Lightning strikes energy question

[Miike012]

Lightning strikes a point on earth, delivering -20.0 C of
charge from cloud to Earth across a potential difference of 1.00 * 10^8 V.

How much energy is delivered to the ground?

U_elec= V*q = 1.00 * 10^8 V* -20.0 = -2.0*10^9 J.

But the book says it is 2.0*10^9 J... why is it positive and not negative?

 

[BruceW]

First, what does U_elec represent in this equation? U_elec= V*q Second, the question tells you the potential difference between Earth and cloud, but it does not tell you which potential is greater. So you need to guess which is greater. (To guess, you just need to think about why the lightning strike happens).