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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!