general, the higher the temperature of a substance, the more it resists the flow of an electric current. This principle can be used to measure the temperature of a substance by observing the extent to which it resists the flow of an electric current.
Another type of thermometer is known as a pyrometer. A pyrometer is a device that detects visible and infrared radiation given off by an object, then converts that information to a temperature reading.
In order to establish a scale against which temperatures can be measured, one first has to select two fixed points from which to begin. Historically, those two points have been the boiling point and freezing point of water. The two points were chosen because water is the most abundant compound on Earth, and finding its boiling and freezing points is relatively easy.
One way of making a thermometer, then, is to begin with a narrow tube that contains a liquid and is sealed at both ends. The tube is then immersed in boiling water, and the highest point reached by the liquid is marked in some appropriate way. Next, the tube is immersed in a mixture of ice and water, and the lowest point reached by the liquid is marked in a similar way. The distance between the lowest point and highest point is then divided into equal sections. The numbers assigned to the lowest and highest point on the thermometer—and the form of dividing the range between them—is what distinguishes one system of measuring temperature from another.
In the early 1700s, for example, German physicist Gabriel Daniel Fahrenheit (1686–1736) decided to assign to the freezing point of water a temperature value of 32 and to the boiling point of water a temperature value of 212. He then divided the distance between these two points into 180 equal divisions, each equal to one degree of temperature. The Fahrenheit (F) system of measuring temperature is still in use today in the United States.
A somewhat more logical system of defining the temperatures on a thermometer was suggested in 1742 by Swedish astronomer Anders Celsius (1701–1744). Celsius suggested assigning the values of 0 and 100 to the freezing point and boiling point of water and dividing the distance between these two into 100 equal parts. The Celsius system is now used throughout the scientific community and in all countries of the world except the United States and Burma.
Both Fahrenheit and Celsius temperature scales have one important inherent drawback: in both cases, negative temperatures can exist. The freezing point of carbon dioxide (dry ice), for example, is −110°F (−78.5°C). But recall the definition of temperature as a measure of the average kinetic energy of the particles that make up a substance. What meaning can be assigned, then, to a negative temperature reading? There is no such thing as negative energy in systems with which we are familiar.
To remedy this problem, a third temperature scale was invented in 1848 by English physicist William Thomson (1824–1907). Thomson set the lowest point on the temperature scale as the lowest possible temperature, absolute zero. Absolute zero is defined as the temperature at which all motion of all particles would cease, a condition in which heat would be absent and, hence, a substance had no temperature. Theoretical calculations suggested to Thomson that the Celsius temperature corresponding to that condition was about −273°C (−459°F). Absolute zero, then, was set at this temperature and assigned the value 0 K. The unit K in this measurement stands for Kelvin, the unit of measure in the absolute temperature system. The term Kelvin comes from William Thomson's official title, Lord Kelvin.
The relationship among the three temperature scales is as follows:
°C = 5/9(°F − 32)
°F = 9/5(°C) + 32
K = °C + 273
°C = K − 273