Persistent current

In physics, persistent current refers to a perpetual electric current, not requiring an external power source. Such a current is impossible in normal electrical devices, since all commonly-used conductors have a non-zero resistance, and this resistance would rapidly dissipate any such current as heat. However, in superconductors and some mesoscopic devices, persistent currents are possible and observed due to quantum effects. In resistive materials, persistent currents can appear in microscopic samples due to size effects. Persistent currents are widely used in the form of superconducting magnets. In electromagnetism, all magnetizations can be seen as microscopic persistent currents. By definition a magnetization can be replaced by its corresponding microscopic form, which is an electric current density: This current is a bound current, not having any charge accumulation associated with it since it is divergenceless. What this means is that any permanently magnetized object, for example a piece of lodestone, can be considered to have persistent electric currents running throughout it (the persistent currents are generally concentrated near the surface). The converse is also true: any persistent electric current is divergence-free, and can therefore be represented instead by a magnetization. Therefore, in the macroscopic Maxwell's equations, it is purely a choice of mathematical convenience, whether to represent persistent currents as magnetization or vice versa. In the microscopic formulation of Maxwell's equations, however, does not appear and so any magnetizations must be instead represented by bound currents. In superconductors, charge can flow without any resistance. It is possible to make pieces of superconductor with a large built-in persistent current, either by creating the superconducting state (cooling the material) while charge is flowing through it, or by changing the magnetic field around the superconductor after creating the superconducting state.