Capacitance: C = Q/V

• Connect the plates to a supply and it pushes electrons onto one plate and pulls them off the other: the plates end up with charges $+Q$ and $-Q$. “The charge stored” means $Q$, the magnitude on each plate — the total is actually zero.

• The capacitance definition carries two marks: the ratio charge / potential difference, and the precision that $Q$ is the charge on one plate while $V$ is the p.d. between the plates (9702/42/O/N/24 Q7(a)).
• One farad is enormous; real capacitors are labelled in $\mu\text{F}$, nF or pF. And capacitance is not only for capacitors: an isolated conducting sphere has $V = Q/4\pi\varepsilon_0 r$, so $C = Q/V = 4\pi\varepsilon_0 r$ — a 2025 question gave a sphere of 69 pF holding 83 pC and asked for its surface potential: $V = 83/69 = 1.2$ V (9702/42/O/N/25 Q6(b)).
• On a graph of $Q$ against $V$ the definition appears as the gradient: a 2025 network question read $C = 400\ \mu\text{C} / 6.0\ \text{V}$ straight off one (9702/42/M/J/25 Q7(b)(i)) (9702/42/F/M/25 Q5(b)).

$C = Q/V$: charge on one plate per unit of p.d. between the plates. Gradient of the $Q$–$V$ line.