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$\sigma_1$ and $\sigma_2$ are the electrical conductivities of Ge and Na respectively. If these substances are heated, then:

1. Both $\sigma_1$ and $\sigma_2$ increase
2. $\sigma_1$ increases and $\sigma_2$ decreases
3. $\sigma_1$ decreases and $\sigma_2$ increases
4. Both $\sigma_1$ and $\sigma_2$ decrease

Correct answer: $\sigma_1$ increases and $\sigma_2$ decreases

Solution

Germanium is a semiconductor, so its conductivity increases on heating due to increased charge carriers. Sodium is a metal, and its conductivity decreases on heating because resistivity increases with temperature.

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