A velocity selector consists of electric field → E = E ^ k and...

Fiven that \overset{\to}{E} = E\overset{^}{k} and \overset{\to}{B} = 12\overset{^}{j}mT

Kinetic energy$= 728eV$

Kinetic energy $= \dfrac{1}{2}m{v}^{2}$

$⇒ 728eV = \dfrac{1}{2} \times 9 . 1 \times {10}^{- 31} \times {v}^{2}$

$⇒ 728 \times 1 . 6 \times {10}^{- 19} = \dfrac{1}{2} \times 9 . 1 \times {10}^{- 31} \times {v}^{2}$

$⇒ v = 16 \times {10}^{6}m{s}^{- 1}$

For electron to move undeflected net force on it should be zero.

$⇒ eE = evB$

$⇒ E = vB = 16 \times {10}^{6} \times 12 \times {10}^{- 3}$

$⇒ E = 192 \times {10}^{3}V{m}^{- 1} = 192kV{m}^{- 1}$