A-Level Physics May/June 2024 Q1(b)(iv): Another square solar panel is placed so that light of the same intensity is incident no…

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  The new panel has shorter sides, so its surface area ($A$) is less than the original panel.

  Input power is the product of intensity ($I$) and area, given by the formula $P_{in} = I \times A$. As the light intensity is the same for both panels, the smaller area of the new panel means it receives a smaller input power ($P_{in}$) than the original panel.

  Efficiency ($\eta$) is the ratio of useful power output to total power input: $\eta = \frac{P_{out}}{P_{in}}$.

  The question states that the useful power output ($P_{out}$) is the same for both panels. Since the new panel has the same $P_{out}$ but a smaller $P_{in}$, its efficiency must be greater than the efficiency of the original panel.

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  How the marks are awarded

  • B1 — Stating that the area of the new panel is less than the original panel, as a consequence of its sides being shorter.
  • B1 — Explaining that the input power to the new panel is less because the area is smaller and the intensity is constant.
  • B1 — Concluding that the efficiency is greater by correctly relating the unchanged useful power output to the reduced input power, based on the definition of efficiency.

  Common mistakes

  • Stating that the efficiency is the same because the power output is the same, completely ignoring the change in area and thus input power.
  • Incorrectly concluding that efficiency is less because the input power is less, showing a misunderstanding of how the denominator affects a fraction.
  • Providing an incomplete explanation, for example, stating 'smaller area means more efficient' without linking it to the reduced input power for the same output power.
  • Confusing intensity with power, leading to incorrect reasoning about the energy incident on the panel.

  Examiner tip: Always break down 'explain' questions by starting from the fundamental definitions of the physical quantities involved, such as intensity and efficiency.

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