low-temperature physics definition of low-temperature physics in the Free Online Encyclopedia.

low-temperature physics, science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures. The temperature temperature, measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used.
..... Click the link for more information. scale used in low-temperature physics is the Kelvin temperature scale Kelvin temperature scale, a temperature scale having an absolute zero below which temperatures do not exist. Absolute zero , or 0°K;, is the temperature at which molecular energy is a minimum, and it corresponds to a temperature of −273.
..... Click the link for more information. , or absolute temperature scale, which is based on the behavior of an idealized gas (see gas laws gas laws, physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion
..... Click the link for more information. ; kinetic-molecular theory of gases kinetic-molecular theory of gases, physical theory that explains the behavior of gases on the basis of the following assumptions: (1) Any gas is composed of a very large number of very tiny particles called molecules; (2) The molecules are very far apart compared to
..... Click the link for more information. ). Low-temperature physics is also known as cryogenics, from the Greek meaning "producing cold." Low temperatures are achieved by removing energy from a substance. This may be done in various ways. The simplest way to cool a substance is to bring it into contact with another substance that is already at a low temperature. Ordinary ice, dry ice (solid carbon dioxide), and liquid air liquid air, ordinary air that has been liquefied by compression and cooling to extremely low temperatures (see liquefaction ). Its commercial preparation involves purification by washing to remove soluble impurities and by passage over calcium oxide (lime) to remove
..... Click the link for more information. may be used successively to cool a substance down to about 80°K; (about −190°C;). The heat is removed by conduction conduction, transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance, in the case of heat, or from a difference in electric potential , in the case of electricity.
..... Click the link for more information. , passing from the substance to be cooled to the colder substance in contact with it. If the colder substance is a liquefied gas (see liquefaction liquefaction, change of a substance from the solid or the gaseous state to the liquid state. Since the different states of matter correspond to different amounts of energy of the molecules making up the substance, energy in the form of heat must either be supplied to
..... Click the link for more information. ), considerable heat can be removed as the liquid reverts to its gaseous state, since it will absorb its latent heat latent heat, heat change associated with a change of state or phase (see states of matter ). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a
..... Click the link for more information. of vaporization during the transition. Various liquefied gases can be used in this manner to cool a substance to as low as 4.2°K;, the boiling point boiling point, temperature at which a substance changes its state from liquid to gas. A stricter definition of boiling point is the temperature at which the liquid and vapor (gas) phases of a substance can exist in equilibrium.
..... Click the link for more information. of liquid helium. If the vapor over the liquid helium is continually pumped away, even lower temperatures, down to less than 1°K;, can be achieved because more helium must evaporate to maintain the proper vapor pressure vapor pressure, pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid.
..... Click the link for more information. of the liquid helium. Most processes used to reduce the temperature below this level involve the heat energy that is associated with magnetization (see magnetism magnetism, force of attraction or repulsion between various substances, especially those made of iron and certain other metals; ultimately it is due to the motion of electric charges.
..... Click the link for more information. ). Successive magnetization and demagnetization under the proper combination of conditions can lower the temperature to only about a millionth of a degree above absolute zero. Reaching such low temperatures becomes increasingly difficult, as each temperature drop requires finding some kind of energy within the substance and then devising a means of removing this energy. Moreover, according to the third law of thermodynamics Carnot cycle after the French physicist Sadi Carnot , who first discussed the implications of such cycles. During the Carnot cycle occurring in the operation of a heat engine, a definite quantity of heat is absorbed from a reservoir at high temperature; part of this heat is
..... Click the link for more information. , it is theoretically impossible to reduce a substance to absolute zero by any finite number of processes. Superconductivity superconductivity, abnormally high electrical conductivity of certain substances. The phenomenon was discovered in 1911 by Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4.2°K;.
..... Click the link for more information. and superfluidity superfluidity, tendency of liquid helium below a temperature of 2.19°K; to flow freely, even upward, with little apparent friction. Helium becomes a liquid when it is cooled to 4.2°K;.
..... Click the link for more information. have traditionally been thought of as phenomena that occur only at temperatures near absolute zero, but by the late 1980s several materials that exhibit superconductivity at temperatures exceeding 100°K; had been found. Superconductivity is the vanishing of all electrical resistance in certain substances when they reach a transition temperature that varies from one substance to another; this effect can be used to produce powerful superconducting magnets. Superfluidity occurs in liquid helium and leads to the tendency of liquid helium to flow over the sides of any container it is placed in without being stopped by friction or gravity.

Bibliography

See A. C. Helden, The Coldest Spot on Earth (1989).