Translation between representations (TBR)
Two bulbs and a charging capacitor
- Unit 11
- 12 points
- About 28 minutes
You can use a calculator on this question, just like on exam day.
A multipart problem about one situation shown in several ways. You draw a diagram, derive equations, sketch or draw graphs, and then explain whether your answers agree with each other or use them to predict what happens when the situation changes. On the exam: Question 2 of 4. New for May 2027: the free-response section is 95 minutes, down from 100, for all 4 questions (50% of your score), one of each type in this order. Calculator and equation sheet allowed. The CED suggests 25–30 minutes. This question type started in May 2025, so Physics 2 free-response questions from 2024 and earlier are built differently.
The question
An ideal 9.0 V battery, a switch S, two identical bulbs A and B, and an initially uncharged 200 μF capacitor are connected as follows. The positive terminal of the battery connects through switch S to one side of bulb A. The other side of bulb A connects to junction J. From junction J, two branches lead to junction K: one branch contains only bulb B, and the other contains only the capacitor. Junction K connects to the negative terminal of the battery. Treat each bulb as a resistor of 30 Ω.
Suggested time: 28 minutes
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Part (a)
3 pointsSwitch S is closed at time t = 0. Rank the brightness of bulbs A and B (i) just after S is closed and (ii) a long time after S is closed. Then (iii) state whether bulb A is brighter, dimmer or the same a long time after S is closed compared with just after. Justify each answer.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
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Part (b)
3 pointsCalculate (i) the current in bulb A just after S is closed, (ii) the current in each bulb a long time after S is closed, and (iii) the charge on the capacitor a long time after S is closed.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (c)
3 pointsOn the same axes, sketch graphs of the current in bulb A and the current in bulb B as functions of time, from t = 0 until long after S is closed. You can't draw here, so describe the graphs you would draw: the axes and their units, the starting and final values, and the shape of each curve.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (d)
3 pointsAfter S has been closed for a long time, it is opened. (i) Describe what happens to the brightness of each bulb, just after S is opened and afterward. (ii) Is the current in bulb B in the same direction as before S was opened, or the opposite direction? Explain how your answers are consistent with your answers to parts (a) and (b).
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
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