Verdict on WPH12/01 (Summer 2025)
The Summer 2025 Unit 2 paper presents a balanced but mathematically demanding assessment of waves, electricity, and quantum physics. Overall, the difficulty sits at a 4-star level, primarily due to complex multi-step calculations in the later questions and the rigorous phrasing required for qualitative marks. While standard questions on resistivity and Snell's law offered accessible entry points, candidates faced significant challenges in the application of the de Broglie wavelength to extremely small angles and the manipulation of non-standard units in power calculations.
Where the Marks are Won and Lost
Marks were highly concentrated in the high-tariff calculation questions. In Question 18(b), candidates had to compute the total energy incident on a screen from UV intensity given in \(\text{mW cm}^{-2}\). Many candidates stumbled during unit conversion; the most elegant method was keeping dimensions in centimeters (calculating area as \(7.15 \text{ cm} \times 14.7 \text{ cm} = 105 \text{ cm}^2\)) and multiplying directly by \(50 \text{ mW cm}^{-2}\) to get \(5.25 \text{ W}\).
Another typical trap lay in Question 13(a), where the path length of the ultrasound pulse required doubling to account for the reflection trip before comparing it with the pulse duration. Failing to use this factor of 2 was a major source of dropped marks.
Examiner Pitfalls & Crucial Misconceptions
Several persistent errors were highlighted in the examiner feedback:
- Stationary Waves in a Closed Tube (Q17a): In this Quality of Written Communication (QWC) question, candidates often missed the fundamental mechanism. To secure full marks, students must state that the wave travels down the tube, reflects from the closed end, and then superposes/interferes with the continuous incident wave. Simply saying 'two waves meet' was insufficient.
- Diffraction Grating Order Feasibility (Q15b_ii): When computing the angle of the second-order maximum for accelerated electrons, candidates obtained an extremely small angle (\(\theta \approx 0.001^\circ\)). Many failed to conclude that because this angle is so small, the maximum would be virtually indistinguishable from the central zero-order maximum, meaning it would not be 'clearly visible'.
- Fixed Energy Levels (Q18c): A common misconception is that increasing the potential difference across a tube automatically increases the frequency of all emitted light. High-performing candidates correctly argued that energy levels in mercury atoms are fixed, meaning the emitted photon frequencies must remain constant unless transitions to higher, previously unreached levels occur.
Strategy and Preparation Predictions
For upcoming series, students should prioritize practicing multi-step unit conversions and getting comfortable with physical units outside the standard SI (such as electronvolts and non-SI intensities). Furthermore, the lack of a prominent double-slit interference or diode/filament lamp I-V characteristic curve question in this series suggests these topics are highly overdue and likely to feature heavily in future sets. Focus revision on sketching standing waves with end corrections and proving physical constants using quantum equations.