Aim for FivePhysics 2

Translation between representations (TBR)

Field and potential of two point charges on a line

  • Unit 10
  • 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

Particle 1, with charge +4.0 nC, is fixed at x = 0 on the x-axis. Particle 2, with charge −1.0 nC, is fixed at x = 0.30 m. Point P is at x = 0.15 m and point R is at x = 0.45 m. Use k = 9.0 × 10⁹ N·m²/C² and take the electric potential to be zero far from both particles.

Suggested time: 28 minutes

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Part (a)

3 points

Describe the electric field at points P and R. For each point, describe an arrow for the field from particle 1 and an arrow for the field from particle 2 (direction and which is longer), and give the direction of the net field. Then calculate the magnitude of the net field at R.

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

0 / 2,500 characters

Part (b)

3 points

(i) There is one location on the x-axis, other than infinitely far away, where the net electric field is zero. Explain which region it must be in and derive its position. (ii) Write an expression for the electric potential V at a point x > 0.30 m, and use it to find where V = 0 in that region.

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

0 / 2,500 characters

Part (c)

3 points

Describe the graph of electric potential V as a function of position x for 0.32 m ≤ x ≤ 1.5 m. Include the axes with units, where V is positive, negative or zero, where V has a maximum or minimum and its approximate value there, and what happens at large x.

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

0 / 2,500 characters

Part (d)

3 points

(i) Explain how your graph in part (c) is consistent with your answers to parts (a) and (b). (ii) A proton is released from rest at x = 0.70 m. Predict which way it moves, and justify your prediction using your graph.

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

0 / 2,500 characters

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