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Homework Help: Question concerning photoelectric effect lab

  1. Sep 29, 2016 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    This isn't really a homework question but I do have to know it for my lab report so I figure this is a good place to post it. So for my lab we had the setup that is displayed in the picture attachments. My question deals specifically with step #9 of the lab instructions. I'm assuming that the negative current is due to the electrons being pulled back to the cathode by the stopping potential after having been knocked off by the photons. Is this correct? of the cathodeThese were the instructions for the lab:

    1. Turn on the mercury lamp and wait for about 5 minutes until it reaches its full intensity. Connect one of the digital multimeters to measure the retarding potential and set it to the 20 V DC range.
    2. Connect the second multimeter to measure the photoelectric cell current (thereby duplicating the reading on the less accurate built-in milliammeter) and set it to the 2 mA DC range.
    3. By turning the wavelength control on the side of the monochromator, different spectral lines will become visible at the exit slit. Those to be used are: yellow (578 nm), green (546 nm), blue (436 nm) violet (405) and ultra-violet (365 nm). The violet appears relatively faint and the ultra-violet is, of course, invisible.
    4. Adjust the wavelength control until the yellow line is visible.
    5. Place the photoelectric cell on the stand forming a light tight seal with the monochromator.
    6. Turn the “voltage adjust” control fully clockwise (maximum retarding potential) and then switch on the power switch.
    7. Cover the entrance slit of the monochromator to prevent light entering and adjust the “zero adjust” until zero current is obtained. This is a very delicate adjustment and you may not be able to obtain a precise zero.
    8. Let the light back into the system. Turn the “voltage adjust” control in a counter-clockwise direction, thereby reducing the retarding potential, until a current of about 1 mA is registered. Now adjust the wavelength control until the current is maximum. You have now optimized the monochromator for 578 nm.
    9. Turn the “voltage adjust” control fully clockwise (maximum retarding potential) Before recording the “stopping potential”, double check the zero setting, blocking the light as before. After you let the light back into the system you will notice a small negative current which is normal. Try to figure out where this current is coming from.
    10. Measure the current as a function of the retarding potential by reducing the retarding voltage first in step of 0.5 V and as soon as you see a noticeable increase decrease the voltage steps in such a way that you have about 10 measurements up to a current of about 0.5 mA.
    11. Having completed this wavelength, turn the “voltage adjust” control fully clockwise and remove the photoelectric cell so that you can adjust the monochromator for the next spectral line. Repeat the above procedures for each line in turn. The ultra- violet line will have to be found by adjusting the wavelength control beyond the position for the violet line until a current is registered. Keep adjusting the “voltage adjust” control to prevent the current reading from going off-scale while seeking the maximum current.

    Attached Files:

  2. jcsd
  3. Sep 29, 2016 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Look up "dark current."
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