Superconductors

- The resistivity of most metals increases with increase in temperature and vice-versa. There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 0⁰Kelvin (-273⁰C). At this stage such metals or compounds are said to have attained superconductivity. Example - mercury becomes superconducting at approximately 4.5 Kelvin (-268.5⁰C). Superconductivity was discovered by Heike Kamerlingh Onnes. The transition from normal conductivity to superconductivity takes place almost suddenly; it occurs over a very narrow range of temperature about 0.05⁰K. The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called transition temperature. Superconductors are used for producing very magnetic fields of about 50 Tesla. Magnetic energy can be stored in large superconductors and drawn as required to counter the voltage fluctuations during peak loading. Superconductors can be used to perform logic and storage functions in computers. As there is no, I²R losses in a superconductor, so power can be transmitted through the superconducting cables without any losses. Superconducting property can be destroyed by applying external magnetic field as in fig (b). In fig (b) H_C is the critical magnetic field and T_C critical temperature.

Conductors: Substances like copper, aluminium, silver which allow the passage of current through them are conductors. The valence band of these substances overlaps the conduction band as shown in fig (b). Due to this overlapping, a large number of free electrons are available for conduction. This is the reason, why a slight potential difference applied across them causes a heavy flow of current through them.