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
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This note was uploaded on 01/05/2012 for the course PHYS 107 taught by Professor N/a during the Spring '08 term at University of Wisconsin Colleges Online.

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PHy107Lect30_6per - Superconductivity From Last Time Solids...

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