Kōkiri Learn

Background reading · Technology

Inside a circuit: voltage, current and resistance

The three ideas that explain every circuit, the symbols engineers use to draw them, and how to keep circuits safe.

Electricity is the flow of tiny charged particles called electrons through a material. A circuit is a complete loop that lets them flow from a power source, through the parts of the circuit, and back again. Break the loop anywhere and everything stops. That is exactly what a switch does.

The water-pipe model

  • Voltage: the push, like water pressure in a pipe. It is measured in volts (V). A AA battery gives about 1.5 V. - Current: how much electricity flows, like how much water passes each second. It is measured in amps (A). - Resistance: how much a part pushes back against the flow, like a narrow bit of pipe. It is measured in ohms (Ω).

    • How they work together

    More push means more flow. More resistance means less flow. Engineers write this as V = I × R, where I stands for current. If you know two of the numbers, you can work out the third.

    Why an LED needs a resistor

    An LED (light-emitting diode) is a tiny light that uses very little power. But connect it straight to a battery and too much current rushes through, and it can burn out. A resistor in the circuit slows the current down to a safe level. Resistors are too small to print numbers on, so they wear coloured bands. Each colour stands for a digit: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.

    Series and parallel

    In a series circuit, parts are in one line, and the same current flows through all of them. If one breaks, they all stop. In a parallel circuit, each part has its own branch, so one can fail while the others keep working. That is why the lights in your house are wired in parallel.

    Drawing circuits

    Engineers everywhere use the same symbols: a long and short line for a cell, a gap with a lever for a switch, a triangle with arrows for an LED, a zigzag or rectangle for a resistor, a circle with M for a motor. A good circuit diagram lets anyone rebuild your circuit, even if they speak a different language.

    Stay safe: battery circuits in class are low voltage. The electricity in wall sockets is 230 V and can kill. Never experiment with mains power.

    Sources and further reading

    Written for Kōkiri Learn students in our own words. Check facts against the sources.

    Used in: Smart Circuits: Electronics and Control