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In the experimental setup shown at the right, green light with a wavelength λ = 540 nm is shone on a metal cathode marked B. The power source can be adjusted to create a potential difference between A and B of a few volts with either A or B being at a higher potential. If the cathode is made of zinc (work function φ = 4.31 eV). No photoelectric current is obtained when the potential difference is zero. Explain why. If the zinc cathode is replaced by a sodium cathode (work function 2.28 eV), will there be a current when the potential difference between A and B is zero? Explain your reasoning.

 
3. The cathode is replaced by a metal cathode that does provide a current with this light when the potential is zero. If the cathode remains illuminated, on the graph below, sketch what the current, I, would look like as the voltage is varied.
   • On your graph, identify any features that depend on the wavelength of the light by writing the letter λ with a circle around it and connecting it to the feature with an arrow. Write a few words to clearly specify what feature you are referring to and why it depends on &lambda.
   • On your graph, identify any features that depend on the intensity of the light by writing the symbol INT with a circle around it and connecting it to the feature with an arrow. Write a few words to clearly specify what feature you are referring to and why it depends on the intensity.

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