Electric current

You press the light switch and the lamp turns on. You plug your phone into its charging cable and the battery fills up. Electric current works for you all day long, mostly completely invisibly. Time to take a close look at what is actually flowing there.

The circuit must be closed

Current only flows if it has a round trip: from the battery through the wire to the lamp and back again. That is why it is called a circuit. As soon as the loop is broken at a single point, for example by an open switch, no current flows anywhere and the lamp stays dark.

Voltage, current, and resistance

Picture the circuit as a water loop. The voltage U is the pressure of the pump, measured in volts (V). The current I is the amount of water flowing per second, measured in amperes (A). The resistance R is a narrow section of the hose that slows the flow, measured in ohms (Ω). Together this gives the formula I = U/R: more pressure means more flow, a narrower section means less flow.

A single battery delivers a fixed voltage, an ordinary AA battery 1.5 V for example. Connect two of them in a row, plus pole to minus pole, and the voltages add up to 3 V. In the water picture that is two pumps in the same hose, pushing together.

Conductors, insulators, and safety

Metals like copper let current through easily, they are conductors. Rubber, plastic, and glass block it, they are insulators. That is exactly why cables have copper on the inside and a plastic coating on the outside. At home, sockets deliver 230 V, which is life threatening. Never put objects or fingers into a socket and keep electrical devices away from water, because your body conducts current too.