In vacuum, the displacement-current density is . Its contribution to magnetic circulation makes the Ampère-Maxwell equation compatible with changing electric charge.
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Displacement current is a concept introduced by James Clerk Maxwell in the context of electromagnetism. It arises in situations where electric fields are changing over time, particularly in regions where conventional current (the flow of charged particles) is not present, such as in capacitors during charging and discharging. In classical electromagnetism, the flow of electric charge generates a magnetic field, as described by Ampère's law.