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Electromagnetic field theory

There arc. howcvcr, situations when thc assumptions of ncither distributcd circuit theory

nor lumped circuit theory are applicable. In these contexts we are obliged to deal directly

in terms of the electromagnetic fields.

The necd for field theory can come about for several reasons. One is that we are not surę just where the currents flow. They may still be on conductors, but the conductors are no longer filamcntary, and we may have morę current on one part of a conductor than

another, and we are not surę by how much.

Secondly the magnetic flux density distribution may be such that it does not arrange itsclf in planes. Thirdly thc displacement currcnt may be comparablc with the conduction

current in the same region. There may even be no conduction current, and the magnetic

field may be crcated purely by displacement current. This is cxactly what happens in electromagnetic wave propagation.

When any of these things occurs we generałly need the fuli field solution as discussed in Chapters 8 to 11.