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You probably have many electrical devices at home that are plugged into wall sockets. You may use many of those devices—such as a refrigerator, lights, and stereo—at the same time. For each appliance to draw the correct amount of current, the circuits in your house must provide the same voltage to every device regardless of whether other devices are connected. In this section you will learn about the differences between series and parallel circuits and how these circuit types apply to the electrical wiring of a house. 
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Series circuits and voltage drop
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A series circuit has only one path that electric current can follow. A series circuit connects the output of the first device to the input of the second device, the output of the second device to the input of the third device, and so on. In a series circuit all the current flows in series through each device, one after another.
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To understand how voltage changes across a resistor, think about three water wheels, each spanning one-third of the height of a waterfall. All the water flows onto each wheel, one after another, just as all the electric current flows through all three of the resistors. The waterfall’s total potential energy, however, only drops by one-third across each of the water wheels. A series circuit of resistors is similar: The total voltage (or potential difference) only partially reduces (or drops) across each individual resistor. If the three resistors are equal, the voltage is reduced (or drops) by ⅓ across each resistor.
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Ohm’s law is often written in the form of equation (17.2) to reflect the voltage drop across a resistor. When 1 A of current flows through 1 Ω resistor, there is a 1 V drop across that resistor. In a circuit with resistors in series there is a voltage drop across each resistor. The total voltage drop over a set of resistors in series equals the sum of the individual voltage drops.
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| (17.2) | | | V | = | voltage drop (V) | | I | = | electric current (A) | | R | = | resistance (Ω) |
| Ohm’s law
voltage-drop form |
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Voltage drops are evidence that electrical energy is being dissipated into other forms of energy, often heat. A drop in voltage is always accompanied by the dissipation of power. (You first learned about electrical power on page 264 in Chapter 9.) Any resistance dissipates electrical power when current flows through it. We will return to the topic of electrical power later in this section.
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If you have two identical resistors connected to a 10 V battery in series, what is the voltage drop across each resistor? - 0 V
- 5 V
- 10 V
- 15 V
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The answer is b, 5 V. Because the two resistors are identical, they will each have the same voltage drop. Furthermore, the total voltage of the battery is 10 V, so each takes 5 V. 
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