The energy density and the potential are part of the fundermental nature of the electric field. Electric potential does the pulling apart of the positive and negative layers of space and the pulled-apart-ness stores energy. I call this the principle of superimposition (as distinct from the principle of superposition) and regard it as the mechanism by which force is exerted at a distance. The potential is not a force, but a doing of work, an amount of energy. You need to go to the basic equations of electrostatics to really derive the energy density formula. As we create extra charges, the model will only continue to work if the polarisation fields of the individual charges can coexist in space. Young came up with the formula. The internal tension must be expressed as a force per unit area of the cross section of the wire and this is called the "stress". These individual fields are able to coexist in space and it is their presence which exerts forces on other charges. Energy Density is the total amount of energy in a system per unit volume. Thus Qe is electric energy density and Qm is magnetic energy density. The potential is not a force, but a doing of work, an amount of energy. It concerns the amount of current flowing across the given area. There is another field present, that of the electric potential. Energy density fields If there is no net charge within the region, then why should an electric field extend beyond it? The process of polarisation results in energy being stored in space. This is responsible for generating magnetic fields. In the case of the stretched piano wire, we can work out the energy stored per unit volume in the wire in any one of three possible ways. The concept seems to be that fields exist both as mathematical artefacts, in which case they have separate existence, and also as a single physical entity which is the sum of the separate mathematical fields. In my model of an electric charge, the charge consists of a polarisation of space towards a point which terminates in a spherical surface. It is at the surface of the charge that a raw edge of polarised space appears. By assuming that the charge on this surface is unable to exert a force at a distance, we can can understand the ability of one charge to exert a force on the other as a property of the polarisation field of the charge. This is sometimes assumed to be addative. This comes from the fact that I see energy as the primary reality and believe that we can only formulate fundamental laws of nature in terms of concepts based on this fact. In my theories it is very important to distinguish between real phenomena and mathematical artefacts. For electromagnetic waves, both the electric and magnetic fields play a role in the transport of energy. This is a scalar field because it does not posses any directional properties. As the name implies both are typified by an energy density. However Feynman writes in Section 27-4 of his well known course: These individual fields are able to coexist in space and it is their presence which exerts forces on other charges.