12–15% of the AP exam. 12 key topics. 4 common errors analyzed.
Unit 15 (Modern Physics) extends the AP Physics 2 curriculum. Emphasize conceptual justification: FRQ scoring rubrics award points for physical reasoning stated in precise sentences.
These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.
Photoelectric effect intensity vs. frequency confusion — students incorrectly predict that increasing light intensity raises the maximum kinetic energy of ejected electrons; intensity affects the number of photoelectrons per second, not their maximum KE; only increasing frequency (above threshold) increases KE_max = hf − φ
Conflating photon energy (E = hf) with photon momentum (p = h/λ) — both equations are on the reference sheet but students apply the wrong one when a problem asks about momentum transfer (e.g., radiation pressure, Compton scattering recoil) vs. energy transfer (e.g., photoelectric threshold frequency)
Bohr model energy level sign errors — the ground state energy is negative (−13.6 eV for hydrogen); students treat energy levels as positive magnitudes and then compute emission photon energy incorrectly by subtracting in the wrong order (|E_lower| − |E_upper| instead of E_upper − E_lower gives a positive photon energy)
Radioactive decay equation balancing errors — failing to conserve both atomic number (Z) and mass number (A) simultaneously in alpha/beta decay equations; beta-minus decay increases Z by 1 while beta-plus decreases Z by 1, and students frequently confuse the two decay modes
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