When turning at $1500 \mathrm{rev} / \mathrm{min}$, a certain generator produces $100.0$ V. What must be its frequency in rev/min if it is to produce $120.0$ V?
Because the amplitude of the emf is proportional to the frequency, we have, for two frequencies $f_{1}$ and $f_{2}$,
$$
\frac{\delta_{1}}{\delta_{2}}=\frac{f_{1}}{f_{2}} \quad \text { or } \quad f_{2}=f_{1} \frac{\delta_{2}}{\varepsilon_{1}}=(1500 \mathrm{rev} / \min )\left(\frac{120.0 \mathrm{~V}}{100.0 \mathrm{~V}}\right)=1800 \mathrm{rev} / \mathrm{min}
$$