Answer
$p = 5.0\times 10^{-28}~C\cdot m$
Work Step by Step
The maximum value of the potential energy is $U = pE$
We can find the magnitude of $p$:
$U = pE = 100\times 10^{-28}~J$
$p = \frac{100\times 10^{-28}~J}{E}$
$p = \frac{100\times 10^{-28}~J}{20~N/C}$
$p = 5.0\times 10^{-28}~C\cdot m$
