10–20% of the AP exam. 12 key topics. 4 common errors analyzed.
Unit 13 (Electromagnetic Induction) is part of the calculus-based AP Physics C: E&M curriculum. Gauss's Law, line integrals, and differential equations appear throughout — set up integrals carefully before evaluating.
These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.
Misapplying Lenz's Law direction — students identify that a current is induced but assign it the wrong direction, often by forgetting to ask 'what change is the induced current opposing?' rather than reasoning from the flux change alone; this is especially error-prone when the source is a moving conductor in a non-uniform field.
Omitting or mis-signing the negative sign in Faraday's Law (EMF = –dΦ_B/dt) when computing the magnitude of an induced EMF — students drop the sign and lose the physical meaning, producing wrong answers when the problem asks for the direction of induced current.
Confusing LR circuit initial/final conditions with RC circuit conventions — in an LR circuit, the inductor acts as an open circuit at t = 0 (if no prior current) and a short circuit at t → ∞, which is the opposite of the capacitor's behavior; students who memorise RC rules without understanding the underlying physics invert the limits.
Treating LC oscillation energy as non-conserved — students compute Q_max or I_max correctly but then fail to use energy conservation (U_E + U_B = constant) to relate charge and current at intermediate times, instead attempting to solve the differential equation from scratch each time.
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