This tutorial illustrates how the flow of water through a system of pipes can be used to understand the flow of current through an electric circuit. Here blue particles represent the flow of water, moving as electrons do through a circuit to create electricity. The rate of water flow, defined as the volume of the fluid moving past a certain point in a fixed amount of time, is comparable to the rate of charge passing a point in a circuit (electric current), which is measured in amperes (coulombs/second).
Adjust the Flow Rate slider to increase or decrease the power of the water pump and the rate of flow through the pipes. The water pump is equivalent to an electric battery or other power source in this analogy, providing the impetus that pushes water through the system, as a battery provides the potential difference (voltage) that results in the movement of electrons through wires (electricity).
The pump pushes water to different parts of the pipe system, which then recombines, just like a parallel circuit funnels electricity along each parallel branch, which then also recombines. The water encounters resistance, just the way electricity in a parallel circuit would. The resistance to the water flow is provided by a coil of long, thin piping and a propeller that turns in response to the current. A coil of narrow wire could serve as a resistor in an electric circuit. Other common resistors found in electric circuits include light bulbs and electric motors. Electrical resistance is measured in ohms, in honor of Georg Ohm, the physicist who experimentally established that there are no perfect conductors.
In the tutorial, the coil furnishes much more resistance than the propeller, so more water will take the path of least resistance. Notice, however, that the flow in the branches adds up to the total amount of water flowing through the system. For instance, when the flow rate is maximized, the first flow meter shows three units of flow, while the flow past the coil is shown to be one unit and the flow past the propeller is two units. In an electric circuit, ammeters measure the flow of current in amperes; the readings would add up to the total current flowing through the circuit.