Charge

Charge ($Q$ or $q$) is the property of matter that makes it respond to electric and magnetic interactions. It is conserved: in a closed system the total charge does not disappear, it only moves between objects.

Charge is quantised in units of the elementary charge $e\approx 1.60\times 10^{-19}\mathrm{C}$. A proton has charge $+e$ and an electron has charge $-e$. The SI unit is the Coulomb.

Important sign rule: like charges repel, opposite charges attract. A positive test charge is the convention used to define field directions and potential differences, even though metal circuits usually move electrons.

Charge flow is Current:

$I=\frac{dQ}{dt}$

Energy per unit charge is Electric potential, and the difference in potential between two points is Voltage.

Quick scale check: $1\mathrm{C}$ is a huge microscopic charge count, about $6.24\times 10^{18}$ elementary charges. That is why normal currents can involve enormous numbers of electrons without the wire becoming strongly charged overall.

Common mistake: saying “positive charges flow” in a metal wire. Conventional current points as if positive charge flowed; electron drift is opposite that direction.