A strong
electric field can ionise the surrounding air and accelerate the charges so
produced to the oppositely charged plates, thereby neutralising the charge
on the capacitor plates, at least partly In other words, the charge of the
capacitor leaks away due to the reduction in insulating power of the
intervening medium The maximum electric field that a dielectric medium can withstand
without break-down (of its insulating property) is called its dielectric
strength; for air it is about 3 × 106 Vm–1 For a separation between
conductors of the order of 1 cm or so, this field corresponds to a potential
difference of 3 × 104 V between the conductors