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Wherever an 'inverse square law' arises the idea of a flux is a useful concept.

In Understanding Physics all three inverse square law forces encountered are treated in a similar way, as summarised in the table below.

This approach, which emphasises certain symmetries common to the different interactions, leads to a simpler, more consistent and integrated treatment of the study of electromagnetism. You need to be aware, however, that this gives rise to some differences in terminology between Understanding Physics and that used in some other textbooks.

As indicated in Section 16.4, many textbooks define electric flux as ∫∫E.dA rather than as ∫∫D.dA. No similar confusion arises in the case of magnetic field where the magnetic flux is always defined as ∫∫B.dA. Unfortunately such texts also tend to call B the 'magnetic field' which, as we saw in the previous Section, conflicts with IUPAP standards and conventions. This can be confusing for the beginner but once you master the material, however, it becomes easy to distinguish between the different aproaches — each approach will give the same answer to any problem provided the approach remains consistent.