Cu and Ge are cooled to 70 K then :-

Options

A. the resistance of Cu will decrease and that of Ge will decrease
B. the resistance of Cu will decrease and that of Ge will increase
C. the resistance of both Cu and Ge decrease
D. the resistance of both Cu and Ge increase

Answer

To solve the question regarding the behavior of copper (Cu) and germanium (Ge) when cooled to 70 K, we can analyze the properties of both materials in relation to temperature changes.

### Step-by-Step Solution:

1. **Identify the Nature of the Materials**:
- Copper (Cu) is a metal and is classified as a good conductor of electricity.
- Germanium (Ge) is a semiconductor.

2. **Understand Resistance Behavior with Temperature**:
- For conductors like copper, the resistance typically **increases with an increase in temperature**. This is due to the increased vibrations of the lattice structure, which impedes the flow of electrons.
- For semiconductors like germanium, the resistance typically **decreases with an increase in temperature**. This is because higher temperatures provide more energy to the charge carriers (electrons and holes), allowing more of them to participate in conduction.

3. **Analyze the Cooling Effect**:
- The question states that both materials are cooled to 70 K.
- Cooling leads to a **decrease in temperature**.

4. **Determine the Effect on Copper**:
- Since copper is a conductor, cooling it down to 70 K will result in a **decrease in resistance**. This is because, at lower temperatures, the lattice vibrations are reduced, allowing electrons to move more freely.

5. **Determine the Effect on Germanium**:
- For germanium, being a semiconductor, cooling it down to 70 K will result in an **increase in resistance**. This is because fewer charge carriers are available to conduct electricity at lower temperatures.

6. **Conclusion**:
- Therefore, after cooling both materials to 70 K, we conclude that:
- The resistance of copper (Cu) will **decrease**.
- The resistance of germanium (Ge) will **increase**.

7. **Final Answer**:
- The correct option is that the resistance of copper will decrease and that of germanium will increase.