Electrical Diagrams Help (page 2)
To understand how electric circuits work, you should be able to read electrical wiring diagrams, called schematic diagrams . These diagrams use schematic symbols . Here are the basic symbols. Think of them as something like an alphabet in a language such as Chinese or Japanese, where things are represented by little pictures. However, before you get intimidated by this comparison, rest assured that it will be easier for you to learn schematic symbology than it would be to learn Chinese (unless you already know Chinese!).
The simplest schematic symbol is the one representing a wire or electrical conductor: a straight solid line. Sometimes dashed lines are used to represent conductors, but usually, broken lines are drawn to partition diagrams into constituent circuits or to indicate that certain components interact with each other or operate in step with each other. Conductor lines are almost always drawn either horizontally across or vertically up and down the page so that the imaginary charge carriers are forced to march in formation like soldiers. This keeps the diagram neat and easy to read.
When two conductor lines cross, they are not connected at the crossing point unless a heavy black dot is placed where the two lines meet. The dot always should be clearly visible wherever conductors are to be connected, no matter how many of them meet at the junction.
A resistor is indicated by a zigzaggy line. A variable resistor, such as a rheostat or potentiometer, is indicated by a zigzaggy line with an arrow through it or by a zigzaggy line with an arrow pointing at it. These symbols are shown in Fig. 12-3.
An electrochemical cell is shown by two parallel lines, one longer than the other. The longer line represents the positive terminal. A battery , or combination of cells in series, is indicated by an alternating sequence of parallel lines, long-short-long-short. The symbols for a cell and a battery are shown in Fig. 12-4.
Fig. 12-4 . (a) An electrochemical cell, (b) A battery.
Some More Symbols
Meters are indicated as circles. Sometimes the circle has an arrow inside it, and the meter type, such as mA (milliammeter) or V (voltmeter), is written alongside the circle, as shown in Fig. 12-5a . Sometimes the meter type is indicated inside the circle, and there is no arrow (see Fig. 12-5b) . It doesn’t matter which way it’s done as long as you are consistent everywhere in a given diagram.
Some other common symbols include the lamp , the capacitor , the air-core coil , the iron-core coil , the chassis ground , the earth ground , the alternating-current (AC) source , the set of terminals , and the black box (which can stand for almost anything), a rectangle with the designator written inside. These are shown in Fig. 12-6.
Fig. 12-6 . More common schematic symbols: (a) incandescent lamp; (b) fixed-value capacitor; (c) air-core coil; (d) iron-core coil; (e) chassis ground; (f) earth ground; (g) ac source; (h) terminals; and (i) and black box.
Most direct current (dc) circuits can be boiled down ultimately to three major components: a voltage source, a set of conductors, and a resistance. This is shown in the schematic diagram of Fig. 12-7. The voltage of the emf source is called E (or sometimes V); the current in the conductor is called I ; the resistance is called R . The standard units for these components are the volt (V), the ampere (A), and the ohm (Ω), respectively. Note which characters here are italicized and which are not. Italicized characters represent mathematical variables; nonitalicized characters represent symbols for units.
Fig. 12-7 . A simple dc circuit. The voltage is E , the current is I , and the resistance is R .
You already know that there is a relationship among these three quantities. If one of them changes, then one or both of the others also will change. If you make the resistance smaller, the current will get larger. If you make the emf source smaller, the current will decrease. If the current in the circuit increases, the voltage across the resistor will increase. There is a simple arithmetic relationship between these three quantities.