Circuits

When electricity flows from one point to another along a closed path (a wire, for example), the electrons flow from a point with a negative charge to one with a positive charge. Any closed path followed by an electrical current is called a circuit (Fig. 27-1). An electrical circuit is a complete conduction path that carries current from a source of electricity to and through some electrical device (or load) and back to the source. Current can't flow unless there is a closed circuit back to the source.

Electrical circuits can be arranged in a couple of different ways. In a series circuit, the parts of the circuit are connected one after another, and the resistances and voltages add up. The current is the same in all points of a series circuit. Old-fashioned Christmas tree lights that all went out when one light blew were in series. When the single light blew out, the circuit was broken, and the electricity couldn't make the trip around the circuit to light the other lights.

When two or more branches or loads in a circuit are connected between the same two points, they are

Hot bus bars

Hot bus bars

Neutral bus bar

Voltage source (electrical panel)

Hot wire Neutral wire.

Neutral bus bar

Voltage source (electrical panel)

Figure 27-1 Circuit.

said to be connected in parallel. Parallel circuits are the standard arrangement in all building wiring. Each parallel group acts as a separate circuit. If one of these smaller circuits is broken, only the devices on that section are affected, and the rest of the circuit continues to circulate electricity.

Sometimes, due to worn insulation on a wire or another problem, an accidental connection is made between points on a circuit. This connection shortens the circuit and lets the electricity take a shortcut back to the source. The electricity doesn't encounter the resistance that would be in the normal wiring, and the current rises instantly to a very high level. This is called a short circuit. If the flow of electricity isn't stopped by a fuse or circuit breaker, the heat generated by the excessive current will probably start a fire.

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