#### THE WATER ANALOGY

Electricity is the flow of electrons — hypothetical particles thought to constitute the basic "stuff" of electricity. Many people have been taught electrical theory using an analogy with water. In a comparison with water flowing through a pipe, the electrons would be the water.

The flow of electricity can be measured much like the flow of water through a pipe. The unit of measurement used is amperes, frequently abbreviated as amps (a). When connected to a circuit, an ammeter will measure the actual amount of current flowing through the circuit. When relatively few electrons flow through a circuit, the amperage is low. When many electrons flow, the amperage is high.

Just as water pressure is measured in units such as pounds per square inch (psi), electrical pressure is measured in units called volts (v). When a voltmeter is connected to a circuit, it is measuring the electrical pressure. The higher the voltage, the more current will flow through the circuit. The lower the voltage, the less current will flow.

While increasing the voltage in a circuit will increase the flow of current, the actual flow depends not only on voltage, but also on the resistance of the circuit. Resistance is the amount of force necessary to push the current through the circuit. The standard unit for measuring resistance is an ohm (W or omega). Resistance in a circuit varies depending on the amount and type of components used in the circuit. The main factors which determine resistance are:

- Material — some materials have more resistance than others. Those with high resistance are said to be insulators. Rubber is one of the best insulators available, as it allows little current to pass. Low resistance materials are said to be conductors. Copper wire is among the best conductors. Most vehicle wiring is made of copper.
- Size — the larger the wire size being used, the less resistance the wire will have. This is why components which use large amounts of electricity usually have large wires supplying current to them.
- Length — for a given thickness of wire, the longer the wire, the greater the resistance. The shorter the wire, the less the resistance. When determining the proper wire for a circuit, both size and length must be considered to design a circuit that can handle the current needs of the component.
- Temperature — with many materials, the higher the temperature, the greater the resistance. This principle is used in many of the sensors on the engine.