波印亭向量
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其指出了能量流的方向，也指出了能量流的規模(magnitude)大小——為通過一垂直於能量流方向之表面的單位面積功率。導得波印亭向量是透過對於能量守恆的考量，並且考慮到磁場不做功。代表波印亭向量的符號為S(採用粗體是因為其為一向量)。此外，以國際單位制(SI units)可表示為： 其中E是電場，H與B分別是磁場與磁通量密度(magnetic flux density)；而μ是周遭介質的磁導率(permeability)。在自由空間中傳遞的電磁波，μ則變為μ0，為自由空間磁導率(permeability of free space)。 For example, the Poynting vector near an ideally conducting wire is parallel to the wire axis - so electric energy is flowing in space outside of the wire. The Poynting vector becomes tilted toward wire for a resistive wire, indicating that energy flows from the e/m field into the wire, producing resistive Joule heating in the wire. Since the electric and magnetic fields of an electromagnetic wave oscillate, the magnitude of the Poynting vector also oscillates. The average of the magnitude over one period of the wave is called the irradiance or intensity, I: For time-harmonic fields with some fixed frequency ω and time-dependence S divided by the square of the speed of light in a vacuum is the density of the linear momentum of the electromagnetic field. The time-averaged intensity <S> divided by the speed of light in a vacuum is the radiation pressure exerted by an electromagnetic wave on the surface of a target. [编辑] 相關條目
[编辑] 外部連結暨參考文獻
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, the Poynting vector varies with the cosine-squared function, at twice the frequency of the electromagnetic wave. Since the average value of this function over one cycle is equal to half its peak value, the time-averaged power density is given by 
, where * denotes the complex conjugate.
