Keberaliran lampau: Perbezaan antara semakan

Superconductivity is an essentially [[quantum mechanics|quantum mechanical]] phenomenon, and cannot be understood simply as the idealization of "[[perfect conductor|perfect conductivity]]" in classical physics.

 

Most of the physical properties of superconductors vary from material to material, such as the [[heat capacity]] and the critical temperature at which superconductivity is destroyed. On the other hand, there is a class of properties that are independent of the underlying material. For instance, all superconductors have ''exactly'' zero resistivity to low applied currents when there is no magnetic field present. The existence of these "universal" properties implies that superconductivity is a [[phase (matter)|thermodynamic phase]], and thus possess certain distinguishing properties which are largely independent of microscopic details.

 

 

Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting [[electromagnet]]s such as those found in [[Magnetic resonance imaging|MRI]] machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years, and theoretical estimates for the lifetime of persistent current exceed the lifetime of the universe.