介质中的电磁能量密度及其损耗

以电磁场理论为基础,从场与介质相互作用的角度详细分析了介质中电（磁）场能量密度的物理意义,将介质中的电磁能量密度分解为电（磁）场能量密度和介质的极（磁）化能量密度.极（磁）化能量密度决定于极（磁）化强度和外场强度.在交变电（磁）场中产生电磁能量损耗的物理机制是,由于非线性介质中的各种阻尼作用,电（磁）偶极矩跟不上外场的变化而出现弛豫损耗,电磁能量被损耗转换为热能.利用极（磁）化能量密度公式导出在简谐交变外场中电磁能量损耗的平均功率密度表达式,该损耗功率密度与介质的相对介电常数（磁导率）的虚部、外场频率和场强的平方成正比.电磁能量密度时变值分解为场能时变值、极（磁）化能时变值和电磁损耗时变值.

Electromagnetic Energy Density and Loss in Medium

Based on electromagnetic field theories,the physical meanings of polarization（magnetization） energy in medium were analyzed from the viewpoint of interactions between electric（magnetic） field and material,and the electromagnetic energy in medium were decomposed into electric（magnetic） field energy and polarization（magnetization） energy of medium.The polarization（magnetization） energy density depended on polarization（magnetization） and the strength of external field.The physical mechanisms of electromagnetic energy loss in alternating electric（magnetic） field were also analyzed.The electric（magnetic） moments fell behind the change of external field because of the damping effects in nonlinear medium which caused the relaxation loss,then electromagnetic energy was converted into thermal energy.The expression of mean power density of electromagnetic energy loss was derived from the expression of polarization（magnetization） energy density in harmonic alternating external field,which was proportional to the imaginary component of relative dielectric constant（permeability）,external field frequency and the square of field strength.The instantaneous expression of electromagnetic energy was decomposed into three parts,which were electromagnetic field energy,polarization/magnetization energy,and loss energy.