# Homework Help: Maxwell's Eqns- HELP - Spherical EM wave

1. Mar 11, 2012

### Sean.Hampson

1. The problem statement, all variables and given/known data

The electric field of a spherical electromagnetic wave in vacuum can be written in the form of:

E(r,θ,phi)= A(sin(θ)/r)*[cos(kr-ωt)-(1/kr)sin(kr-ωt)]phi

Show that E is consistent with ALL of Maxwell's equations in vacuum and find the associated magnetic field.

2. Relevant equations

Maxwell's Equations

3. The attempt at a solution

I calculated Curl E, which came out as

(Acos(θ)/r^2sin(phi))*(sin(kr-ωt)/ω) +(2Asin(theta)/r^2)*(sin(kr-ωt)/ω) -(kAsin(θ)/r)*(sin(kr-ωt)/ω)+(Asin(θ)/kr^3)*(cos(kr-ωt)/ω)...

I then tried to take the integral of this with respect to t to find B. although I ran into trouble, unknowing what to do with theta or r....

Any help would be much appreciated, maths isn't a strong point so I struggle with these type of questions.

2. Mar 11, 2012

### Clever-Name

E has to be a vector, what direction is it in? Does the phi at the end indicate a unit vector? The curl should not be such a long expression.

3. Mar 11, 2012

### Sean.Hampson

Yes the Phi indicates a unit vector,

so since we're working in the direction of phi, the E x Curl is

|r ... theta ...phi |
|d/dr...[(1/r*sin(phi))*d/dtheta] ...[(1/r) d/dphi] |
|0... 0 ...E |

I've tried to represent it well here, I hope it makes sense. I just petformed the Determinant and I got that long line out. I also used maple and got something quite similar :/ any suggestions ?

4. Mar 11, 2012

### Clever-Name

I'm not so sure the determinant trick works for calculating the curl in spherical coordinates. As far as I know it only applies in cartesian, though I could be wrong. Looking at the expression from the Griffiths text for curl in spherical coordinates, we only have $E_{\phi}$ so it reduces to:
$$\nabla \times {E} = \frac{1}{rsin\theta} \left( \frac{\partial}{\partial\theta}(sin\theta E_{\phi}) \right) \hat{r} - \frac{1}{r}\left( \frac{\partial}{\partial r}(r E_{\phi})\right) \hat{\theta}$$

I want you to work that out yourself, I don't think it works out to the same expression that you got. I underestimated the size of the curl, I anticipated it only depending on one term.. I was wrong =(. For future problems just refer to the expression in the cover of Griffiths if that's the text you're using.

As for 'what to do with the theta or r', you're integrating with respect to time, so they don't even come into the picture, just integrate as if they were constants.

Last edited: Mar 11, 2012