Determining Electric and Magnetic field given certain conditions

guyvsdcsniper
Messages
264
Reaction score
37
Homework Statement
Refer to attached image
Relevant Equations
comlplex wave equation,
Screen Shot 2022-10-06 at 11.06.55 AM.png

I am unsure of my solutions and am looking for some guidance. a.)The real part of the wave in complex notation can be written as ##\widetilde{A} = A^{i\delta}##. Writing the Complex Wave equation, we have ##\vec E(t) = \widetilde{A}e^{(-kz-\Omega t)} \hat x##. Therefore the real part is ##\vec E(t) =Ae^{(-kz-\Omega t+\delta)} \hat x##. The negative in front of kz indicates it is a left traveling wave.

b.) The unit vector of ##\hat B = \frac{(\hat x - 2\hat z)}{\sqrt{5}}##. I know that ##\hat E## must be perpendicular to ##\hat B##, so simply,
##\hat E = \frac{(\hat x + 2\hat z)}{\sqrt{5}}##

c.) I am not so sure about this problem. I know that ##\vec E = \widetilde{E}_oe^{i(ky-wt)}\hat x##
Griffiths states ##\widetilde{B}_o = \widetilde{E}_o/c## and ##v=c/n##.

So ##\vec B = \frac{c\widetilde{E}_o}{1.7}e^{i(ky-wt)}\hat z##
 
Physics news on Phys.org
guyvsdcsniper said:
a.)The real part of the wave in complex notation can be written as ##\widetilde{A} = A^{i\delta}##. Writing the Complex Wave equation, we have ##\vec E(t) = \widetilde{A}e^{(-kz-\Omega t)} \hat x##. Therefore the real part is ##\vec E(t) =Ae^{(-kz-\Omega t+\delta)} \hat x##. The negative in front of kz indicates it is a left traveling wave.
The problem statement said the wave travels in the ##-x## direction. Your answer isn't sinusoidal. It decays with time. What is ##\delta##?

b.) The unit vector of ##\hat B = \frac{(\hat x - 2\hat z)}{\sqrt{5}}##. I know that ##\hat E## must be perpendicular to ##\hat B##, so simply,
##\hat E = \frac{(\hat x + 2\hat z)}{\sqrt{5}}##
It doesn't look like ##\hat E \cdot \hat B=0##.
 
vela said:
The problem statement said the wave travels in the ##-x## direction. Your answer isn't sinusoidal. It decays with time. What is ##\delta##?
You're right. I should have ##\vec E(t) =Acos{(-kz-\Omega t+\delta)} \hat x##
##\delta## is the phase
So I need to make ##\hat x## be ##\hat - x## as well as account for it is a wave traveling left by the ##-kz## in the ##cos##?

I assumed the ##-kz## in the ##cos## accounted for the negative direction.
 
guyvsdcsniper said:
I assumed the ##-kz## in the ##cos## accounted for the negative direction.
In the ##-z## direction, not the ##-x## direction.
 
Hint: The wave travels in the negative ##x## direction and the ##\vec{B}##-field (sic!) is polarized in ##z##-direction. So the complex ansatz for ##\vec{B}## is (using the HEP physicists' convention concerning the signs in the exponential)
$$\vec{B}=A \vec{e}_z \exp(-\mathrm{i} \Omega t-\mathrm{i} k x), \quad \Omega,k>0.$$
Now just use the source-free Maxwell equations to get the dispersion relation ##\Omega=\Omega(k)## and the ##\vec{E}##-field!
 
I figured it out. I had a big misunderstanding on the equations I was using but took the time to read through my book and was able to come to the correct answer. Thanks all for the help!
 
Thread 'Need help understanding this figure on energy levels'
This figure is from "Introduction to Quantum Mechanics" by Griffiths (3rd edition). It is available to download. It is from page 142. I am hoping the usual people on this site will give me a hand understanding what is going on in the figure. After the equation (4.50) it says "It is customary to introduce the principal quantum number, ##n##, which simply orders the allowed energies, starting with 1 for the ground state. (see the figure)" I still don't understand the figure :( Here is...
Thread 'Understanding how to "tack on" the time wiggle factor'
The last problem I posted on QM made it into advanced homework help, that is why I am putting it here. I am sorry for any hassle imposed on the moderators by myself. Part (a) is quite easy. We get $$\sigma_1 = 2\lambda, \mathbf{v}_1 = \begin{pmatrix} 0 \\ 0 \\ 1 \end{pmatrix} \sigma_2 = \lambda, \mathbf{v}_2 = \begin{pmatrix} 1/\sqrt{2} \\ 1/\sqrt{2} \\ 0 \end{pmatrix} \sigma_3 = -\lambda, \mathbf{v}_3 = \begin{pmatrix} 1/\sqrt{2} \\ -1/\sqrt{2} \\ 0 \end{pmatrix} $$ There are two ways...
Back
Top