In hydrogen atom, energy of first excited state is `- 3.4 eV`. Then, `KE` of the same orbit of hydrogen atom is.

Explanation

Total energy of the electron in n =2 (1st excited state) `= -3.4 eV`
`E_("total") = K.E. + P.E. = (1)/(2) m v^(2) + ((-Ze^(2))/(r))`
But force of altraction on the nucleus = Centripetal force
i.e., `(Ze^(2))/(r^(2)) = (mv^(2))/(r) or mv^(2) = (Ze^(2))/(r)`
`:. E_("total") = (1)/(2) (Ze^(2))/(r) - (Ze^(2))/(r) = - (Ze^(2))/(2r) = -3.4 eV` (Given)
`:. K.E. = (1)/(2) mv^(2) = (1)/(2) (Ze^(2))/(r) = 3.4 eV`