Question

In a photoelectric experiment a parallel beam of monochromatic light with power of 200W is incident on a perfectly absorbing cathode of work function 6.25eV. The frequency of light is just above the threshold frequency so that the photoelectrons are emitted with negligible kinetic energy. Assume that the photoelectron emission efficinecy is 100% A potential difference of 500V is applied between the cathode and the anode. All the emitted electrons are incident normally on the anode and are absorbed. The anode experiences a force F = n × 10^–4N due to the impact of the electrons. The value of n is........ Mass of the electron m_e = 9 × 10^–31kg and 1.0eV = 1.6 × 10^–19J.?

Answer 1

Power = nhv n = number of photons per second

As photon is just above threshold frequency KE_max is zero and they are accelrated by potential difference of 500V.

Since efficiency is 100%, number of electrons = number of photons per second
As photon is completely absorbed force exerted = nmv