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Superposition

While it may seem obvious, it bears saying that the electric fields due to several different "source" charges or the magnetic fields due to several different "source" current elements are just added together (vectorially, of course) to make the net $\Vec{E}$ or $\Vec{B}$ field. Horrible as it might seem, this might in principle not be true - we might have to "add up" such fields in some hopelessly more complicated way. But it didn't turn out that way in this universe. Lucky us!