Maxwell’s Displacement Current

• Can we understand why this “displacement current” has the form it does?

• Consider applying Ampere’s Law to the current shown in the diagram.
  • If the surface is chosen as 1, 2 or 4, the enclosed current = I
  • If the surface is chosen as 3, the enclosed current = 0! (ie there is no current between the plates of the capacitor)

Big Idea: The added term is non-zero in this case, since the current I causes the charge Q on the capacitor to change in time which causes the Electric field in the region between the plates to change in time. The “displacement current” ID = e0 (dfE/dt) in the region between the plates = the real current I in the wire.

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