# Equivalent form of a vector area of a surface

I am interested in showing that the vector area $$\int_{\mathcal{S}}da$$ can be equivalently given by $$\int_{\mathcal{S}} da = \frac{1}{2}\oint(r \times dI).$$ I am mostly interested in getting a geometric idea of what this means with reference to the following which is stated in the literature I am working through: "Draw the cone subtended by the loop at the origin. Divide the conical surface up into infinitesimal triangular wedges, each with vertex at the origin and opposite $dI$, and exploit the geometrical interpretation of the cross product".

The idea I have is that it has something to do with $r \times dI$ being the area of a parallelogram, with direction perpendicular to the surface thereof. So then we have some kind of connection between the path integral and the surface integral. I'm specifically interested in the geometric interpretation of the right-hand side, I don't see the connection with a cone or conical section.

Thanks for any assistance.

• Is this related to that ? In any case, you should have a look at Stokes' theorem. – Zubzub Jul 7 '16 at 13:37
• The geometric explanation is beautiful. To prove it, the link that Zubzub is useful that needs to use Stokes' theorem, where you can replace v with norm n. There is one mistake, though, in the last equation. – user115350 Jul 7 '16 at 14:16
• @user115350 Do you have a reference to some notes where I can get a geometric idea of what this means, specifically the right side of the equation. I have edited my question. – user100431 Jul 7 '16 at 14:22

## 1 Answer

Applying the proof from this post I can see that taking $\vec{v} = \vec{c}$, where $c$ is a constant vector , that we get my result above:

$$\vec{c} \cdot\int d\vec{a} =\int \vec{c} \cdot d\vec{a}$$ then using Stokes Theorem as in this post we get $$\oint (\vec{c} \times \vec{r}) \cdot d\vec{I} = \int \nabla \times (\vec{c} \times \vec{r}) \cdot d \vec{a} = \int 2 \vec{c} \cdot d \vec{a}$$ therefore $$\int \vec{c} \cdot d \vec{a} = \frac{1}{2} \oint (\vec{c} \times \vec{r}) \cdot d\vec{I} = \frac{1}{2} \oint \vec{c} \cdot (\vec{r} \times d \vec{I}) = \vec{c} \cdot \frac{1}{2} \oint (\vec{r} \times d \vec{I}).$$ Since $\vec{c}$ is an arbitrary constant the result follows. But I am interested in the geometric interpretation mentioned above?

• I don't have reference or I guess the reference should be the definition of cross product and the definition of integral. $\arrow r \times d\arrow l$ can be the infinitesimal area for the integration. – user115350 Jul 7 '16 at 16:44