JEE Main · 2004 · 24 Apr 2004 · Offline · Current Electricity

A piece of copper and another of germanium are cooled from room temperature to 77 K, the resistance of

(A)each of them increases
(B)each of them decreases
(C)copper decreases and germanium increases
(D)copper increases and germanium decreases✓ Correct

Answer: (D)

Solution

Copper is a metal, so cooling it reduces the lattice vibrations that scatter the electrons and its resistance falls as the temperature drops. Germanium is a semiconductor, so cooling it leaves fewer carriers raised into the conduction band and its resistance rises as the temperature drops. The two therefore respond to cooling in opposite ways, copper's resistance decreasing while germanium's increases.

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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