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AP Physics C Electricity and Magnetism · Question of the Day

AP Physics C Electricity and Magnetism Question of the Day

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Sunday, September 13, 2026

A uniform electric field is directed vertically upward. A student holds a flat, rectangular sheet of paper with area AA. To produce the largest possible negative electric flux through the paper, how should it be oriented?

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Question of the Day

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A uniform electric field is directed vertically upward. A student holds a flat, rectangular sheet of paper with area AA. To produce the largest possible negative electric flux through the paper, how should it be oriented?

  1. Held horizontally, with the paper's normal vector pointing upward.
  2. Held horizontally, with the paper's normal vector pointing downward. (correct answer)
  3. Held vertically, with its normal vector pointing horizontally.
  4. Held at a 45° angle to the vertical direction.

Explanation: Electric flux is given by ΦE=EAcos⁡θ\Phi_E = EA \cos\theta. To get the largest negative value, cos⁡θ\cos\theta must be -1, which occurs when θ=180°\theta = 180°. This means the electric field vector and the surface normal vector must be anti-parallel. Since the electric field points upward, the paper's normal vector must point downward. This corresponds to holding the paper horizontally.