A piece of copper and a piece of germanium are cooled from the room temperature down to 77 K.
Correct answer: C. The resistance of copper decreases and that of germanium increases.
- A. The resistance of each of them increases.
- B. The resistance of each of them decreases.
- C. The resistance of copper decreases and that of germanium increases.
- D. The resistance of copper increases and that of germanium decreases
Explanation
Copper is a metal, so cooling reduces lattice vibrations and decreases its resistance. Germanium is a semiconductor, and cooling greatly reduces its thermally generated charge carriers, so its resistance increases. The likely choice is b, because students may apply the metallic temperature rule to both materials and overlook germanium's semiconductor behavior.
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About Resistance and Resistivity
Resistance measures opposition to current in a conductor and follows R = V/I for an ohmic conductor, while resistivity is an intrinsic property of the material. Questions use R = ρL/A, examine effects of length, cross-sectional area and temperature, and distinguish resistance, resistivity, conductance and conductivity.
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