Electricity is delivered to homes and other buildings from the power distribution grid. Inside these buildings, the electric charge moves throughout a large circuit, which is made up of many smaller circuits. The hot end of the wire leads to the power plant.
The neutral end of the wire leads to the ground. Since the hot wire is connected to a high source of energy, and the neutral end is connected to the ground, there is a voltage across the circuit. The charge moves when the circuit is closed. This is called an alternating current because the current is rapidly changes directions.
The electricity delivered from the power distribution grid is a consistent voltage (in the U.S. this is 120 and 240 volts), however the current within the building (the resistance) can vary dramatically.
All of the different devices using power put up a certain mount of resistance, which is called the load. It is the resistance that makes the device work.
For example, the filament inside some light bulbs are very resistant to the flow of the charge. The charge has to work extra hard to heat the filament, making the bulb glow – which is also why this type uses more energy.
The wiring in a building is set up so that the hot and neutral never touch. The charge running through the circuit must always pass through an appliance first, which is the resistor in this case. That means the electrical resistance in the device or appliance is able to limit how much of a charge can pass through.
When the voltage and resistance are constant, the current must be constant, as well. For safety reasons, appliances are designed to keep the current at low levels. If the charge was allowed to run free, the wires in the appliance and in the building could heat up to unsafe levels, resulting in a fire.
How circuit breakers work to keep you safe is by "cutting the juice" any time the current jumps to unsafe levels.
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