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The gravitational force is therefore completely overwhelmed by the electromagnetic force with respect to scattering in the ionized ISM.  \subsection{Part c \textit{Following Yuan-Sen Ting}} \textit{following Anjali Tripathi}}   \href{In class we derived}{http://ay201b.wordpress.com/2011/04/12/course-notes/#excitation_processes} the timescale for neutral-neutral (nn) and neutral-ion (ni) interaction between particles in the neutral ISM.