Electric Circuits: Parallel vs. Series, Diagrams and Components

FIELDS OF STUDY: Electronics; Electromagnetism

ABSTRACT: This article describes four basic parts and two basic configurations of elements used in constructing electrical circuits. The elements of an electrical circuit use the current flowing through them to produce a desired result. The basic method of calculating the total resistance of series and parallel resistors is described.

Principal Terms

• capacitor: an electrical part consisting of two conductors separated from each other by a nonconductor, allowing it to store electrical charge temporarily.
• diodes: devices in which an anode and a cathode, or the transistor equivalent, control the direction of current flow.
• elements: the parts of an electrical circuit, each of which has a specific function.
• ground: a direct connection to a larger body by which excess current is carried away, preventing errant electrical potentials from being generated in the circuit.
• hot conductor: in an electrical circuit, the conductor that brings current into a component element (the conductor having the higher electrical potential).
• inductor: a conducting coil in which the current generates a proportional magnetic field that, in turn, impedes the flow.
• neutral conductor: in an electrical circuit, the conductor that accepts the current coming out of the component elements (the conductor having the lower electrical potential).
• parallel: circuits or segments of circuits in which any two different elements are joined at two common points side by side.
• resistor: a device or material that resists the flow of electrons through it.
• series: circuits or segments of circuits in which any two different elements are joined only at one common point end to end.

The Basic Electric Circuit

A circuit is a structure in which some property moves through an interconnected series of stages from a starting point to a finishing point. In an electric circuit, that property is the flow of electrons through a conductor under the influence of an electromotive force (EMF). The EMF results from the difference in electrical potential between two points, such as the terminals of a battery or the connection slots of a wall outlet. The difference in electrical potential between two points is the voltage (V). The flow of electrons through the circuit is called the current. It always moves in the direction from a point of higher electrical potential to one of lower potential. The current is measured in units called amperes (A or amp).

Electrical current is either direct current (DC) or alternating current (AC). Direct current is produced by a constant voltage that drives current flow in one direction only. Alternating current is produced by a potential that varies in a cyclic manner between a maximum positive potential and an equivalent negative potential. The current flow thus switches direction as the applied potentials alternate between positive and negative. Switches, wires, and conductors are "nonpolarized," meaning that they work equally well regardless which way the current flows. Devices that function differently based on the flow are "polarized."

An electrical circuit is useful only when it does a desired function. Simply connecting two points together with a conductor creates a "short" circuit that will drain electrical current from one point to the other freely. A functional circuit consists of various elements that act with the current to produce a desired result. The basic component elements are the resistor, the capacitor, and the inductor. Devices called diodes also carry out important functions in directing the current flow. Each different element has a standard symbol to identify it easily in a circuit diagram. Each wire is represented by a straight line. The circuit diagram is essentially a map of the circuit that shows how each element is connected in relation to the other elements and to the voltage source. The important functional characteristic of each element, such as its resistance or capacitance, is typically also shown to be the respective element. Conventional current diagrams indicate current flowing from positive to negative, while electron flow diagrams show it flowing from negative to positive. Because the direction of flow affects how polarized devices work, it is important to know the type of diagram one is using.

Resistors resist current flow. They also change the electrical potential from one end of the resistor to the other as a "voltage drop." In complex circuits, resistors are generally used to produce specific electrical potentials or voltages. This permits the use of a wide variety of devices that require different voltages in the overall circuit function.

An inductor has a similar role. An inductor is a simple coil of wire. Current flow through the coil generates a magnetic field. This in turn "induces" a voltage that impedes the current flow and is directly proportional to the current flowing through the inductor. Inductors are generally used to stabilize an electrical signal within a complex circuit.

Because of its construction, a capacitor is a nonconductive element in any circuit. Capacitors consist of two conducting surfaces separated by a nonconductor. A capacitor can store electrical charge, releasing it when the driving potential is removed. The amount of charge that a capacitor can store depends primarily on the applied voltage and the size of the conductive surfaces inside the capacitor. The discharge from even a small capacitor can be very damaging or even fatal. The electrical spark of a Taser weapon, for example, is produced by the discharge of capacitors. In complex circuits, capacitors are often connected between the main circuit and a ground to stabilize or "condition" the electrical signal in the circuit.

Diodes were originally vacuum-tube devices that used a cold anode and a heated cathode to drive current flow in one direction only. Transistor-based devices use semiconductor structures to achieve the same function. Diodes are used in both DC and AC circuits. In a DC circuit, diodes prevent errant currents from flowing in the wrong direction. In AC circuits, diodes "rectify" the alternating current by allowing the flow of current in one direction and blocking the current flow in the opposite direction. By combining different elements, diodes in an AC circuit can produce the equivalent of a DC current.

Circuit Structures

The elements in an electrical circuit can be combined in many different ways, but all circuits consist of just two basic structures. Elements may be connected in series or in parallel. In a series circuit, elements are connected head to tail so that the same current flows through each element. Electrical work is done at each element and decreases the voltage across the element according to its resistance (R). This "work" is the power consumed by the element, expressed as watts. In a parallel circuit, various elements are connected side by side at common points. The voltage measured across all of the parallel elements is the same. The total current entering a parallel circuit is the same as the current leaving, but a different current can flow through each parallel element. Complex circuits are constructed using a combination of series and parallel sections, forming a series-parallel circuit.

In all circuits, the hot conductor represents the high potential by which current flows from the voltage source. The neutral conductor represents the lower potential by which current flows back toward the voltage source. In a three-conductor system, the third conductor leads to the ground. To prevent short circuits from forming, the different conductors are isolated from each other using insulators, devices or materials that are very poor conductors of electrical current.

Calculating the Load

The electrical service in buildings is primarily a parallel-circuit system. This ensures that the voltage supplying all of the outlets is the same throughout the system. This cannot be done with a series-circuit system as the voltage at each outlet would decrease according to the number of devices that are drawing current. The load on a circuit is the sum of the power being drawn by the devices operating in the circuit. A parallel circuit distributes the load so that all of the devices receive the correct voltage supply. A series circuit can properly supply only one functioning device at a time.

The resistances in a series circuit add together according to the expression

RT = R1 + R2 + R3 + etc.

where each R value is expressed in ohms (Ω). In a parallel circuit, the total resistance is distributed as the reciprocal values, according to the expression

1/RT = 1/R1 + 1/R2 + 1/R3 + etc.

Circuit Complexity

Electrical circuits range from extremely simple to extremely complex. Analog circuits, through which current flows constantly, are generally much simpler in structure. Digital circuits, such as those in integrated circuit and computer-processor chips, are extremely complex. Many different elements besides resistors, capacitors, inductors, and diodes are typically used in common circuits, and each additional element makes the analysis and calculation of circuit properties that much more complex.

Bibliography

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Dorf, Richard C., and James A. Svoboda. Introduction to Electric Circuits. 9th ed. Hoboken: Wiley, 2014. Print.

Nilsson, James William, and Susan A. Riedel. Electric Circuits. New York: Prentice, 2008. Print.

Pulfrey, David L. Understanding Modern Transistors and Diodes. New York: Cambridge UP, 2010. Print.

Schulz, Alexander L. Capacitors: Theory, Types, and Applications. Hauppauge: Nova Science, 2010. Print.

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