# Principle of parallel plate capacitor

In a capacitor , capacitance is given by
$$C=\frac{Q}{V}$$

I learned from various sources that V is the potential difference between charged plate and supporting plate and not just the potential of charged plate . Assuming it is true , I can't understand the principle of parallel plate capacitor ,that the supporting plate helps in decreasing V and thus increasing the capacitance because:-

Potential difference can't actually be increasing. It is more convincing to say that the potential of charged plate is decreasing while potential difference is always remaining constant according to the following video

Can anyone point out why the professor says more charge can be stored , even when potential difference remain constant (not decreasing )and only potential is decreasing, while potential is not actually the part of above equation.

• Absolute values of potential don't have meaning. Potential energy is always defined only up to an additive constant, so differences are the only meaningful thing to talk about. – jacob1729 May 29 '18 at 13:33
• @jacob1729 But, in that video potential difference always remain constant , right ? Then ,how can it help in increasing charge stored in the plate – salvin May 29 '18 at 13:37
• I've looked at the first two thirds of this video, and wouldn't recommend using it to learn about capacitors. There's one outright mistake, and that is trying to apply the equation for the potential due to a point charge to a uniformly changed flat plate. But, quite apart from that, the whole approach via bringing up an earthed plate to a charged plate is, in my view, unnecessarily complicated. As others have said, it's the potential $difference$ between the plates that matters. Of course you may have your own reasons for approaching capacitors in this way – in which case, good luck! – Philip Wood May 29 '18 at 14:14

Capacitors are, today, understood in terms of the pd between their plates, which produces an electric field between the plates, and this electric field can be related (by Gauss's theorem) to the charge on each plate. From the analysis emerges $Q=CV$, together with an expression for $C$ in terms of the geometry of the gap between the plates and the nature of the 'dielectric' between the plates. We don't bring the absolute potential of either plate into the argument.