Electric circuits: current, voltage, resistance
Three quantities, one equation, and two rules for series and parallel. Here's the whole of basic circuits — plus the water analogy that makes every part of it intuitive.
Circuits feel abstract because you can't see what's moving. The fix is a picture: think of charge flowing like water in pipes, pushed by a pump (the battery). Everything else follows from three quantities.
The three quantities
| Quantity | Symbol | Unit | Water picture |
|---|---|---|---|
| Current | ampere (A) | Rate of water flowing | |
| Voltage | volt (V) | Pressure from the pump | |
| Resistance | ohm (Ω) | A narrow section of pipe |
Current is the flow of charge; voltage is the push that drives it; resistance is anything that slows it down.
Ohm's law
The three are tied together by one equation:
More voltage pushes more current through the same resistor. More resistance means less current for the same voltage. Rearrange it as needed: or .
Series vs parallel — the two rules that matter
Series (one single loop, components in a line):
- The current is the same everywhere — there's only one path.
- Voltages add up to the battery voltage. Resistances add: .
Parallel (the path splits into branches):
- Each branch gets the full battery voltage.
- The currents in the branches add up to the total. Total resistance drops below the smallest branch.
Why a series circuit dims when you add bulbs
Add a bulb in series and total resistance rises, so by the current falls — and every bulb shares that smaller current, so all of them dim. In parallel, each new branch gets its own full voltage, so the others don't dim at all. This is why house wiring is parallel.
Decide series or parallel before any maths. Trace the circuit and ask: does the current have one path or does it split? That single question chooses every rule you'll use next.
Last revised 5 March 2025.