Meet the most famous formula in electricity: $V = I\,R$ — read aloud: V equals I R. $V$ is voltage in volts, the push. $I$ is current in amperes, the flow. $R$ is resistance in ohms (Ω — the Greek letter omega), how much the path squeezes. Push harder, more flows. Squeeze harder, less flows. That is the entire law, and it runs every circuit in this chapter.

Water helps: voltage is the height of the water slide, current is how much water rushes down, resistance is how narrow the slide gets. To find a current, read the law backwards: $I = \dfrac{V}{R}$ — the push divided by the squeeze. Volts over ohms comes out in amperes, every single time.