# Homework Help: Divergence and curl of vector field defined by \vec A = f(r)vec r

1. Oct 11, 2008

### FourierX

1. The problem statement, all variables and given/known data
A vector field is defined by A=f(r)r
a) show that f(r) = constant/r^3 if $$\nabla$$. A = 0
b) show that $$\nabla$$. A is always equal to zero

2. Relevant equations
divergence and curl relations

3. The attempt at a solution
I tried using spherical co-ordinates to solve this. But I am not sure if i am totally right.

Last edited: Oct 11, 2008
2. Oct 11, 2008

### G01

I would also use spherical coordinates on this problem.

I cannot say if you are right since I haven't seen any of your work. I'll need to see some work if you want more specific advice.

3. Oct 11, 2008

### olgranpappy

Certainly parts (a) and (b) can not be consistent as you have written them... i suppose part (b) should be a curl not a divergence?

4. Oct 11, 2008

### FourierX

here is what i've done and where i got stuck:

$$\nabla$$A = $$\frac{1}{r^{2}}$$ $$\frac{\partial}{\partial r} (r^{2}f(r))$$

on simplifying this i got:

f(r) = $$\frac{-1}{2} r f'(r)$$

then i integrated with a hope to get an expression for f(r) but did not end up with what i needed i.e. f(r) = constant/ r^{3}

5. Oct 12, 2008

### nicksauce

Well the way you interpreted it, you have $$\mathbf{A}=f(r)\hat{r}$$, but the way the question is stated it is $$\mathbf{A}=f(r)\mathbf{r}=$$, or $$\mathbf{A}=f(r)r\hat{r}$$. This extra factor of r should give the answer required.

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