The water analogy

This page explains how the analogy between electricity and a flow of water can be helpful.

Summary

A flow of water has some similarities to a flow of electricity but is easier to visualise. The analogy can therefore be helpful in understanding certain electrical concepts.

Safety

Water is wet. If it leaks out or gets spilled it can make a mess.

Water can be fatal if inhaled. Learn to swim.

The water analogy

The flow of water is often a useful (though not perfect) analogy for the flow of electricity, as shown (humourously) in this xkcd cartoon.

Water flow and electrical current

Water flow (in litres/minute, or whatever you like) is analogous to the electrical current in Amps. A current of 1 Amp flowing around a circuit corresponds to 6.25 billion billion electrons per second passing any given point.

If a river can flow through several alternate channels, the flow will distribute itself according to the width of the channels, with most flowing through the widest. In the same way, if electricity is given several paths it can follow, the most current will flow through the path offering the least resistance.

Water pressure and voltage

Water pressure is rather like voltage, which is in fact the electrical "pressure" driving a current around a circuit against certain resistance.

If a water pipe contains a constriction, there will be a greater pressure drop across the constriction than over the unconstricted free flowing length. In the same way, there is a voltage (electrical pressure) drop across any resistance in a circuit, the greatest voltage drop across the greatest points of resistance.

A complex circuit diagram, for example for a radio, is often drawn with the positive supply (perhaps 9V) as a line across the top, known as the positive rail, and the negative or ground rail (at 0V) as a line across the bottom. The various components (resistors, capacitors, coils and transistors, or valves in a vintage radio) are strung between the two. As a rough guide then, points in the circuit nearer the top will be at a higher voltage than points lower down.

A proviso

Unlike water, electricity can't just spill out (unless the voltage is very high or the insulation is poor), but can only flow if it can flow all the way around a circuit, perhaps through many different paths on the way. A closed central heating system might therefore be a more accurate analogy. The pump produces a pressure to drive the water around the system just as a battery produces a voltage to drive a current around. A central heating system will include a boiler to heat the water before it takes various paths through the radiators, like paths through an electrical circuit, before returning to the pump.

Going further

To learn more, continue reading through the section Understanding how stuff works (or doesn't) on the Main Page, in particular Electric circuits, volts amps watts and ohms.