Germanium can be used to measure the energy of incident photons. Consider a 660-keV gamma ray emitted from ${ }^{137} \mathrm{C}$. Given that the band gap in germanium is $0.72 \mathrm{eV}$, how many electron-hole pairs can be generated as this gamma ray travels through germanium?

For the npn-transistor amplifier, suppose $R_b=2.00 \mathrm{k} \Omega$ and $R_L=10.0 \mathrm{k} \Omega$. Suppose further that a $10.0-\mu \mathrm{A}$ ac base current $i_{\mathrm{b}}$ generates a $0.500-\mathrm{mA}$ ac collector current $i_c$. What is the voltage gain of the amplifier?

The base current in an $n p n$ transistor circuit is $25.0 \mathrm{~mA}$. If $88.0$ percent of the electrons entering the base from the emitter reach the collector, what is the base current?

A metal is a good conductor because the valence energy band for electrons is $(a)$ completely full. (b) full, but there is only a small gap to a higher empty band. (c) partly full. (d) empty. (e) None of these is correct.

When a sample of pure copper is cooled from $300 \mathrm{~K}$ to $4 \mathrm{~K}$, its resistivity decreases more than the resistivity of a sample of brass when it is cooled through the same temperature difference. Why?