Your First Circuit

The loop is everything

An electric circuit is exactly what the word says: a closed circle. Charge leaves the battery’s + terminal (following convention from last lesson), travels through wires and components, and returns to the − terminal, where the battery pumps it back up to full energy and sends it around again. Every useful circuit has at least a source (something that supplies energy — a battery here) and a load (something that usefully spends it — a lamp, a motor, a chip).

Because the loop is completely full of charge, the flow everywhere in a simple loop is the same — you can’t have more current entering the lamp than leaving it, any more than a circle of marbles can bunch up. Break the loop at any point and the current stops everywhere, instantly. That is all a switch is: a deliberate, re-closable break.

Reading schematics

Engineers draw circuits with standard symbols connected by lines (the lines are ideal wires — zero resistance, just connections). The physical layout on your desk can look completely different from the drawing; what matters is only what connects to what. Here are the symbols you’ll need:

The two failure modes

Open circuit: the loop is broken — a switch is off, a wire came loose, a bulb filament snapped. Current: zero. Nothing happens. Annoying, but safe.

Short circuit: the opposite and far more dramatic failure. If a stray wire connects the battery’s terminals without passing through a load, almost nothing limits the flow. The current is held back only by the battery’s tiny internal resistance, so it becomes enormous — the wire and the battery heat up fast, and things can melt or catch fire. The word “short” literally means the current found a shorter path than the one you built for it.