Contents
Research
Reduction of temperature of metals lowers electrical resistance.
Electrical conductors are materials, namely metals, which allow an electrical current to flow through them. The ease at which the electrons of a sample of metal are able to flow is its conductivity. Many factors influence how well a metal conducts electricity, including size, purity, and temperature. Under certain conditions, some metals become superconductors, that is, metals that allow electrons to flow with zero resistance. Resistance plays a limiting role in electronics as it determines the speed with which electricity can flow in a circuit. Lowered resistance will result in faster devices. Practical means of lowering resistance is therefore important to the electronics and computer industries.
Resistance in conductors is caused by collisions of the electrons with atoms in the crystal lattice of the material, and with phonons, which are quantums of lattice energy. As an electron is pulled through free space by an electric field, the constant force of the field is applied, causing the particle to undergo constant acceleration. Electric current, however, is known to be constant throughout a circuit. The apparent constant velocity is actually the result of repeated acceleration, followed by collisions in which the electron encounters as it flows through the conductor has a major effect on the materials conductivity.
The number of collisions undergone by an electron can be expressed in the terms of its mean free path. In laymen's terms, the mean free path is simple the average distance a given electron can move before it runs into the crystal lattice, or is scattered by a phonon. Just as a car will go faster given a longer distance to accelerate, an electron's average velocity will be greater if it is allowed to accelerate over a longer distance -- the mean free path. As the mean free path increases, resistance is lowered.
Lowering the temperature of a conductor reduces the energy of the conductor, and therefore reduces the number of phonons. The decreased concentration of phonons reduces the electron scattering, increasing conductivity. Because of reduced phonon collisions, electrical resistance is proportional to temperature (T) at low temperatures, while at higher temperatures, the resistance is proportional to temperature (T). A classical physical explanation of the relationship between temperatures and resistance also exists. In a metal, the atoms are arranged in a crystal lattice. As temperature increases, the atoms vibrate more rapidly and more randomly about their given positions in the lattice. The opposite is true at lower temperatures. The atoms vibrate less, and are more closely confined to their positions. The apparent random motion of particles at high temperatures causes more collisions to occur, slowing the electrons further.
In the 19th century, it was known that resistance decreased with temperature. At this time, however, it was unknown what would happen to resistance as the temperature approached absolute zero. Lord Kelvin, for whom the Kelvin temperature scale is named, theorized that the flow of electrons would cease, as the particles of matter would be frozen in place. In 1911, Kelvin's theory was disproved by Heike Kamerlingh Onnes, who discovered superconductivity by running an electrical current through a supercooled mercury wire. Superconductors exist under certain conditions in which phonon collisions have no effect on electron motion, effectively reducing resistance to (about) zero. Through quantum theory, electrons with opposite spins can occupy the same energy level, forming Cooper pairs. If one electron in such a pair collides with a photon, the scattering effect is "equalized" by the other electron in the pair and the system doesn't scatter, but retains it's initial path. The first super conductor was discovered at liquid helium temperatures, about 4K. The discovery of superconductors at convenient temperature may eventually lead to potential applications, such as superconduction electrical transmission.
Though principle of quantum mechanics it has been shown that electrical resistance and temperature are proportional. Some substances are able to superconduct, with zero resistance, below certain temperatures. The temperature dependence of the electrical properties of various conductors may find applications in technology, improving the performance of electricity transmission, and the speed of electronic devices.
