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Two conducting spheres of radii R1 and R2 are kept widely separated from each other. What are their individual capacitances? If the spheres are connected by a metal wire, what will be the capacitance of the combination? Think in terms of series−parallel connections.

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Solution

We need to calculate the capacitance of an isolated charged sphere.
Let us assume that the charge on the sphere is Q and its radius is R.

Capacitance of the charged sphere can be found by imagining a concentric sphere of infinite radius consisting of −Q charge.
Potential difference between the spheres = 14π0QR − 0 = 14π0QR

Capacitance is the ratio of the magnitude of the charge on each conductor to the potential difference between them.
C=Q14π0QR=4π0R
Therefore, the capacitances of spheres of radii R1 and R2 are C1 and C2, respectively. They are given by

C1=4π0 R1C2=4π0 R2

If the spheres are connected by a metal wire, the charge will flow from one sphere to another till their potentials become the same.
As there potentials become the same, the potential difference between the conductors for both the capacitors also becomes the same. Thus, it can be concluded that the capacitors are connected in parallel.

Thus, the capacitance of the combination is given by
Ceq = C1 + C2

=4π0 R1+R2

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