# Homework Help: Surface integral

1. Mar 12, 2010

### likephysics

1. The problem statement, all variables and given/known data
This is not a HW prob. Just a question.
When doing surface integrals, why should the area element ds be dotted with the normal. I don't get it .
$$\oint$$A.n ds

2. Relevant equations

3. The attempt at a solution

2. Mar 12, 2010

### gabbagabbahey

It doesn't have to be, in general. There are other types of surface integrals as well. The type you've listed above is designed to determine the flux of the vector field A through the surface you are integrating over. The dot product is there because, by definition of flux, you are only concerned with the component of A that is perpendicular to the surface at each point on the surface.

3. Mar 12, 2010

### likephysics

Ok. Thanks. I can live with it now.