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An electric circuit is a path in which electrons from a voltage or current source flow. Electric current flows in a closed path called an electric circuit. The point where those electrons enter an electrical circuit is called the "source" of electrons. The point where the electrons leave an electrical circuit is called the "return" or "earth ground". The exit point is called the "return" because electrons always end up at the source when they complete the path of an electrical circuit.
The part of an electrical circuit that is between the electrons' starting point and the point where they return to the source is called an electrical circuit's "load". The load of an electrical circuit may be as simple as those that power electrical appliances like refrigerators, televisions, or lamps or more complicated, such as the load on the output of a hydroelectricpower generating station.
Circuits use two forms of electrical power: alternating current (AC) and direct current (DC). AC often powers large appliances and motors and is generated by coal, hydroelectric, nuclear and other large power plants. DC powers battery, PoE and USB operated vehicles, tools, toys and small appliances, as well as electronic circuits with LEDs (e.g., lighting, monitors, TVs), sensors and computer logic (e.g., networking, cell phones, tablets, laptops). DC power can easily be converted from AC or produced directly from solar, wind and other generators.
Electronic circuits usually use low voltage direct current sources. The load of an electronic circuit may be as simple as a few resistors, capacitors, and a lamp, all connected together to create the flash in a digital camera. Or an electronic circuit can be complicated, connecting thousands of resistors, capacitors, and transistors to create the microprocessors that make computers possible.
Circuit and wiring diagrams
Electrical and electronic circuits can be complicated. Making a drawing of the connections to all the component parts in the circuit's load makes it easier to understand how circuit components are connected. Drawings for electronic circuits are called "circuit diagrams". Drawings for electrical circuits are called "wiring diagrams". Circuit diagrams and wiring diagrams are usually drawn by skilled draftsmen, and then printed. But they can also be simple pencil sketches drawn by technicians or other workers.
Wiring and circuit diagrams use special symbols recognized by everyone who uses the drawings. The symbols on the drawings show how components like resistors, capacitors, inductors, motors, outlet boxes, lights, switches, and other electrical and electronic components are connected together. The diagrams are a big help when workers try to find out why a circuit does not work correctly.
Resistors and other circuit elements can be connected in series or in parallel. Resistance in series circuits is the sum of the resistances.
The current flowing in an electrical or electronic circuit can suddenly increase when a component part fails. This can cause serious damage to other components in the circuit, or create a fire hazard. To protect against this, a fuse or a device called a "circuit breaker" can be wired into a circuit. The circuit breaker will open, or "break", the circuit when the current in that circuit becomes too high, or the fuse will "blow". This gives protection.
Ground-fault-interrupt (GFI) devices
The standard return for electrical and electronic circuits is the earth ground. When an improperly designed electrical or electronic device fails, it may open the return circuit to the earth ground. The user of the device could become a part of the device's electrical circuit by providing a return path for the electrons through the user's body instead of the circuit's earth ground. When the user's body becomes part of an electrical circuit, the user can be seriously shocked, or even killed by electrocution.
To prevent the danger of electrical shock and the possibility of electrocution, ground fault interrupt devices detect open circuits to earth ground in attached electrical or electronic devices. When an open circuit to earth ground is detected, the GFI device immediately opens the voltage source to the device. GFI devices are similar to circuit breakers, but are designed to protect humans rather than circuit component.