Past exam of the physics 2 paper of the University of Oxford / 2016 / 9 / b / Solution

The electric field magnitude falls as $1/r^2$ throughout the vacuum gap, so its maximum occurs just outside the inner sphere:ThusInside the metal the electric field is zero. The one-sided gap value is the relevant value for dielectric breakdown; the ideal zero-thickness conducting boundary itself should not be assigned a single field value across its surface-charge discontinuity.