Experimental design and analysis (LAB)
Planck's constant from the photoelectric effect
- Unit 15
- 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 Planck's constant and the work function of the metal surface inside a phototube. Use e = 1.60 × 10⁻¹⁹ C and c = 3.00 × 10⁸ m/s.
Part A: Available equipment
A phototube (a sealed, evacuated tube with a metal cathode that light can reach and a collecting anode), five light sources that each emit light of a single known wavelength, an adjustable DC power supply that can make the anode negative relative to the cathode, a voltmeter, a sensitive ammeter that can measure currents as small as 10⁻¹¹ A, and connecting wires.
Source: Hypothetical lab setup
Part B: Table 1. Another group's data (same phototube)
| Wavelength λ (nm) | Stopping potential ΔV_s (V) |
|---|---|
| 365 | 1.51 |
| 405 | 1.17 |
| 436 | 0.94 |
| 546 | 0.38 |
| 578 | 0.25 |
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 Planck's constant. 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 Planck's constant and the work function. 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 pointUsing the data in Table 1, indicate which quantities you would graph to produce a straight line that could be used to determine Planck's constant, and calculate any values you would plot that aren't in the table.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
Part (B(ii))
2 pointsPlot the quantities from B(i) 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 experimental values for Planck's constant and for the work function of the metal.
Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).
0 / 2,500 characters
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