Question about photocell experiment

[Clara Chung]

Consider a set up with a power supply(dc voltage) ,and a photocell (metal plate + collector) ,and an ammeter. Frequency of light is constant.

When V gradually increases from 0 (anode linked to the collector, anode to the metal plate), the photoelectric current is said to be increasing until it reaches the saturated current. (from my book)

But from my exercise, it says it won't increase because the current is not contributed by the dry cell, but by radiation. Once the photoelectrons are emitted, they tend to go round the circuit(because there is a potential no matter its value) and return to the illuminated metal. The dry cell cannot increase the number of photoelectrons.

Which one is correct?

 

[Tom.G]

The light hitting the photocell knocks some electrons off of the metal in random directions. The externally applied voltage (from the dry cell in your description) attracts the electrons. At low applied voltage there is not enough electric field strength to attract all the electrons that were knocked off. A higher applied voltage will attract more of the free electrons, as shown by the higher current. Eventually, with a high enough applied voltage, all the free electrons flow to the collector in the photocell. This is the "saturated current" the book refers to. To get a higher current you need to increase the light intensity (more photons per second) to knock off more electrons.