Q.During a thunder storm, the movement of water molecules within the clouds creates friction, partially causing the bottom part of the clouds to become negatively charged. This implies that the bottom of the cloud and the ground act as a parallel plate capacitor. If the electric field between the cloud and the ground exceeds the dielectric breakdown of the air ($3\times10^6\ \text{V m}^{-1}$), lightning will occur.

Parallel Plate Capacitance and Plate Separation

A parallel plate capacitor stores charge on two conducting plates of area $A$ separated by a distance $d$. Its capacitance depends only on this geometry (and the medium between the plates):

Intuition: capacitance is how much charge the plates hold per volt. Pull the plates apart (increase $d$) and $C$ decreases; the field must reach across a bigger gap for the same charge.

The key distinction — is the battery connected? Two variables ($Q$, $V$) are linked by $Q = CV$, but only one is held fixed depending on the circuit:

• Battery (cell of emf $E$) still connected (key closed): the plates stay at $V = E$. So $V$ is constant. Changing $d$ changes $C$, and therefore $Q = CV$ changes — charge flows through the circuit to keep $V$ fixed.
• Battery disconnected first (key open): the charge $Q$ on the isolated plates has nowhere to go, so $Q$ is constant. Changing $d$ changes $C$, and therefore $V = Q/C$ changes. …