Max Speed Electron Ejected from Chromium Metal by Light of 250 nm Wavelength

[Cursed]

Homework Statement

The work function of chromium metal is 7.2 x 10^-19 J. What is the maximum speed an electron can be moving if it is ejected from chromium metal by light of wavelength 250 nm? (Answer: u= 4 x10⁵ m/s)

Homework Equations

\Phi = h v_{0}

KE = h v_{light} - \Phi = \frac{m_{e} u^{2}_{e}}{2}

\Delta E_{light}= \Phi + KE


h is Planck's constant
v_{0} is the characteristic frequency
m_e is the mass of an electron (9.1 x 10^-31 kg)
u_e is the speed of the electron

The Attempt at a Solution

7.2\times10^{-19} J = \frac{(9.1\times10^{-31} kg) (u^{2}_{e})}{2}

u_e \approx 1.3 \times 10^6 m/s

 

[Redbelly98]

Homework Statement

The work function of chromium metal is 7.2 x 10^-19 J. What is the maximum speed an electron can be moving if it is ejected from chromium metal by light of wavelength 250 nm? (Answer: u= 4 x10⁵ m/s)

Homework Equations

\Phi = h v_{0}

KE = h v_{light} - \Phi = \frac{m_{e} u^{2}_{e}}{2}

Keep this equation in mind.

\Delta E_{light}= \Phi + KE


h is Planck's constant
v_{0} is the characteristic frequency
m_e is the mass of an electron (9.1 x 10^-31 kg)
u_e is the speed of the electron

The Attempt at a Solution

7.2\times10^{-19} J = \frac{(9.1\times10^{-31} kg) (u^{2}_{e})}{2}

u_e \approx 1.3 \times 10^6 m/s

You have left out the energy of the photon, the hv_light in your earlier equation.