1. The problem statement, all variables and given/known data I wonder if anyone could explain me how to arrive at the equation (1.24). I have attached the part of the book where this appears. 2. Relevant equations I have attached the part of the book where this appears. 3. The attempt at a solution I have tried to make the second integral zero by performing the gradient and then the remaining algebra but i can't get to this, i suppose that the second integral is zero because in the expression (1.24) it doesn't appear. Using the term that has been expanded i tried to arrive at the integrand in (1.24) but i think that i have to make the second integral zero before this.