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venatorr
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1. The problem statement
The problem is from the textbook Mathematics for Physicist by S.M. Lea. it's problem 2.35
The power radiated per unit solid angle by a charge undergoing simple harmonic motion is
where the constant K is
and
is the amplitude/c. Using the Residue Theorem, perform the time average over one period to show that
where [itex]z=e^{i\theta}[/itex]
Residue theorem:
the time average of a function f(t) is
My guess is that I have to convert the integral over time to an integral over θ and than convert it to an integral on the complex unit circle using the formula converting cosθ and sinθ to z. I can then do the integral using the Residue theorem
So i first converted it into an integral over theta
since [itex]\omega = \stackrel{~.}{\theta}[/itex]
However I'm stuck with the term ωt in the sin and cos and I don't know how to convert them into theta. Since the speed is not constant in simple harmonic motion, I can't simply say ωt = θ. I also don't think I can say that ωt is constant with respect to theta.
The problem is from the textbook Mathematics for Physicist by S.M. Lea. it's problem 2.35
The power radiated per unit solid angle by a charge undergoing simple harmonic motion is
[itex] \frac{dP}{dΩ} = K \sin^{2}θ \frac{cos^{2}(ωt)}{(1+β \cosθ \sin(ωt))^{5}}[/itex]
where the constant K is
[itex]K=\frac{e^{2}~c~β^{4}}{4~\pi~a^{2}}[/itex]
and
[itex]β=\frac{a\omega}{c}[/itex]
is the amplitude/c. Using the Residue Theorem, perform the time average over one period to show that
[itex]\left\langle \frac{dP}{dΩ} \right\rangle = \frac{K}{8}\sin^{2}\theta \frac{4+\beta ^{2}\cos^{2}\theta}{(1-\beta^{2}\cos^{2}\theta)^{10}}[/itex]
[itex] \textit{}[/itex]
Homework Equations
[itex]\cos\theta=\frac{1}{2}\left( z +\frac{1}{z}\right)[/itex]
[itex]\sin\theta=\frac{1}{2i}\left( z -\frac{1}{z}\right)[/itex]
where [itex]z=e^{i\theta}[/itex]
Residue theorem:
[itex]\oint_{C}f~dz = 2\pi i \sum Resf(z_{n})[/itex]
The Attempt at a Solution
the time average of a function f(t) is
[itex]\frac{1}{T}\int_{0}^{T}f(t)[/itex]
My guess is that I have to convert the integral over time to an integral over θ and than convert it to an integral on the complex unit circle using the formula converting cosθ and sinθ to z. I can then do the integral using the Residue theorem
So i first converted it into an integral over theta
since [itex]\omega = \stackrel{~.}{\theta}[/itex]
[itex] K \sin^{2}θ \frac{cos^{2}(ωt)}{(1+β \cosθ \sin(ωt))^{5}}dt [/itex]
[itex] = K \sin^{2}θ \frac{cos^{2}(ωt)}{(1+β \cosθ \sin(ωt))^{5}} \frac{d\theta}{\omega}[/itex]
[itex] = K \sin^{2}θ \frac{cos^{2}(ωt)}{(1+β \cosθ \sin(ωt))^{5}} \frac{d\theta}{\omega}[/itex]
However I'm stuck with the term ωt in the sin and cos and I don't know how to convert them into theta. Since the speed is not constant in simple harmonic motion, I can't simply say ωt = θ. I also don't think I can say that ωt is constant with respect to theta.