Everything in this chapter is built from five quantities, and the unit on the page tells you which one you are holding before you have thought about it at all. Learn to read the unit first.

Charge is the stuff itself, measured in coulombs (C). Current is how fast it flows: $Q = I t$, read aloud Q equals I t, where $Q$ is the charge in coulombs, $I$ is the current in amperes (A) and $t$ is the time in seconds. One ampere is one coulomb crossing a section every second, so the ampere already has a second buried in it — which is why an amp-hour is a charge and not a current, and why 1 A·h is 3600 C.

Voltage is the push, in volts (V): energy per unit charge, and always measured BETWEEN two points, never at one. Resistance is the opposition, in ohms (Ω): $V = I R$ — V equals I R — with $V$ the voltage across a component in volts, $I$ the current through it in amperes and $R$ its resistance in ohms. An ohm is a volt per ampere, by definition.

And power is the rate energy is converted, in watts (W): $P = V I$ — P equals V I — where $P$ is the power in watts and $V$ and $I$ are the same two quantities as before. One watt is one joule per second. Units guide, they do not confess: a number wearing watts is certainly not a current, but a number wearing amperes might still be the wrong current. The check runs one way only.