Register to reply 
Help on Photoelectric Effect problemby Reshma
Tags: photoelectric effect 
Share this thread: 
#1
Oct2207, 08:32 AM

P: 777

a] A beam of light of wavelength 400nm and power 1.55mW is directed at the cathode of a photoelectric cell. If only 10% of the incident photons effectively produce photoelectrons, find the current due to these electrons.
b]If the wavelength of light is now reduced to 200nm, keeping the power same, the kinetic energy of the electrons is found to increase by a factor of 5. What are the values of the stopping potentials for the two wavelengths? My Work: a]Given the wavelength of incident light I calculated the incident energy as [itex]h\nu = h{c\over \lambda} = 3.1 eV[/itex] The doubt in my mind is over the term "effective". If 10% of the incident photons is utilized for producing photoelectrons, then it should be equal to the Kinetic energy of the electrons (KE). Hence, [itex]KE = 0.1 h\nu[/itex] in eV. Power = Voltage x Current Current(I) = 1.55mW/0.31V Please correct me if I am wrong. b] The stopping potential [itex]V_s[/itex] is related to the kinetic energy by [itex]KE_{max} = eV_s[/itex] and we have the relationship: [tex]{h\over e} = \frac{V_{S2}  V_{S1}}{\nu_2  \nu_1}[/tex] I am a little confused over whether I should use this relation or simply add 5 to the KE determined from the previous exercise and divide it by e to obtain the stopping potential. 


#2
Oct2207, 08:41 AM

Emeritus
Sci Advisor
PF Gold
P: 11,155

You have the incident power and the energy of each photon. From this, you can calculate the incident photon rate striking the metal. Assuming 10% of the photons incident on the metal produce photoelectrons, you can find the rate of electron emission, and hence the current produced. 


#3
Oct2607, 08:39 AM

P: 777




Register to reply 
Related Discussions  
Difference between Identical , Equal , Equivalent  Calculus & Beyond Homework  9  
Is relativistic effect of length contraction physically real ?  Special & General Relativity  92  
The Photoelectric effect and classical physics  Introductory Physics Homework  9  
A very peculiar magnetic effect (chemist yells help! )  Classical Physics  1  
Oneoff: Black drop problem an SR effect?  Special & General Relativity  6 