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If we have two conducting spheres of equal but opposite charge and they are at a large distance from each other, why can we consider this system as a capacitor? According to my textbook, this is valid. It doesn't quite go into detail with this.

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  • $\begingroup$ Technically one conducting sphere by itself can be considered a capacitor. $\endgroup$
    – lalala
    Commented Mar 10, 2019 at 7:26
  • $\begingroup$ Any shape conductors separated by a dielectric can be considered a capacitor $\endgroup$ Commented Mar 8, 2020 at 16:11

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First let’s state some fundamental things. The electric field of a conducting sphere (outside the sphere) varies by the inverse square law for distance. And is independent of the dimensions of the sphere and depends on the charge. If you place this sphere in an empty vacuum space you would find that even at large distance the field is non zero. Only at theoretical infinity is the field zero. But at a very large distance you can approximate the sphere as a point sphere.

Consider 2 such spheres kept at a very large distance. Since the spheres are very large you can place them at very large distance apart. Only if you are close will you realize the sizes of each. If you apply a voltage across them at a certain distance you will find that they act a a capacitor holding charges at a certain distance apart. This will create a potential difference between the 2 spheres.

You can consider the same analogy for a very thin parallel plate capacitor. Only if you go to the atomic scale will you realize that the short distance between the plates is actually very large form the atoms viewpoint.

Hope this helps.

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Capacitor is not nothing but a system of two conducting materials having equal and opposite charges of magnitude Q. The potential difference across the two materials (V) is related as Q=CV where C is called the capacitance of the system. So there's is no good reason to not take the situation mentioned by you as a capacitor.

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