In electric circuits, what SI unit measures electric charge?

The coulomb is the SI unit that measures electric charge in electric circuits.

One coulomb represents approximately 6.242 × 10^18 elementary charges. It is defined through the ampere: a current of one ampere transfers one coulomb of charge past a point each second. This relationship is written as Q = It, where Q is charge, I is current, and t is time.

The unit honors French physicist Charles-Augustin de Coulomb, whose experiments on electric charges helped establish the inverse-square law of electrostatic force. The coulomb is used for both positive and negative charge, although the charge carried by electrons is negative and the charge of protons is positive.

A common mix-up is confusing charge with current. Charge is the quantity of electricity, measured in coulombs; current describes the rate at which charge flows, measured in amperes. Capacitors are often rated in farads, which measure capacitance rather than charge itself.