Physics Asked by Djamillah on August 14, 2021
I have some problems with getting the complex (time dependent) dipole moments of some dipoles in a configuration. I eventually want to get the electric and magnetic fields of the configuration, but my biggest confusion is about the dipole moments. I wonder:
I can give you an example:
Consider the following configuration. I want to calculate the complex dipole moments of the two dipoles (and later the electric field they are creating).
The dipoles have the dipole moments
$vec{p}_{1}=p_0cos(omega t)hat{z}$
$vec{p}_{2}=p_0cos(omega t-pi/2)hat{z}$
I would then use the Euler identity
$e^{ix}=cos(x)+isin(x)$
$cos(x)=frac{e^{ix}+e^{-ix}}{2}$
$sin(x)=frac{e^{ix}-e^{-ix}}{2i}$
which would give me the following dipole moments
$vec{p}_{1}=p_0frac{e^{iomega t}+e^{-iomega t}}{2}hat{z}$
$vec{p}_{2}=p_0frac{e^{i(omega t-pi/2)}-e^{-i(omega t -pi/2)}}{2}hat{z}$
However, according to my book the correct complex dipole moments should be
$vec{p}_{1}=p_0hat{z}$
$vec{p}_{2}=p_0e^{-ipi/2}hat{z}$
I can see that the author has removed the time dependent part, but I still think my result looks wrong. I feel quite confused about the complex dipole moments so I hope someone can make it a little bit clearer.
If you use the identity
$$cos(alpha - beta) = cosalpha cosbeta + sinalphasinbeta$$
You can see that for $beta = pi/2$ some terms will be zero. What you are left with can then be written in terms of $p_0$ (which itself it time varying).
You can actually get the same result by taking your expression and realizing that
$$costheta = frac{e^{itheta}+e^{-itheta}}{2}$$
and
$$sintheta = frac{e^{itheta}-e^{-itheta}}{2}$$
I agree that it's a little bit confusing, but the math works out. You were very close. Having a few trig identities at your fingertips (recognizing them in what is written down) really helps with problems such as these.
Answered by Floris on August 14, 2021
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