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Physical interpretation of j in E&M field equations 
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#1
Nov1112, 07:21 PM

P: 83

Hi all. I've been trying to study microwave and electromagnetic engineering . I'm not sure how I should interpret j in some of the field equations. For example, for the field equations for a rectangular waveguide resonant cavity are:
E[itex]_{y}[/itex] = E[itex]_{0}[/itex] sin[itex]\frac{\pi x }{a}[/itex] sin [itex]\frac{l \pi z}{a}[/itex] H[itex]_{x}[/itex] = [itex]\frac{j E_{0}}{Z_{TE}}[/itex] sin[itex]\frac{\pi x}{a}[/itex] cos [itex]\frac{l \pi z}{d}[/itex] H[itex]_{z}[/itex] = [itex]\frac{j \pi E_{0}}{k \eta a}[/itex] cos[itex]\frac{\pi x}{a}[/itex] sin [itex]\frac{l \pi z}{d}[/itex] What is the physical interpretation of j and j in the H field in the x z direction? Does that indication that they are 90° out of phase of the E field? Does it indicate phase in the sense of time or space? Or should i think of them as derivatives of phasors? I know that the fields are derived from the more general phasor form of Maxwell's equations: ∇ × E =  jωμH ∇ × H = jωεE for which jω = [itex]\frac{\partial E}{ \partial t }[/itex] and E is E[itex]_{0}[/itex] e[itex]^{j \omega t}[/itex] which makes sense to me as I believe you can interpret jω as the sinusoidal frequency. But once the E and H fields have been derived as above it's no longer jω just j, so I've lost the sense of there meaning in the H fields. Could someone please explain how I should interpret them. Or anything else I seem to have screwed up in my thinking. Thanks a lot. 


#2
Nov1112, 08:13 PM

Sci Advisor
PF Gold
P: 2,725



#3
Nov1112, 09:50 PM

P: 83

Thanks, but I don't think that's correct in this instance. In my book current and current density are usually denoted at J_{0} or I_{0}. I'm fairly certain that in this instance j is the same as the imaginary unit, i, indicating that it's coefficient is the imaginary part of a complex number. Also these field equations were originally derived from;
∇×E = jωμH ∇×H = jωεE so everything should be in terms of the E and H fields. 


#4
Nov1112, 11:40 PM

Sci Advisor
PF Gold
P: 2,725

Physical interpretation of j in E&M field equations



#5
Nov1212, 03:16 AM

P: 3,898

But I think you should write the formulas in vector form, I am only guessing the direction of the vectors by the subscripts like Ex is in x direction. What you gave are only the scalar value of the vectors. Regarding to the disappearance of the jωt, your equation is in phasor form where [tex]\vec E= Re[\tilde E e^{j\omega t}] \;\hbox{ where } \;\tilde E=\hat y E_0e^{j\beta R}[/tex] BTW, [tex]\frac {\partial E}{\partial t}= j\omega E_0 e^{j\omega t} \;\hbox { not }\; j\omega[/tex] Where your Eo has to contain the phasor function. 


#6
Nov1212, 03:31 AM

P: 83

Thanks guys.
OK Yungman, so you think the j represents a 90° phase shift in the orientation of the coordinates of the H fields. That's kind of something I was thinking, but I can't figure out how a 90° shift like j puts you on the x axis, while a 90° shift from j puts you on the z axis. Also i was thinking because the field were derived from; ∇×E = jωμH ∇×H = jωεE that they must be related to the frequency jω, although ω does not seem to be factored into the fields in this case. Looking at what dlgoff posted seems kind of close, but that would seem to indicate that the H field are purely imaginary, and can not be measured. I don't think this is the case though. I could post the full derivation from the textbook I'm referencing if that would help?? 


#7
Nov1212, 04:26 AM

P: 3,898

Remember if [tex] E= jE_0 e^{j\beta R}= E_0e^{j(\beta R  \frac {\pi}{2})}[/tex] [tex] E= jE_0 e^{j\beta R} = E_0e^{j(\beta R + \frac {\pi}{2})}[/tex] Where R is at the direction of propagation. This only mean the peak of the E and H is off by 90 degree along the direction of propagation, NOT the angle between E and H. Edited: equations has been modified. 


#8
Nov1212, 11:45 AM

P: 3,898

I draw out a TEM wave propagates in z direction. j is just a simple phase shift of the H. It has nothing to do with H in y direction and E and H are in perpendicular direction to each other.



#9
Nov1212, 01:56 PM

P: 83

Yeah, that interpretation seems to make more sense. What is also confusing to me though is I thought that in general the E and H fields were in phase unless in a medium with a complex impedance. These seem to show that in a vacuum that the E and H fields are 90° out of phase. I'll have to dig further in the book and do a little more research to see if i can figure out what's going on. Thanks a lot for the hard work on helping me with this!



#10
Nov1212, 02:14 PM

P: 3,898

In fact, you should leave the equation in exponential form to take into account of the j before translating to sin and cos function as j represents a phase shift of ∏/2 and is together with the βR. so you should get something like [itex]sin(\frac{πR}{a}+π/2)[/itex] From your equation: E[itex]_{y}[/itex] = E[itex]_{0}[/itex] sin[itex]\frac{\pi x }{a}[/itex] sin [itex]\frac{l \pi z}{a}[/itex] H[itex]_{x}[/itex] = [itex]\frac{j E_{0}}{Z_{TE}}[/itex] sin[itex]\frac{\pi x}{a}[/itex] cos [itex]\frac{l \pi z}{d}[/itex] H[itex]_{z}[/itex] = [itex]\frac{j \pi E_{0}}{k \eta a}[/itex] cos[itex]\frac{\pi x}{a}[/itex] sin [itex]\frac{l \pi z}{d}[/itex] The direction of propagation is in xz plane as your propagation constant has [itex] \sin\frac{\pi x }{a} \sin \frac{l \pi z}{a}[/itex]. Obviously it is TE wave as E is normal to the direction of propagation and not H. 


#11
Nov1212, 02:36 PM

P: 83

Yes. The equations are for fields in a cavity but do not specifiy if they are in a lossless medium or not. That would depend on β and Z_{0} I think. Also these are TE waves, not TE waves.



#12
Nov1212, 02:40 PM

P: 3,898




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