# two ground balls deep in the ground

 $J (r = a) = \frac{I}{4 \pi a^2}$ $E (r = a) = \frac{I}{4 \pi \sigma} \frac{1}{a^2}$ $U_p = \int_a^{x \rightarrow \infty} E (r) \mathrm{d} r = \frac{I}{4 \pi \sigma} \int_a^{x \rightarrow \infty} \frac{1}{r^2} \mathrm{d} r = \frac{I}{4 \pi \sigma} \Big( \frac{1}{a} - \frac{1}{\infty}\Big) = \frac{I}{4 \pi \sigma a}$ $R_{p1} = \frac{U_p}{I} = \frac{I}{4 \pi \sigma} \frac{1}{a}$