Using Ohm's Law (U = I R) and the formula for charge (q = I T), we can find the unknown voltage U.

First, let's find the current (I) using the formula q = I * T:
I = q / T = 100 Кл / 20 сек = 5 A

Now, we can calculate the voltage (U) using Ohm's Law:
U = I R = 5 A 2 Ом = 10 V

Therefore, the voltage U is 10 V.