電磁気学こぼれ話—超伝導応用のための電磁気学の問題 第2話—

This article focuses on Poynting’s vector which is defined by a vector product of electric and magnetic fields and plays an important role in considering a flow of electromagnetic energy between power supplies and various types of electromagnetic devices in electrical, electronical and information networks. Poynting’s vector is introduced from a law of energy conservation and its surface integral on an arbitrary domain is equal to the electromagnetic power inside it. This relation can be applied to any system in which there is an interest in the energy flow. Validity of the speculation that Poynting’s vector directly gives the energy flow is examined in various cases. It is pointed out that Poynting’s vector is not equal to the energy flow in some cases. Also discussed is the condition under which the equality holds.