I was reading an article about resistivity when I faced this equation:
The equation is in one dimension and $P$ and $O$ are points on the $x$ axis.
The Electric field is oriented in the $+x$ direction ans has the formula $c\frac1r$ where $c$ is a coefficient and $r$ is the distance form the origin of the coordinate system.
The vector $\vec r_P - \vec r_O$ has the same direction as that of the electric field.
Now, Why do we put $P$ below the integral and $O$ above it? Can someone explain the equation for me?