The kinetic energy of the electron is [tex]K= \frac{1}{2}mv^2 [/tex] where [tex]m=9.1 \cdot 10^{-31} kg[/tex] is the mass of the electron and v its speed. Since we know the value of the kinetic energy, [tex]K=4.1 \cdot 10^{-18} J[/tex], we can find the value of the speed v: [tex]v= \sqrt{ \frac{2K}{m} }= \sqrt{ \frac{2\cdot 4.1 \cdot 10^{-18}J}{9.1 \cdot 10^{-31}kg} } = 3\cdot 10^6 m/s[/tex]
