A parallel$-$plate capacitor with plate separation d $=1\mathrm{.}0$ cm has $29,000$ V applied when free space is the only dielectric. Assume that air has a dielectric strength of $30000$ V$/$cm$.$ Show why the air breaks down when a thin piece of glass $($r $=6\mathrm{.}5)$ with a dielectric strength of $290,000$ V$/$cm and thickness is inserted between the plates