Aim for FivePhysics 2

Qualitative/quantitative translation (QQT)

Pulling apart the plates of a charged capacitor

  • Unit 10
  • 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. 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 15–20 minutes. This question type started in May 2025, so Physics 2 free-response questions from 2024 and earlier are built differently.

The question

An air-filled parallel-plate capacitor is connected to a battery of potential difference ΔV₀ until it is fully charged, holding charge Q₀ and storing energy U₀. The capacitor is then disconnected from the battery. Using insulated handles, a student slowly pulls the plates apart until their separation is twice its original value. No charge leaks off the plates, and edge effects can be ignored.

Suggested time: 18 minutes

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Part (a)

3 points

As the plates are pulled apart, does (i) the charge on each plate, (ii) the electric field between the plates, and (iii) the energy stored in the capacitor increase, decrease or stay the same? Justify each answer. 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 points

Derive an expression for the energy U stored in the capacitor after the separation is doubled, in terms of U₀.

Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).

0 / 2,500 characters

Part (c)

2 points

(i) Explain how your result from part (b) agrees with your reasoning in part (a). (ii) Suppose instead the capacitor had stayed connected to the battery while the plates were pulled apart. Predict whether the stored energy would increase, decrease or stay the same, and by what factor.

Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).

0 / 2,500 characters

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