# Thread: Checking the divergence theorem on a sphere

1. ## Checking the divergence theorem on a sphere

I have not been able to solve this problem. Can someone please help me find the error?

2. ## Re: Checking the divergence theorem on a sphere

I think your problem is that $\bigtriangledown \cdot r^2 \hat r$ is not 2r, but rather is 4r. In spherical coordinates you have:

$\bigtriangledown \cdot \vec v = \frac 1 {r^2} \frac {\partial (r^2 v_r)}{\partial r} + \frac 1 {r \sin \theta} \frac {\partial (v_\theta \sin \theta)}{\partial \theta} + \frac 1 {r \sin \theta} \frac {\partial v_\phi}{\partial \phi}$

For the case of $\vec v=r^2 \hat r$ this becomes:

$\bigtriangledown \cdot \vec v = \frac 1 {r^2} \frac {\partial (r^4)}{\partial r} = 4r$

Use this to determine $\int _{Vol} ( \bigtriangledown \cdot \vec v ) dV$ and it turns out to be $4 \pi R^4$.

3. ## Re: Checking the divergence theorem on a sphere

Thank you very much!