Problem 4.136

A solenoid with inductance $L=7 \mathrm{mH}$ and active resistance $R=44 \Omega$ is first connected to a source of direct voltage $V_{0}$ and then to a source of sinusoidal voltage with effective value $V=V_{0} .$ At what frequency of the oscillator will the power consumed by the solenoid be $\eta=5.0$ times less than in the former case?