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AP Physics Featured Question: Brightness of Lightbulbs
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by Greg Jacobs
Woodberry Forest School
Woodberry, Virginia
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| | Introduction
News flash: A 60-watt (W) lightbulb does not necessarily dissipate 60 W.
This bulletin was delivered quite emphatically to the 40,000 or so students taking the AP Physics B Exam in 2002. Question 3 asked students what happens when lightbulbs are connected in series, causing consternation throughout the nation.
The lightbulbs that we buy at the store are all intended for use with a standard house plug, which provides 120 volts (V). These bulbs all have a power rating that assumes a 120 V source, so our students have never had to consider how the bulbs behave with any other connection.
No matter what specific questions are asked, the following three points can serve as general themes for all lightbulb problems:
- The resistance of a circuit component does not change depending on what voltage or current it uses.
- The brightness of a lightbulb depends on the power it dissipates.
- A power rating indicates a maximum appropriate power for the bulb to dissipate, but the bulb may dissipate more or less power when it is connected to different voltage sources.
For example, a bulb rated for 100 W at 120 V means that it is designed for use with a 120 V potential difference, in which case the bulb will dissipate 100 W. If we connect this bulb to a lower or higher voltage source, the power it dissipates will change; dissipating more than 100 W might cause the bulb to burn out.
By carefully approaching this featured question and the follow-up quiz, teachers will drill the three general lightbulb themes into their students' brains.
Correlation to the Topic Outline in the Course Description
III.C.1. Current, resistance, power
III.C.2. Steady-state direct current circuits with batteries and resistors only
Question as a Homework Assignment
Consider two commercially available, standard incandescent lightbulbs, each rated for use with a standard 120 V wall outlet. Bulb A is rated at 100 W; bulb B is rated at 60 W. (For the questions below, ignore any effects of temperature on resistance.)
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Voltage |
Current |
Resistance |
Power |
| Bulb A |
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| Bulb B |
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a. Consider that the bulbs are each plugged into 120 V wall outlets in the same room. Fill in the chart above, giving the voltage across, current through, resistance of, and power dissipated by each bulb. Indicate which bulb is brighter.
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Voltage |
Current |
Resistance |
Power |
| Bulb A |
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| Bulb B |
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b. Now consider that these two bulbs are connected in series with each other and with a 120 V wall outlet. Fill in the chart above for each bulb in the series circuit. Indicate which bulb is brighter.
c. Discuss the differences in the answers to (a) and (b). In particular, explain why and under what conditions it is possible for a 60 W bulb to be brighter than a 100 W bulb.
 Handwritten Solution (.pdf/244KB)
Greg Jacobs teaches AP Physics B and C at Woodberry Forest School in central Virginia. He is a graduate of Haverford College, and has a master's degree in engineering from Northwestern University. When he is not teaching, Greg broadcasts Woodberry Forest varsity baseball games over the Internet; he is a reporter for STATS, Inc., covering baseball, basketball, and football; and he is a Reader and consultant for the College Board's AP Physics program. Greg lives on campus at Woodberry with his wife Shari and their son Milo Cebu.
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