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Chapter 10: Q21CQ (page 467)
What is Cooper pair, and what role does it play in superconductivity?
Cooper pairs are a pair of electrons at states slightly above Fermi energy and attract each other mediated via photons.
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The diagram shows a bridge rectifier circuit. A sinusoidal input voltage is fed into four identical diodes. each represented by the standard diode circuit symbol. The symbol indicates the direction of conventional current flow through the diode. The plots show input and output voltages versus time. Note that the output voltage is strictly in one direction. Explain
(a) how this circuit produces the unidirectional output voltage it does, and
(b) what features in the output plot indicate that the band gap of the diodes is about half an electronvolt, (It might seem that about one volt is correct, but consider how many diodes are on and in series at any given instant. In fact, although not the usual habit, it might be more accurate to plot the output voltage shifted upward relative to the input.)
The accompanying diagram shows resistivity (reciprocal of conductivity) data for four solid materials from 77Kto 273K. scaled so that the maximum value plotted for each material is 1. Two are metals, one of which undergoes a transition between ordered and disordered spins in this temperature range. Speculate as to which plots correspond to these two metals and what the other two materials might be. Explain your reasoning.
For the four kinds of crystal binding 鈥� covalent, ionic, metallic, and molecular- how would the destiny of valence electrons vary throughout the solid? Would it be constant, centered on the atoms, or largest between the atoms? Or would it alternate, with a net charge density positive at one atom and negative at the next?
The "floating magnet trick" is shown in Figure 10.50. If the disk on the bottom were a permanent magnet, rather than a superconductor, the trick wouldn't work. The superconductor does produce an external field very similar to that of a permanent magnet. What other characteristic is necessary to explain the effect? (Him: What happens when you hold two ordinary magnets so that they repel, and then you release one of them?)
Question: The magnetic field at the surface of a long wire radius R and carrying a current I is . How large acurrent could a 0.1 mm diameter niobium wire carry without exceeding its 0.2 T critical field?
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