20.5

Comparing the Brightness of Light Bulbs

Student Notes

This simulation gives you an excellent way to see whether you completely understand the ideas of resistors in series and parallel. Make sure you predict what will happen before turning the circuit on. Were all your predictions correct? With the switch open, what did you predict about bulb C? Why does bulb C do this when the switch is open? With the switch open, which bulb is brighter, bulb A or bulb B? Why is this? Which bulb, A or B, has more current through it? When the switch is closed, what happens to bulb B? Why? With the switch closed, how do bulbs B and C compare? Why? When the switch is closed, what happens to the brightness of bulb A? How can you explain this? One thing to think about is what happens to the overall resistance in the circuit when the switch is closed.

Self-test

The three bulbs in the simulation are identical. What is their resistance? If just one of these bulbs was connected to the 120-volt battery, how much power would be dissipated in the bulb? For the purposes of the simulation the resistance of the bulbs is assumed to be constant. This is not actually true - how (and why) does the resistance of a bulb change as the bulb gets brighter? You may need to do some further reading, but you can find the answer in your textbook.

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