Q279 : Usage of GPR, Scalar Potential, Gradient of Scalar Potential and Vector Potential in HIFREQ

Question
How are the GPR, Scalar Potential, Gradient of Scalar Potential and Vector Potential in HIFREQ defined? Since only the electromagnetic fields are meaningful, why have these quantities in the first place?

## The concepts of GPR and Scalar Potential

The GPR is the "Grid Potential Rise" and is the Scalar Potential of metallic conductors. The Scalar Potential can be computed at any point in space (including at the surface of conductor); the GPR is therefore just a special case of the Scalar Potential. The reason the two concepts are kept separate is that the GPR has an important physical meaning, being related to the impedance of the metallic structure. The Scalar Potential at arbitrary points in space doesn't have such an interpretation. There is also a practical consideration. For the Scalar Potential computations, you need to provide the position of the points in space where the potential is to be computed. By specifying that you want the GPR to be computed, you automatically instruct the program to compute the scalar potential at the surface of all conductors.

## The "Gradient of Scalar Potential" and "Vector Potential"

The "Gradient of Scalar Potential" is equal to the electric field, without the vector potential. Even though only E and H have any real physical meaning, at low frequencies, the Gradient of Scalar Potential is as well-defined as the Scalar Potential itself. Its usage is limited; it can sometimes be used at low frequencies when a separation of the electric field into a capacitive and an inductive part is desired, the vector potential providing the inductive part. (The values obtained in this way can, for instance, be inserted into a circuit model).

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