Cant understand this double integral

diazona
Homework Helper
1) Do you understand the meaning of the sum a couple of lines above it?
$$k\sum_i \Delta q_i \sum_j \Delta q_j \frac{(\vec{r}_j - \vec{r}_i)}{(\vec{r}_j - \vec{r}_i)^3}$$

2) Do you understand why the sum is equivalent to the integral, when the number of charges is very very large?
$$k\int_0^L \lambda \mathrm{d}l_1 \int_{2L}^{3L} \lambda \mathrm{d}l_2 \frac{1}{(l_2 - l_1)^3}$$

the sum is like Coulomb law but what is the meaning
of sum from i and sum from j
??

i dont know why the this sum equals this integral
??
i know that integral is a sum too
bu thy look different
there is no i and j in the integral
no L in the sum
?

?????????