Electric circuits are essentially paths through which electrical current moves. The two basic types of electric circuits are the direct-current, or DC circuit, in which the current flows only in one direction, and the alternating-current, or AC circuit, in which current alternates at a given frequency (usually 60 Hz in North America and 50 Hz in Europe). Circuits consist of localized circuit elements connected by conducting paths (e.g., wires). The three basic circuit elements are resistors, capacitors, and inductors. An example of the simplest circuit possible is provided by resistors connected across an emf source either in series or in parallel. When resistors are connected in series, all the current flowing through one resistor also flows through each of the other resistors and the total resistance of the circuit is the sum of the individual resistances of the resistors. A parallel circuit, however, is one that contains a junction. The current flowing through the circuit divides, as more than one path is available. Accordingly, only part of the current flows through any resistor or circuit element. The higher the resistance of a given path in a parallel circuit, the less current will flow through it. As a result, the voltage across each branch of a parallel circuit is the same, but the current may not be the same.
This is especially useful in understanding basic circuits, which apply the same voltage across each circuit element (e.g., a device or appliance) whose individual loads draw different amount of current, depending on their power requirements.
The behavior of electrical circuits is described by Kirchoff's laws. The current law states that the sum of the currents flowing into a junction is equal to the sum of the currents flowing out. The voltage law states that the sum of the emf's in a closed circuit is equal to the sum of the potential difference of the components. These laws can be illustrated by applying them to a parallel circuit, such as a simple one consisting of two resistors, R and R. Since there are two junctions in the circuit, the current is not the same everywhere in the circuit. According to Kirchoff's current law, the current flowing into the junction is equal to the amount of current leaving the junction. From Ohm's law, the potential differences across each resistor are equal to R I and R I, respectively. Because the circuit is parallel, there are two possible paths, or two circuits, for the current to flow through. Because there is only one resistor in each circuit, the potential difference across it must be equal to the emf. Kirchoff's voltage law applied to the first circuit is then: emf = R I, and emf = R I for the second one. For the overall circuit, this yields: emf = R I = R I.
The total resistance of a series circuit is equal to the sum of the resistance of each of the circuit elements. This is not the case for a parallel circuit. In a parallel circuit, the total resistance is equal to the potential difference across the parallel segment divided by the current going through it: R = V/I.
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