We use symbols to represent the components of an electric circuit
Thus far, this unit of The Physics Classroom tutorial has focused on the key ingredients of an electric circuit and upon the concepts of electric potential difference, current and resistance. Conceptual meaning of terms have been introduced and applied to simple circuits. Mathematical relationships between electrical quantities have been discussed and their use in solving problems has been modeled. Lesson 4 will focus on the means by which two or more electrical devices can be connected to form an electric circuit. Our discussion will progress from simple circuits to mildly complex circuits. Former principles of electric potential difference, current and resistance will be applied to these complex circuits and the same mathematical formulas will be used to analyze them.
Electric circuits, whether simple or complex, can be described in a variety of ways. An electric circuit is commonly described with mere words. Saying something like "A light bulb is connected to a D-cell" is a sufficient amount of words to describe a simple circuit. On many occasions in Lessons 1 through 3, words have been used to describe simple circuits. Upon hearing (or reading) the words, a person grows accustomed to quickly picturing the circuit in their mind. But another means of describing a circuit is to simply draw it. Such drawings provide a quicker mental picture of the actual circuit. Circuit drawings like the one below have been used many times in Lessons 1 through 3.
A final means of describing an electric circuit is by use of conventional circuit symbols to provide a schematic diagram of the circuit and its components. Some circuit symbols used in schematic diagrams are shown below.
A single cell or other power source is represented by a long and a short parallel line. A collection of cells or battery is represented by a collection of long and short parallel lines. In both cases, the long line is representative of the positive terminal of the energy source and the short line represents the negative terminal. A straight line is used to represent a connecting wire between any two components of the circuit. An electrical device that offers resistance to the flow of charge is generically referred to as a resistor and is represented by a zigzag line. An open switch is generally represented by providing a break in a straight line by lifting a portion of the line upward at a diagonal. These circuit symbols will be frequently used throughout the remainder of Lesson 4 as electric circuits are represented by schematic diagrams. It will be important to either memorize these symbols or to refer to this short listing frequently until you become accustomed to their use.