# Homework Help: Poynting vector

1. May 14, 2005

### robert25pl

I have to solve many electromagnetics problems but some of them are not clear to me. For example,

For what values of the parameters is the Poynting vector time independent?
If given electric field of a uniform plane wave propagating in the positive z direction in vacuum.

E is given. So I have to find H and then just P = E x H ?

2. May 14, 2005

### dextercioby

Well,u'll need to cancel the time dependence of the Poynting vector.Compute it's phase and set the time part to 0.

Daniel.

3. May 14, 2005

### robert25pl

Ok this is my E

$$E = E_{1} cos(\omega t - \beta z) \vec{i} + E_{2} sin(\omega t - \beta z) \vec{j}$$

4. May 14, 2005

### dextercioby

Okay.Compute the B and then the P.And set the time dependence of P to zero.

Daniel.

5. May 14, 2005

### robert25pl

Will my answer change because E in vacuum?

6. May 14, 2005

### dextercioby

It shouldn't matter too much,you're interested in the phase.Anyway,do those derivatives,that integration and then see what else needs to be done.

Daniel.