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conantee
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« on: December 31, 2009, 12:37:29 PM »

23. Which of the following statements concerning the electrical conductivities at room temperature of a pure copper sample and a pure silicon sample is NOT true?
(A) The conductivity of the copper sample is many orders of magnitude greater than that of the silicon sample.
(B) If the temperature of the copper sample is increased, its conductivity will decrease.
(C) If the temperature of the silicon sample is increased, its conductivity will increase.
(D) The addition of an impurity in the copper sample always decreases its conductivity.
(E) The addition of an impurity in the silicon sample always decreases its conductivity.
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« Reply #1 on: December 31, 2009, 12:51:25 PM »

This is a kind of solid-state physics problem. However, you have learned some facts since in high school. So just read through and do a little guess
(A) is true (that's why copper is called conductor and silicon is called semiconductor).
(B) is true (this is what we have learned in high school. At even lower temperature, conductor may become superconductor, where the resistivity is zero).
(C) is true (if you don't know solid-state physics, you may keep this choice for guess. Well, it is true because the electron in semiconductor can be free by gaining energy from thermal energy).
(D) we're not so sure about this. Just leave it for guess. (The answer, though, is true.)
(E) is false. We know that doping, adding other elements in semiconductor, improves its conductivity. The doped semiconductor can be categorized as P or N type.

Hence, the correct answer is (E), and 55 of 100 people did it right.
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