A Typical Car Electrical System

Apart from the main charging, starting and ignition circuits, there are other circuits that power lights, electric motors, the sensors and gauges of electrical instruments, heating elements, magnetically operated locks (if fitted), the radio and so on.

All circuits are opened and closed either by switches or by relays - remote switched operated by electromagnets.

The energy needed to push current through a resistance is transformed into heat. This can be useful, for example in the very thin filament of a light bulb, which glows white hot.

However, a component with a high current consumption must not be connected using wires which are too thin, or the wires will overheat, blow a fuse, or burn out.

All the electrical units of measurement are interrelated: a pressure of 1 volt causes a current of 1 amp to flow through a resistance of 1 ohm. Volts divided by ohms equal amps. For example, a light bulb with a resistance of 3 ohms, in a 12 volt system, consumes 4 amps.

Electrical System - Current, Voltage and Resistance

The electrical system of a car is a closed circuit with an independent power source the battery. It operates on a small fraction of the power of a household circuit.

Current flows along a single cable from the battery to the component being powered, and back to the battery through the car's metal body. The body is connected to the earth terminal of the battery by a thick cable.

This type of circuit is called an earth-return system any part of it connected to the car body is said to be earthed.

The strength of the current is measured in amperes (amps); the pressure that drives it round the circuit is called voltage (volts). Modern cars have a 12 volt battery. Its capacity is measured in amp/hours. A 56 amp/hour battery should be able to deliver a current of 1 amp for 56 hours, or 2 amps for 28 hours.

If the battery voltage drops, less current flows, and eventually there is not enough to make the components work.

Current, voltage and resistance

The extent to which a wire resists the flow of current is called resistance, and is measured in ohms.

Thin wires conduct less easily than thick ones, because there is less room for the electrons to travel through.

In a negative (-) earth-return system, the current flows from the positive (+) terminal of the battery to the component being operated. The component is earthed to the car body, which is earthed to the negative (-) terminal of the battery.