A parallel plate capacitor consists of two equal and oppositely charged metal plates separated by a small distance, as seen in the photograph below. The voltage across the plates that created this charge separation on the plates is measured by the voltmeter in the picture, which is wired across the plates.

Now suppose that a thin circular, uncharged metal sheet, with the same diameter as the plates of the capacitor, is inserted between the plates of the capacitor. What happens to the voltage across the plates as measured by the voltmeter when the uncharged metal plate is inserted between the plates of the capacitor?

• (a) The voltage across the plates will increase.
• (b) The voltage across the plates will decrease.
• (c) The voltage across the plates will remain the same.

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The answer is (c): the voltage across the plates will remain the same, as compared with the original configuration in the photographs below.

  

The inserted plate becomes charged with equal but opposite charges distributed on its sides. The original capacitor then becomes two capacitors in series, where the sum of the two series capacitors is the same as that of the original single capacitor. However, the voltage across the original plates cannot change due to insertion of an uncharged plate, so the potential difference across the plates must remain unchanged.