1 \subsection{General form}
3 The procedure here is exactly the same as the previous section. The
4 integral normalizing $G_0$, however, becomes a little more
7 Fourier transforming \cref{eq:DHO} and applying \cref{eq:four-deriv} we have
9 (-m\omega^2 + i \gamma \omega + k) x(\omega) &= F(\omega)
10 \label{eq:DHO-freq} \\
11 (\omega_0^2-\omega^2 + i \beta \omega) x(\omega) &= \frac{F(\omega)}{m} \\
12 |x(\omega)|^2 &= \frac{|F(\omega)|^2/m^2}
13 {(\omega_0^2-\omega^2)^2 + \beta^2\omega^2} \;,
16 where $\omega_0 \equiv \sqrt{k/m}$ is the resonant angular frequency
17 and $\beta \equiv \gamma / m$ is the drag-aceleration coefficient.
18 \index{Damped harmonic oscillator}\index{beta}\index{gamma}
19 \nomenclature{$\omega_0$}{Resonant angular frequency (radians per second)}
22 We compute the \PSD\ by plugging \cref{eq:DHO-xmag} into \cref{eq:psd-def}
25 = \normLimT \frac{2 |F(\omega)|^2/m^2}
26 {(\omega_0^2-\omega^2)^2 + \beta^2\omega^2} \;.
31 Plugging \cref{eq:GOdef} into \cref{eq:DHO-psd-F} we have
33 \PSD(x, \omega) = \frac{G_0/m^2}{(\omega_0^2-\omega^2)^2 +\beta^2\omega^2}\;.
36 Integrating over positive $\omega$ to find the total power per unit time yields
38 \iOInf{\omega}{\PSD(x, \omega)}
40 \iInfInf{\omega}{\frac{1}{(\omega_0^2-\omega^2)^2 + \beta^2\omega^2}}
41 = \frac{G_0}{2m^2} \cdot \frac{\pi}{\beta\omega_0^2}
42 = \frac{G_0 \pi}{2m^2\beta\omega_0^2}
43 = \frac{G_0 \pi}{2m^2\beta \frac{k}{m}} \\
44 &= \frac{G_0 \pi}{2m \beta k} \;.
46 The integration is detailed in \cref{sec:integrals}.
47 By the corollary to Parseval's theorem (\cref{eq:parseval-var}), we have
49 \avg{x(t)^2} = \frac{G_0 \pi}{2m^2\beta\omega_0^2} \;. \label{eq:DHO-var}
52 Plugging \cref{eq:DHO-var} into the equipartition theorem
53 (\cref{eq:equipart}) we have
55 k \frac{G_0 \pi}{2m \beta k} &= k_BT \\
56 G_0 &= \frac{2}{\pi} k_BT m \beta \;. \label{eq:GO}
59 So we expect $x(t)$ to have a power spectral density per unit time given by
61 \PSD(x, \omega) = \frac{2 k_BT \beta}
62 { \pi m \p[{(\omega_0^2-\omega^2)^2 + \beta^2\omega^2}] }\;.
67 As expected, the general form \cref{eq:DHO-psd} reduces to the
68 extremely overdamped form \cref{eq:ODHO-psd}. Plugging in for
69 $\beta\equiv\gamma/m$ and $\omega_0\equiv\sqrt{k/m}$,
71 \lim_{m\rightarrow 0} \PSD(x, \omega)
72 &= \lim_{m\rightarrow 0} \frac{2 k_BT \gamma}
73 { \pi m^2 \p[{(k/m-\omega^2)^2 + \gamma^2/m^2\omega^2}] }
74 = \lim_{m\rightarrow 0} \frac{2 k_BT \gamma}
75 { \pi \p[{(k-m\omega^2)^2 + \gamma^2\omega^2}] } \\
78 \frac{\gamma k_BT}{k^2 + \gamma^2\omega^2} \;.