Qualitative/quantitative translation (QQT)
Adding a parallel resistor to a real battery
- Unit 11
- 8 points
- About 18 minutes
You can use a calculator on this question, just like on exam day.
A shorter multipart problem that connects reasoning in words with an equation. You make and justify a claim without equations, derive an equation from a physics principle, and then explain whether the two agree or use them to predict what happens in a changed situation. On the exam: Question 4 of 4. Section II has 4 questions in 95 minutes (50% of the exam score), one of each type in this order; calculator and equation sheet allowed. The CED suggests 15–20 minutes.
The question
A battery with emf ε and internal resistance r is connected to resistor 1, which has resistance R. Resistor 2, which also has resistance R, is connected in parallel with resistor 1 through a switch that is initially open. The connecting wires have negligible resistance.
Suggested time: 18 minutes
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Part (a)
3 pointsThe switch is closed. (i) Does the potential difference across the battery's terminals increase, decrease, or stay the same? (ii) Does the power dissipated in resistor 1 increase, decrease, or stay the same? Justify your answers. Do not use equations in this part; reason in words.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (b)
3 pointsDerive expressions, in terms of ε, r, and R, for (i) the terminal potential difference before and after the switch is closed, and (ii) the power dissipated in resistor 1 before and after the switch is closed.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (c)
2 pointsExplain how your expressions from part (b) agree with your reasoning in part (a).
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
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