Question

When Cu and Ge are cooled to $-150^{O}C$ then resistance of Cu -- and that of Ge --.
A. increases, increases
B. decreases, increases
C. decreases, decreases
D. decreases, increases

Answer 1

Hint: Check whether the given elements are metal, non-metal or semiconductor. Use the concept of thermal coefficient of resistance for the metals and semiconductors. Hence, check the effect of temperature on the resistance of the given elements.

Complete step by step answer:
The thermal coefficient of resistance for the metals is positive which means that the resistance of the metals is directly proportional to the temperature.
Hence, the resistance increases with increase in temperature and decreases with decrease in the temperature.
The thermal coefficient of resistance for the semiconductors is negative which means that the resistance of the semiconductors is inversely proportional to the temperature.
Hence, the resistance decreases with increase in temperature and increases with decrease in the temperature.
Cu is the symbol for the element copper. Copper is a metal.
Ge is the symbol for the element germanium. Germanium is a semiconductor.
So, when the temperature of the copper is decreased to cool it, its resistance also decreases and when the temperature of the germanium is decreased to cool it, its resistance increases.

Therefore, when Cu and Ge are cooled to $-150^{O}C$ then resistance of Cu decreases and that of Ge increases.

So, the correct answer is “Option D”.

Additional Information:
In metals, electrons are the carriers. As the temperature is increased, the electrons gain extra energy and start colliding with each other increasing the orderly transfer of the electrons.
Hence, increase in temperature increases resistance in metals and vice versa.
In the semiconductors, electrons and holes are the charge carriers.

Note:
One can also say that the resistivity of metals increases with increase with temperature and decreases with increase in temperature for the semiconductors.
As the temperature of semiconductor is increased, the number of charge carriers available increases with increase in conductivity and decrease in resistivity and vice versa.