Question

A surface of work function 2.2 eV is illuminated with ultra-violet light of wavelength lambda = 270 nm. When a detector plate is held at a voltage V_p = +12.6 volts with respect to the surface, a current between the surface and the detector plate is observed. A current will also flow for which ONE of the following conditions on V_p? -2.4 < V_p volts V_p < -2.2 volts V_p < -3.2 volts -5.4 < V_p < -2.2 volts V_p > -5.4 volts

Answer 1

Work Function E = 2.2 eV, Vp = 12.6 volts, λ =270 nm

Using Photoelectric effect, K.E = (hc/λ) - W

It is evident that current flows as long as electrons are moving, i.e KE>0 => (hc/λ) > W

Maximum Kinetic Energy = (hc/λ) - W

                                     = [ (1.2398*10^-6) / (270*10^-9) ] - 2.2 eV

                                     = 4.59 eV - 2.2 eV

                                     = 2.39 eV

So clearly stopping potential = KE max / -e = -2.39 V = - 2.4 V

It is very much evident that Vp > -2.4 volts, current will cease

So option (A) is correct -2.4 volts < Vp