thermal conduction Transfer of heat energy resulting from differ¬ ences in temperature between adjacent bodies or adjacent parts of a body. In the absence of a heat pump, the energy will flow from a region of higher temperature to a region of lower temperature. The transfer of energy occurs as a result of collision among the particles of the matter involved. The rate of transfer of energy is proportional to the cross-sectional area of contact and to the difference in temperature between the two regions. A substance of high thermal conductivity, such as copper, is a good ther¬ mal conductor; one with low thermal conductivity, such as wood, is a poor thermal conductor. See also convection, radiation.
thermal energy Internal energy of a system in thermodynamic equi¬ librium (see thermodynamics) by virtue of its temperature. A hot body has more thermal energy than a similar cold body, but a large tub of cold water may have more thermal energy than a cup of boiling water. Thermal energy can be transferred from one body, usually hotter, to a second body, usually colder, in three ways: conduction (see thermal conduction), con¬ vection, and RADIATION.
thermal expansion Increase in volume of a material as its tempera¬ ture is increased, usually expressed as a fractional change in dimensions per unit temperature change. When the material is a solid, thermal expan¬ sion is usually described in terms of change in length, height, or thick¬ ness. If a crystalline solid has the same structural configuration throughout, the expansion will be uniform in all dimensions. Otherwise, there may be different expansion coefficients and the solid will change shape as the temperature increases. If the material is a fluid, it is more useful to describe the expansion in terms of a change in volume. Because the bonding forces among atoms and molecules vary from material to material, expansion coefficients are characteristic of elements and com¬ pounds.
© 2006 Encyclopaedia Britannica, Inc.
1898 I thermal radiation ► Thespis
thermal radiation Process by which energy is emitted by a warm surface. The energy is electromagnetic radiation and so travels at the speed of light and does not require a medium to carry it. Thermal radiation ranges in frequency from infrared rays through visible light to ultraviolet rays. The intensity and frequency distribution of the emitted rays are determined by the nature and temperature of the emitting surface; in gen¬ eral, the hotter the object, the shorter the wavelength. A hotter object is a better emitter than a cooler one, and a blackened surface is a better emit¬ ter than a silvered one. An example of thermal radiation is the heating of the Earth by the Sun.
thermal spring See hot spring
Thermidorian Reaction (1794) Revolt in the French Revolution against the Reign of Terror that was initiated on 9 Thermidor (July 27). Weary of the mounting executions (1,300 in June alone), deputies in the National Convention decreed the arrest of Maximilien Robespierre, Louis de Saint-Just, and other members of the Committee of Public Safety. They and others were guillotined, inaugurating a brief “White Terror” against the radical Jacobin Club. The Directory period followed soon afterward.
thermocouple or thermal junction or thermoelectric ther¬ mometer Temperature-measuring instrument consisting of two wires of different metals joined at each end. One junction is placed where the temperature is to be measured, and the other is kept at a constant lower (reference) temperature. A measuring instrument is connected in the elec¬ trical circuit. The temperature difference causes the development of an electromotive force that is approximately proportional to the difference between the temperatures of the two junctions. Temperature can be read from standard tables, or the instrument can be calibrated to display tem¬ perature directly.
thermodynamics Study of the relationships among heat, work, tem¬ perature, and energy. Any physical system will spontaneously approach an equilibrium that can be described by specifying its properties, such as pres¬ sure, temperature, or chemical composition. If external constraints are allowed to change, these properties generally change. The three laws of thermodynamics describe these changes and predict the equilibrium state of the system. The first law states that whenever energy is converted from one form to another, the total quantity of energy remains the same. The second law states that, in a closed system, the entropy of the system does not decrease. The third law states that, as a system approaches absolute zero, further extraction of energy becomes more and more difficult, even¬ tually becoming theoretically impossible.
thermoluminescence Emission of light from certain heated sub¬ stances as a result of previous exposure to high-energy radiation. The radiation causes displacement of electrons within the crystal lattice of the substance. Upon heating, the trapped electrons return to their normal, lower-energy positions, releasing energy in the process. The longer the substance is exposed to radiation, the greater is the energy released. By measuring the amount of light given off, the duration of exposure to radiation can be determined; thus, thermoluminescence has been used to determine the age of various minerals and archaeological artifacts.
thermometry Uhor-'ma-mo-treX Science of measuring the temperature of a system or the ability of a system to transfer heat to another system. Temperature measurement is important to a wide range of activities, including manufacturing, scientific research, and medicine.
thermonuclear bomb See hydrogen bomb thermonuclear weapon See nuclear weapon
Thermopylae Uhor-'ma-po-leV, Battle of Battle in northern Greece (480 bc) in the Persian Wars. The Greek forces, mostly Spartan, were led by Leonidas. After three days of holding their own against the Persian king Xerxes I and his vast southward-advancing army, the Greeks were betrayed, and the Persians were able to outflank them. Sending the main army in retreat, Leonidas and a small contingent remained behind to resist the advance and were killed to the last man.
thermoreception Sensory capacity (see sense) to detect the tempera¬ ture of the environment and the body. It helps keep body temperature stable by regulating autonomic responses to temperature changes (see homeostasis). Temperature sensations are generated by separate heat and cold receptors (nerve endings) in the skin. Blood-sucking insects and some snakes (e.g., the pit viper) locate prey by its thermal radiation.
thermostat Device that detects temperature changes for the purpose of maintaining the temperature of an enclosed area essentially constant. The thermostat generates signals, usually electrical, to activate relays, valves, switches, and so on when the temperature rises above or falls below the desired value. Thermostats are used to control the flow of fuel to a burner, of electric current to a heating or cooling unit, or of a heated or cooled gas or liquid into the area it serves. They are also used in fire-detection warning systems.
theropod Vthir-s-.padX Any species of bipedal, carnivorous saurischian in the suborder Theropoda. The chicken-sized Compsognathus,the small¬ est known adult dinosaur, probably weighed 2-4 lb (1-2 kg); the tyran- nosaurs weighed tons. The theropods also included Allosaurus, Deinonychus, Megalosaurus, Oviraptor, and Velociraptor. Theropod remains recovered have been from the Late Triassic through the Late Cre¬ taceous Epoch (227-65 million years ago) from all continents except Ant¬ arctica. Their well-developed hind legs provided support and locomotion; their short forelimbs had mobile hands, probably for grasping and tear¬ ing prey. Despite the group’s name, which means “beast (i.e., mammal) foot,” theropod feet usually resembled those of birds. It is widely believed that all modern birds are descended from one line of small theropods.