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Units Electromagnetic Waves, Frequency domain

Posted Jan 7, 2021, 5:45 a.m. EST Wave Optics Version 5.5 2 Replies

Johannes Schultz
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Hi, i am wondering about the unit declaration of the quantities using the Electromagnetic Waves, Frequency domain module of COMSOL. E.g., for the electric field V/m (correct unit in the time domain) is indicated by COMSOL but i am expecting Vs/m because we are in frequency space. Is the calculated field automatically transformed back (inverse fourier transformation) to the time domain after the calculation by COMSOL? It would be nice if someone could explain me this behaviour.

Thanks, JS

2 Replies Last Post Jan 8, 2021, 12:40 p.m. EST
Lars Dammann COMSOL Employee

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Posted: 4 years ago Jan 7, 2021, 10:13 a.m. EST

Hi,

COMSOL uses phasors in the frequency domain, so if you type as a voltage somewhere, this actually gets interpreted as . You can get the physical voltage back by inserting a time t and applying a real operator, so the expression above becomes . To phase shift the voltage, you can write e.g. which then becomes a sine function instead of a cosine, and so on.

So the magnitude of the complex values is really the amplitude of the harmonic signal and the complex phase is the phase shift relative to the global phase. So since it is an amplitude, we can measure it in V.

There is also a chapter on phasors in the reference manual or possibly the AC/DC user's guide, you can find it by just searching the help for "phasor".

I hope that makes it more clear, let me know if you have any questions.

Hi, COMSOL uses phasors in the frequency domain, so if you type 1V as a voltage somewhere, this actually gets interpreted as 1V\cdot e^{i\,\omega\, t}. You can get the physical voltage back by inserting a time t and applying a real operator, so the expression above becomes 1V\cdot\cos\left(\omega\, t\right). To phase shift the voltage, you can write e.g. -1iV which then becomes a sine function instead of a cosine, and so on. So the magnitude of the complex values is really the amplitude of the harmonic signal and the complex phase is the phase shift relative to the global phase. So since it is an amplitude, we can measure it in V. There is also a chapter on phasors in the reference manual or possibly the AC/DC user's guide, you can find it by just searching the help for "phasor". I hope that makes it more clear, let me know if you have any questions.
Johannes Schultz
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Posted: 4 years ago Jan 8, 2021, 12:40 p.m. EST

Thanks a lot for the explanation. Now everything is clear.

Best, JS

Thanks a lot for the explanation. Now everything is clear. Best, JS

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