In this video, he derives an expression for the potential difference between the two radii R2 and R1 where R2>R1
He arrives at the derivation by integrating the electric field with respect to radius. But his integration limits are from R1 to R2 (2:18 in video)
Whats the reason behind this?
I'm not understanding the basis of putting a lower value at the top of the integration and a higher value at the bottom, as I've always done the opposite.