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Solutions to the Follow-Up Quiz
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by Greg Jacobs
Woodberry Forest School
Woodberry, Virginia
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| | Two lightbulbs, one rated 60 W at 120 V and the other rated 100 W at 120 V, are arranged in two different circuits.
1. Calculate the resistance of each bulb.
Answer:
2. Explain what the 60 W and 100 W ratings mean.
Answer: The bulbs are designed to be connected directly to a 120 V source. When you operate the bulbs as designed, they will dissipate the rated power. If they dissipate more than the rated power, they are likely to burn out.
3. Which of the following quantities determines the brightness of a lightbulb?
Answer: c. Power
4. Which of the following quantities must be the same for each bulb in a series circuit?
Answer: a. Current
5. If both lightbulbs are connected in parallel to a 120 V source, which is brighter? Explain your answer briefly.
Answer: The bulb rated at 100 W will be brighter. When both bulbs are connected to the rated voltage, they will dissipate the rated power. The brightness of a bulb depends on the power it dissipates, so the 100 W bulb will be brighter than the 60 W bulb.
6. If both lightbulbs are connected in parallel to a 50 V source, which is brighter? Explain.
Answer: Now the bulbs will each be dimmer than when they're connected to 120 V. But the 100 W bulb will still be brighter. No matter what a bulb is connected to, its resistance is a property of the bulb. Parallel bulbs must take the same voltage. By the equation for power 
, the bulb with the lower resistance will dissipate more power. The 100 W bulb has a smaller resistance, so it will be the brighter bulb.
7. If both lightbulbs are connected in series to a 120 V source, which is brighter? Explain.
Answer: Bulbs connected in series each take the same current. Power is also given by the equation
P = I2R. With the same current, the bulb with the bigger resistance will dissipate more power. Since the 60 W bulb has the larger resistance, it will be brighter.
8. Rank the brightness of the following bulbs, with 1 being the brightest and 4 being dimmest. Justify your answer with an explanation and/or a calculation in the space below.
Calculate the power dissipated by each bulb.
Answer: ___60 W bulb connected to 50 V source
___100 W bulb connected to 50 V source
In series: The equivalent resistance of the two bulbs in series is 380W. When connected to 120 V, the current in the circuit is 
. This same current flows through both bulbs.
___ 60 W bulb in series with 100 W bulb and 120 V source
___ 100 W bulb in series with 60 W bulb and 120 V source
Now rank according to power dissipated:
4 (10 W) 60 W bulb connected to 50 V source
2 (18 W) 100 W bulb connected to 50 V source
1 (25 W) 60 W bulb in series with 100 W bulb and 120 V source
3 (14 W) 100 W bulb in series with 60 W bulb and 120 V source
9. Explain briefly why, in your house, a 100 W bulb will always be brighter than a 60 W bulb.
Answer: In a house, all electrical outlets are connected in parallel to 120 V. Bulbs that you buy in the store are rated for use on a standard household 120 V circuit. So if you plug your lamps into a standard outlet, you always use the bulbs at their rated voltage and thus at their rated power. A 100 W bulb at home will always dissipate more power than a 60 W bulb and so will always be brighter.
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