Problem 4.94

Due to a certain cause the free electrons in a plane copper plate shifted over a small distance $x$ at right angles to its surface. As a result, a surface charge and a corresponding restoring force emerged, giving rise to so-called plasma oscillations. Find the angular frequency of these oscillations if the free electron concentration in copper is $n=0.85 \cdot 10^{29} \mathrm{~m}^{-1}$.