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| -rw-r--r-- | supcon.tex | 16 |
1 files changed, 8 insertions, 8 deletions
@@ -13,7 +13,7 @@ Solids can be roughly divided into four classes, according to the way they conduct electricity. They are: Metals, Semiconductors, Insulators and Superconductors. The behavior of these types of materials is explained by quantum mechanics. Basically, when atoms form a solid, the atomic levels of the -electrons combine to form bands. That is over a finite range of energy there +electrons combine to form bands. That is, over a finite range of energies there are states available to electrons. Since only one electron can occupy a given state, the {\bf Pauli Exclusion Principle}, electrons will fill these states up to some maximum, the Fermi Energy: $E_f$. A solid is a metal if it has an @@ -22,7 +22,7 @@ making a metal a good conductor of electricity. If the solid has a band which is completely full, with an energy gap to the next band, that solid will not conduct electricity very well, making it an insulator. A semiconductor is between a metal and insulator: while it has a full band (the valence band), -the next band (the conduction band) is close enough in energy and so that the +the next band (the conduction band) is close enough in energy, and so the electrons can easily reach it. Superconductors are in a class by themselves. They can be metals or insulators at room temperature. Below a certain temperature, called the critical temperature, the electrons ``pair'' together (in @@ -47,13 +47,13 @@ dissipation. resistance. Superconductors have a critical temperature, above which they lose their superconducting properties. - Another striking demonstrations of superconductivity is the \textbf{Meissner effect}. - Magnetic fields cannot penetrate superconducting surface, instead a + Another striking demonstration of superconductivity is the \textbf{Meissner effect}. + Magnetic fields cannot penetrate superconducting surfaces, instead a superconductor attempts to expel all magnetic field lines. It is fairly simple to intuitively understand the Meissner effect, if you imagine a perfect conductor of electricity. If placed in a magnetic field, Faraday's Law says an induced current which opposes the field - would be setup. But unlike in an ordinary metal, this induced current does not dissipate in + would be set up. But unlike in an ordinary metal, this induced current does not dissipate in a perfect conductor. So, this induced current would always be present to produce a field which opposes the external field. In addition, microscopic dipole moments @@ -101,10 +101,10 @@ superconductor disk. Record what you observe. \item Try demonstrating a \emph{frictionless magnetic bearing}: if you carefully set the magnet rotating, you will observe that the magnet continues to rotate for a long time. Also, try moving the magnet across the superconductor. Do you feel any resistance? If you - feel resistance, why is this. + feel resistance, why is this? \item Using tweezers, take the disk (with the magnet on it) out of the - nitrogen (just place it on side of disk), allowing it to + nitrogen and place it beside of the container, allowing it to warm. Watch the thermocouple reading carefully, and take a reading when the magnet fails to levitate any longer. This is a rough estimate of the critical temperature. Make sure you record it! @@ -146,7 +146,7 @@ At room temperature, you should be reading a non-zero V=0 (R=0). At a critical temperature, you will see a voltage (resistance) appear. -\item Repeat this measurement several time to acquire significant number of data points +\item Repeat this measurement several times to acquire a significant number of data points near the critical temperature (6.4-4.5 mV). Make a plot of resistance versus temperature, and make an estimate of the critical temperature based on this plot. |