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A dielectric slab partly inserted between the plates of a charged capacitor is

  • A.pushed out, the system energy rising as it enters
  • B.unaffected, no force acting on an uncharged dielectric
  • C.drawn in only if the slab carries a net charge of its own
  • D.drawn further in, the system energy falling as it enters

Answer: D

Inserting the slab raises the capacitance, and at constant charge that lowers the stored energy, so the slab is pulled into the region of stronger field. The same conclusion holds at constant voltage once the work done by the source is properly accounted for, which is a classic examination trap.

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