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Maxwell's Laws Tour

Displacement Current

James Clerk Maxwell's laws did not get their name because he thought of all of them. In truth he only added a part to Ampere's law. However this small part turned out to be the missing link to complete the whole chain. Maxwell was not a experimental scientist but was rather a brilliant thinker and mathematician. He saw the conection between Ampere's, Faraday's, and Gauss's law and was able to write equations for those whose mathematical prowess was less than his.

Finaly Maxwell saw that there was a missing link to complete the chain. For example it was pointed out that a circuit with a capacitor still supported a current but that no physical current passed through the capacitor. if a surface was placed between the plates of a capacitor no current would pass through it, therefore no magnetic field should be present at the boundary of the surface. However, the surface could be deformed so that a magnetic field did exist around the boundary of the surface thus Ampere's law seemed to be invalid. Maxwell's answer to the problem was a that a current did flow through the capacitor, a current he called a displacement current.

This displacment current turned out to be the time rate of chage of the electric field between the plates of the capacitor. By adding this to Ampere's law Maxwell had coupled all the laws together and put them in neat mathematical packages. Can you think of any electromagnetic phenomenon that would not be possible without Maxwell's displacement current?