The Physics 212 Puzzler

A parallel plate capacitor is given a charge Q. After charging, the capacitor is removed from the circuit so that the charge remains fixed. The potential energy stored on the capacitor is

PE = 1/2QV = 1/2Q2/C = 1/2CV2.

A student inserts a dielectric into the space between the plates, effectively reducing the electric field between the plates (by a factor equal to the dielectric constant K).

  1. Has the electrical energy stored in the capacitor increased or decreased?

  2. Did the student do positive or negative work on the dielectric?

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