4 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. desired function. The conductor (most often a wire) carries the current from the source to the device and back to the source. Other compo- nents found in a circuit are controllers (such as switches) and circuit protection (such as a fuse or circuit breaker). A very basic circuit is shown in Figure 1-4. There are several types of circuits. The various types are differentiated by the arrangement of the resistors and other elements. There are series circuits, parallel circuits, and complex circuits. 1.2.1 Series Circuits A series circuit, Figure 1-5, is a circuit in which the loads are connected in such a way that there is only one current path. As shown in the illus- tration, the current must pass through the fi rst resistor, then the second, and then the third before returning to the source. The following lists some of the characteristics of series circuits: • The current (I) is the same through every element and every resistor in the circuit. I TOTAL = I 1 = I 2 = I 3 = I N • The voltage across each resistor is a product of the current times the individual resistance. Recall Ohm’s law (E = I × R). The voltage across each resistance can be different. However, the total voltage must equal the sum of the individual voltages across the resistors. E TOTAL = E 1 + E 2 + E 3 + … + E N = E T • The total resistance of the circuit is the sum of the individual resistances. RTOTAL = R 1 + R 2 + R 3 + … + R N = R T • Ohm’s law is applicable to any part of the circuit or to the circuit as a whole. This is extremely important to remember. It is the key to understanding and solving circuit problems. Volts E I2×R E __ R P __ I 2 Watts P Amperes I Ohms R P __ E E __ I P __ R E 2 __ R E 2 __ P P __ I I×R I×E P×R Goodheart-Willcox Publisher Figure 1-3. Mathematical relationship of power, voltage, current, and resistance. Within each quadrant, the three outer expressions are equal to the inner quantity. Circuit protection Source Conductor Device Controller Goodheart-Willcox Publisher Figure 1-4. A simple electric circuit. The source supplies electromotive force, which moves current through the conductor to the other components. Fuse DC generator Resistor Switch Only one pathway Goodheart-Willcox Publisher Figure 1-5. Series circuits have only one pathway for current.