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PHy107Lect30_6per - Superconductivity From Last Time Solids...

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1 Wed. Oct 19 Phy107 Lecture 30 From Last Time… Solids are large numbers of atoms arranged in a regular crystal structure. Each atom has electron quantum states, but interactions shift the energies. End result is each type atomic electron state (e.g. 1 s ) corresponds to a broadened ‘band’ of energy levels in a solid. Band filling determines electrical properties Partially full bands = metal Bands completely full or empty = insulator / seminconductor Substitutional doping of a semiconductor leads to a material useful in electronic devices. Wed. Oct 19 Phy107 Lecture 30 Superconductivity Superconductors are materials that have exactly zero electrical resistance. But this only occurs at temperatures below a critical temperature, T c In most cases this temperature is far below room temperature. Hg, mercury Wed. Oct 19 Phy107 Lecture 30 Brief History of Superconductivity Discovery in 1911 Theoretical explanation in 1957 High-temperature superconductivity 1986 Wed. Oct 19 Phy107 Lecture 30 Persistent currents How zero is zero? • EXACTLY! Can set up a persistent current in a ring. The magnitude of the current measured by the magnetic field generated. No current decay detected over many years! Persistent supercurrent Flux lines Wed. Oct 19 Phy107 Lecture 30 Critical current If the current is too big, superconductivity is destroyed. Maximum current for zero resistance is called the ‘critical’ current. For larger currents, the voltage is no longer zero, and power is dissipated. Current Voltage Critical current Wed. Oct 19 Phy107 Lecture 30 Superconducting elements Many elements are in fact superconducting But the critical temperatures are quite low.
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2 Wed. Oct 19 Phy107 Lecture 30 Elemental Critical Temperatures Wed. Oct 19 Phy107 Lecture 30 Temperature Scales All these are near -450˚F This is because they are near 0˚ K (Kelvin) Kelvin is an absolute temperature scale 0 K is the coldest temperature possible This is -459.67 ˚F This is because temperature describes the average internal kinetic energy of the system.
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