Qualitative/quantitative translation (QQT)
Forces between two parallel current-carrying wires
- Unit 12
- 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
Two long, straight, parallel wires lie in the plane of the page, a distance d apart. Wire 1 is on the left and carries current I toward the top of the page. Wire 2 is on the right and carries current 2I, also toward the top of the page.
Suggested time: 18 minutes
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Part (a)
3 points(i) Do the wires attract each other, repel each other, or exert no force on each other? (ii) Is the magnitude of the force that wire 1 exerts on wire 2 greater than, less than, or equal to the magnitude of the force that wire 2 exerts on wire 1? (iii) At one point between the wires the net magnetic field is zero. Is that point closer to wire 1, closer to wire 2, or at the midpoint? 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).
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Part (b)
3 pointsDerive expressions for (i) the magnitude of the force per unit length on each wire and (ii) the distance x from wire 1 to the point where the net magnetic field is zero. Write your answers in terms of I, d and physical constants, as appropriate.
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 expressions from part (b) agree with your reasoning in part (a). (ii) The current in wire 2 is reversed so that it points toward the bottom of the page, with the same magnitude 2I. Predict the direction and magnitude of the force per unit length on wire 1.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
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