You have met this shape before. Newton's gravitation put two masses over a squared distance; Coulomb's law puts two CHARGES over a squared distance, and the algebra is a twin: $F = \dfrac{k_e q_1 q_2}{r^2}$. Read aloud: F equals k-e q-one q-two over r squared. Letter by letter — $F$ is the electrostatic force in newtons; $q_1$ and $q_2$ are the two charge magnitudes in coulombs (the subscripts only NUMBER the two charges, 1 and 2, with no order implied — swap them and the formula does not notice); $r$ is the separation between their centres, in metres; and $k_e$ is Coulomb's constant, $9.0 \times 10^{9}\ \mathrm{N \cdot m^2/C^2}$ on this paper. Whichever letter the question leaves blank is the one you solve for.

Now the part worth carrying out of the room: gravity between two protons is unimaginably feebler than the electric force between them — by something like $10^{36}$. Gravity only wins on the large scale because it never cancels, and charge does. Which is also why the coulomb is a monster of a unit: two full coulombs a metre apart would shove with nine BILLION newtons. Real bench charges live in microcoulombs, $1\ \mu\mathrm{C} = 10^{-6}\ \mathrm{C}$, and dropping that prefix is this chapter's most reliable way to lose two marks.

Where does charge come from? A current, running for a while: $Q = It$ — Q equals I t — with $Q$ the charge in coulombs, $I$ the steady current in amperes, and $t$ the time in seconds. One ampere IS one coulomb per second, so this formula is really just that sentence, rearranged.