What is the physical significance of the poynting vector?
Answers
Pointing vector gives us the direction of electro-magnetic field travelling in space. But in static electro magnetic fields we don't find and travelling fields. But, even then pointing vector has a significant role in static fields.
As mentioned earlier pointing vector is not confined to give information of direction of fields but also it says about the angular momentum of the field. According to feynman pointing vector is really means a momentum flow. It is needed to maintain conservation of angular momentum.
There are many cases where we can't account for conservation of angular momentum in static fields with out considering pointing vector. For example consider a magnetic material placed in another non magnetic material (assume them in concentric rings(inner magnetic material, outer non magnetic material)). Initially they are at rest (Zero angular momentum). If we supply positive and negative charges to outer and inner layers respectively, there comes a field between them. This field will try to get some momentum in magnetic material.
So, initially we started with Zero magnetic momentum and we are in static fields even then we end up with some non zero moment. This disagree with conservation. Now if we analyse pointing vectors for this case we get its moment conserved. Hence pointing vector is needed for angular momentum conservation in Static electric fields.