## Illustration 30.1: Complete Circuits

This Illustration examines open and closed circuits by considering a circuit composed of three lightbulbs. These lightbulbs behave very much like resistors and are often represented using the resistance symbol, . Restart.

Note that even though these lightbulbs are identical, their brightness can vary. The relationship between voltage, current, and brightness is discussed later in this chapter, but you should notice that the brightness of a bulb depends on the voltage across it.

A burned-out filament results in an open circuit in that branch and is equivalent to a very large resistance. First predict what will happen when one or more lightbulbs burn out and then click on the links to test your prediction. The animation will show you both the bulbs (which ones light and which ones do not) and the voltages (voltage is given in volts).

Remember that if there is a voltage drop across a "good" lightbulb (one that is not broken), it should light up. Notice that when bulb 1 is out, none of the lightbulbs light. If only bulb 2 or bulb 3 is out, notice that the other bulbs light. If bulbs 2 and 3 are both out, notice that bulb 1 does not light up. Make sure that you can explain these observations and the associated voltage readings.

By looking at the voltage across each bulb (resistor), you should be able to analyze the circuit. Notice that you cannot determine which lightbulb is broken simply by looking at the bulbs because multiple conditions produce the same visual clues.

Illustration authored by Anne J. Cox.

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