Electrical Circuits
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Simple circuit.

Electrical circuits are designed to perform electrical work in a controlled manner. They can be compared to a small city; the roads are like wires, the stoplights are like switches, and businesses are like electrical devices where work happens. And cars are like electrons that deliver the workers to the workplace. Electrical circuits can be very basic, consisting of a power supply, a fuse, a switch, a component that performs work, and wires connecting them all together Figure 36-6. The power source—for example, the battery—creates a potential difference across its terminals measured in volts. It pushes a flow of electrons (current flow), measured in amps, when the switch is in the closed position completing the path. The current flows through the fuse into the circuit wires to the lamp where the resistance of the lamp filament causes it to glow, producing light. The current continues to flow through the lamp filament and the return pathway through the wires back to the battery to complete the circuit. When the switch is moved to the open position, the current path is broken and current flow stops, turning the lamp filament off.

A circuit can be much more complex than the one just described. But even then, most circuits contain a power source, circuit protection device, control mechanism, load, and connecting wires. As you understand the principles of electricity and how it behaves, you will be able to understand more complicated circuits.

Technician Tip
One thing to know about current flow in a circuit: It doesn’t flow like dominoes, with a little bit of delay between each domino. Current flow is more like ball bearings lined up touching each other inside a closely fitting pipe. As you push on a ball bearing at one end, the ball bearing at the other end falls out. Current flow works the same way, except the effect of current flow travels at the speed of light. Thus, if you add an electron at one end of a wire that is 186,000 miles (300,000 km) long, an electron at the other end of that wire will be pushed out 1 second later. That is why we say that current flow stays the same throughout a series circuit. As one electron is moved, all of the others move with it.