# Electric and magnetic field problems (curl/divergence)

1. Apr 29, 2013

### ParoxysmX

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

Consider the electric field E(t,x,y,z) = Acos(ky-wt)k

1. Find a magnetic field such that $\partial_t$B + $\nabla$ X E = 0
2. Show that $\nabla$ . E = 0 and $\nabla$. B = 0
3. Find a relationship between k and w that enables these fields to satisfy

$\nabla$ X B = $\mu_{0}$$\epsilon_{0}$$\frac{\partial E}{\partial t}$

3. The attempt at a solution

Really the problem here is the first one. I understand (sort of) the curl operator, but how do you find $\nabla$ X E? Would you start with a matrix of

[i j k
0 kAcos(ky-wt) 0
0 0 Acos(ky-wt)]

Then find the determinant, which is Acos(ky-wt)(1+k)i - 0j + 0k?

Last edited: Apr 29, 2013
2. Apr 29, 2013

### bossman27

No, that matrix is not correct. The cross product of the del operator $\nabla$ and a vector function is just an alternate convention for denoting curl. You can why here: http://en.wikipedia.org/wiki/Del#Curl. This should also explain why divergence can be denoted: $\nabla \cdot$

So your matrix should be: $\left[ \begin{array}{ccc} \hat{i} & \hat{j} & \hat{k} \\ \frac{\partial}{\partial x} & \frac{\partial}{\partial y} & \frac{\partial}{\partial z}\\ 0 & 0 & {\scriptsize A\cos(ky-wt)} \end{array} \right]$

The rest of the problem should be relatively trivial once you know what the operators do.

Last edited: Apr 29, 2013
3. Apr 29, 2013

### ParoxysmX

Ah I see. Thanks for your help.