Problem #$5[10$ marks$]$ A $7$Li$+$ ion $($charge $=+$e$)$ has a mass of $1\mathrm{.}16$ x $10-26$ kg$.$ It is accelerated through a potential difference of $500$ V and subsequently enters a uniform magnetic field of magnitude $0\mathrm{.}400$ T perpendicular to the ion$$s velocity. $($a$)($b$)$ Find the speed of the ion. Find the magnitude of the magnetic force acting on the ion.