A capacitor is made of two pieces of metal separated by an insulator. When the capacitor becomes charged there will be a potential difference (p.d.) across the two pieces of metal.
The capacitance of a capacitor is measured in Farads ($F$), usually in $\mu F$ or $nF$. The capacitance of a conductor depends on its construction, not the charge on it or the p.d. across it.
The charge ($Q$) stored by a conductor is directly proportional to the p.d. ($V$) across its plates. The constant of proportionality is the capacitance of the conductor:
$$ C=\frac{Q}{V}\quad or\quad Q=VC $$
where:
<aside> 🗒️ This means that the capacitance of the capacitor is numerically equal to the amount of charge it stored when the p.d. is 1$V$)
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Parallel plates:
If we take an electron from the left-hand plate and transfer it to the right-hand plate, we have to do work in moving the electron since the electrical force acting on it opposed this motion.
The more charge that is stored on the plates, the more difficult it will be to move the electron since the electric field between the plates will be larger.
The work that is done in placing charge on the plates of a capacitor is stored as potential energy in the charged capacitor. The more charge that is stored on the capacitor, the greater the stored potential energy.
The equations for the energy stored in a capacitor are:
$$ E=\frac{1}{2}QV\quad E=\frac{1}{2}CV^2\quad E=\frac{1}{2}\frac{Q^2}{C} $$
where: