Chapter 7 Series Circuits 59 Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. Series Circuit Characteristics Consider the example of a highway between two towns. This highway has four narrow bridges connected by stretches of road, as shown in Figure 7-1. All the traffi c moves in one direction and must cross the bridges one after the other. The bridges can be considered to be in series with each other. Before each bridge is a traffi c sign, “Slow down, narrow bridge.” You reduce your speed because the bridge has slowed down your progress. You can compare this traffi c situation to a simple electrical circuit, Figure 7-2. The highway has been replaced by an electrical conductor or wire. The bridges have been replaced with resistors, R. Your travel in the direction of the arrows has been replaced by a fl ow of electrons from negative to positive terminals of the power source. Resistors R1, R2, R3, and R4 are all in series with the power source. Each R has a certain resistance to the fl ow of electrons between points A and B. The total resistance in the series circuit, then, is equal to the sum of all the resistors. RT = R1 + R2 + R3 + R4 where RT equals the total resistance. As you study Figure 7-2, you observe that all electrons entering the circuit at point B, must fl ow through R4, R3, R2, and R1 before reaching point A. Also, all electrons going in at point B must come out at point A. None are lost along the way. So, one can conclude that the electron fl ow, or current, must be the same at any point in the circuit. IT = IR1 = IR2 = IR3 = IR4 where IT is the total current of the circuit, IR1 is the current through resistor 1, IR2 is the current through resistor 2, etc. This formula was devised by Gustav Kirchhoff and is referred to as Kirchhoff’s current law. It can be stated: Whatever current fl ows into a circuit junction (or connection) must fl ow out. Highway Bridges Goodheart-Willcox Publisher Figure 7-1. The bridges on the highway are in series. A B R1 R2 R3 R4 Goodheart-Willcox Publisher Figure 7-2. The resistors are in series. Gustav Robert Kirchhoff (1824–1887) Kirchhoff was educated in Russia, but spent much of his professional life in German institutions of higher learning. He is known for pioneering work in electrical network theory. He and chemist Robert Wilhelm Bunsen developed the spectrometer, a device that can determine the chemical composition of various materials. Kirchhoff was confi ned to a wheelchair due to an accident early in his life. Scientist Profi le