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The electric circuitry of the average car.

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The electric circuitry of the average car

By Danielle Atlas

When the manufacturing of cars had just begun, electricity was used only to ignite the fuel inside the engine. By the late 1920's, the electric starter replaced the hand crank, electric headlights made acetylene lamps outdated and the louder electric horn replaced the squeak of the hand-squeezed air horn. Today, a car requires an elaborate electrical system of circuits just to produce, store, and distribute all the electricity it requires simply for everyday operation.

In the average car, electric circuits are to perform a variety of tasks – starting the car, recharging the battery and turning on headlights are only a few. The electrical circuits of the car are also used for radios, electric door locks, window defrosters, electric windows and other accessories. Many of these tasks can be performed using the same circuit.

In order to ensure that the circuits therefore, are not overloaded, each circuit has a fuse, to ensure that if the circuit does surge or overload, the car will not be severely damaged.

Almost everything that is powered electrically in an average car is ‘grounded’ to the frame or the body of the car. These are called "grounding points" and complete electrical circuits around the car. Therefore, an earth symbol depicted on a car’s electric circuit is not literally sending the electricity to the ground but rather sending the current through the conductive metal of the car’s frame.

The Battery

The major component in the electrical system of the car is the battery.

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The electrical current travels through an ignition coil. The coil is generates the high voltages required to create a spark. It is essentially a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil. The secondary coil is wrapped around it. The secondary coil normally has hundreds of times more circles of wire than the primary coil. Current flows from the battery through the primary coil. The ignition key is then removed, breaking the circuit. This causes the magnetic field around the primary coil – as it is an electromagnet - to collapse. The secondary coil is then surrounded by a powerful magnetic field. This field induces a current in the coils because of the number of coils in the secondary winding, and this current has an extremely high voltage – about 20,000V. The secondary coil distributes this voltage to the distributor.

The distributor must distribute the high voltage from the coils to the spark plugs in the cylinders of the engine. This is done by the cap and rotor, which are contained in the distributor. The coil is connected to the rotor, which spins inside the cap. This creates an electric pulse, which arcs across the small gap between the rotor and the contact wire (they don't actually touch). The pulse then continues down the wire of the circuit to the spark plug on the appropriate cylinder.

The spark plug forces electricity in the circuit to traverse a gap.

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The various switches allow the headlights to be used in different ways. When the main lighting switch completes the circuit to the headlamps, the low beam lights the way for city driving and for use when meeting oncoming traffic on the highway. When a dimmer switch is activated, the single filament headlamps go "on," along with the high beam of the two filament headlamps. The next activation of the dimmer switch returns the head lighting system to low beams only on the two filament lamps.

The electrical circuits of an average car enable the driver to perform many tasks easily. Processes such as ignition and starting the engine, recharging the battery and turning on the headlights of the car have improved as time has passed, and will continue to be improved and modernised in the future.


  • About.com http://www.autorepair.about.com/
  • Beginner’s guide to how a car works – starting and electrical systems http://www.sexydog.freeserve.co.uk/driven/guidesections/guide_a.html
  • Electrical system http://www.autoeducation.com/autoshop101/electrical.htm
  • Exhibit cross reference http://www.exploratorium.edu/xref/exhibits/automotive_ignition.html
  • Questions in the box http://hydro4.sci.fau.edu/~rjordan/phy2044/Q_A/ans_5.htm
  • How automobile ignition systems work http://auto.howstuffworks.com/ignition-system.htm
  • How car engines work http://auto.howstuffworks.com/engine4.htm
  • The electrical system – how it works in your car http://www.mycarwizard.com/autoshop/electext.html
  • Wiring your car – parts 2 to 10 http://www.constructorscarclub.org.nz/Articles/wiringpart2.html
  • Physics through Applications, Jardine,J. Oxford Press, 1989.

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