Experimental design and analysis (LAB)
Measuring a magnet's field with a current balance
- Unit 12
- 10 points
- About 27 minutes
You can use a calculator on this question, just like on exam day.
A two-part lab question. First you design an experiment to answer a question: what you change, what you measure, the equipment and how you would analyze the data. Then you get a data table from a similar experiment, choose what to graph (often so the points fall on a straight line), plot it and use the slope, the intercept or the graph's shape to answer a question. On the exam: Question 3 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 and its sources
Students want to determine the strength of the magnetic field in the gap between the poles of a U-shaped permanent magnet. In the gap, the field is uniform and horizontal.
Part A: Available equipment
The U-shaped magnet, a digital balance that reads to 0.01 g, a stiff straight wire held by an insulated clamp so that a 4.0 cm length of it can pass horizontally through the gap between the poles without touching the magnet, an adjustable DC power supply, an ammeter, a ruler, and connecting wires.
Source: Hypothetical lab setup
Part B: Table 1. Another group's data (magnet on the balance, 4.0 cm of wire in the gap, perpendicular to the field; balance zeroed with no current)
| Current I (A) | Change in balance reading Δm (g) |
|---|---|
| 0.50 | 0.25 |
| 1.00 | 0.48 |
| 1.50 | 0.74 |
| 2.00 | 0.97 |
| 2.50 | 1.23 |
Source: Hypothetical data
Suggested time: 27 minutes
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Part (A(i))
3 pointsDescribe an experimental procedure you could use, with only the equipment listed in Part A, to collect data that would allow you to determine the magnetic field strength in the gap. Include what you would measure, what you would change, and any steps you would take to reduce experimental uncertainty.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (A(ii))
2 pointsDescribe how you would use your data to determine the magnetic field strength. Include what you would graph and how you would use the graph.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (B(i))
1 pointThe magnetic force acts on the wire, but the balance supports the magnet, not the wire. Explain why the balance reading changes when there is a current in the wire.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (B(ii))
2 pointsUsing the data in Table 1, plot the quantities you would use to determine the magnetic field strength and draw a best-fit line. You can't draw here, so describe the graph you would draw: what goes on each axis (with units), the scale, the points you would plot, and the best-fit line.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (B(iii))
2 pointsUse your best-fit line to calculate an experimental value for the magnetic field strength in the gap.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
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