# 5.4: Concentric Spherical Capacitor

Unlike the coaxial cylindrical capacitor, I don’t know of any very obvious practical application, nor quite how you would construct one and connect the two spheres to a battery, but let’s go ahead all the same. Figure \(V.\)4 will do just as well for this one.

The two spheres are of inner and outer radii *a *and *b*, with a potential difference *V* between them, with charges \(+Q\) and \(-Q\) on the inner and outer spheres respectively. The potential difference between the two spheres is then \(\frac{Q}{4\pi\epsilon}\left (\frac{1}{a}-\frac{1}{b}\right )\), and so the capacitance is

\[C=\frac{4\pi \epsilon}{\frac{1}{a}-\frac{1}{b}}.\label{5.4.1}\]

If \(b \to \infty\) we obtain for the capacitance of an isolated sphere of radius *a*:

\[C=4\pi \epsilon a.\label{5.4.2}\]

*Exercise*: Calculate the capacitance of planet Earth, of radius 6.371 × 10^{3} km, suspended in free space. I make it 709 \(\mu\text{F}\) - which may be a bit smaller than you were expecting.