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B Photoelectric effect suddenly stopped working help

  1. Nov 27, 2016 #1
    my son did a duality of light science fair project .. did tons of work.. still likes doing the experiments at home

    anyway.. first off his wool and ebonite rod stopped putting a static charge on his christmas tree tinsle that he has on the end of a piece of a metal hanger sticking out of a metal soda can all propped up on some foam blocks.. so he started using a plastic bag instead .. this worked great .. charged it up even more than the wool..

    so he got that working again

    then his uv hand held light stopped removing the charge.. it doesn't have any effect at all any more.... it lights up but it doesn't seem to do anything.. can a uv light look lit up but not produce any uv light?

    the thing has worked for him dozens of times and just suddenly stopped this past week

    i really can't afford to go buy another light just to try it


    thanks for any input
     
  2. jcsd
  3. Nov 27, 2016 #2

    berkeman

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    Staff: Mentor

    Is the humidity high right now? :smile:
     
  4. Nov 27, 2016 #3
    Wow. That sucks. The wool and ebonite will stop working when they get dirty. You can wash them. However you worked around that and so that isn't your biggest problem.

    The UV light ionizes air which then neutralizes the surface. If it stops working either it stopped putting out UV light or the air became harder to ionize. The ability to ionize air can be affected by humidity. You'll get more ions at a longer distance from the lamp when the air is dry. However while humid air would make things noticeably slower, they wouldn't just stop working. The UV lamp will decrease in UV output over time basically because the quartz envelope changes and becomes less transparent to UV light. Getting dirt or oil on the lamp can greatly reduce the life of the lamp. You should never handle a UV bulb with your bare hands. Perhaps the bulb was handled and became opaque to UV light? Unfortunately you can't just clean that.
     
  5. Nov 27, 2016 #4
    You can't see UV light, but UV pointers and similar usually produce some visible blue light as well as the UV.
    If you can see that bluish light then the device is at least partially functional.
    If it's battery operated a low battery might cause the more energetic UV to be reduced while the less energetic blue light is not.
    One way to check if the UV is present would be to shine the light on something fluorescent, like those reflector strips for bikes maybe.
    High denomination bank notes in some countries have 'invisible ink' incorporated in them, which only can be seen under UV illumination.
    Or fluorescent paint is not expensive, and available in hobbyist stores.
    If UV is present the fluorescent object should light up really brightly due to UV exciting the material causing it emit strongly in the visible range,
     
  6. Nov 27, 2016 #5
    Thanks everyone..

    the humidy swings high and low here but is no higher now than it was when it was all working right

    at first i was thinking it was just cause the plastic bag put more static in than the wool.. the tinsel was sticking out more i mean.. but no matter how long he holds the light there..no dice... when you touch the can the charge goes away right away like always..

    it was just a cheap folding uv like looks like a flip phone .. i think maybe it has become less transparent to the uv light
     
  7. Nov 29, 2016 #6
    is it possible rubbing the ebonite on a plastic bag puts too much charge in to the tinsel? cause it does seem to put a lot more than the wool did.. i mean they really shoot out..like sticking straight out

    i was able to scrounge up enough to order a different uv light off of amazon .. still probably not the best one but i wanted one that would ship fast
    already got it.. this one https://www.amazon.com/gp/product/B00RUE0GO4/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

    same thing it does nothing? and now the kiddo says they want him to bring his project back cause he advanced to a regionals with it.. next friday.. did i buy a bad lamp? is there a good lamp i could get fast? that doesn't cost a ton of money... is the problem not the lamp at all?

    i made a new can set up to see if it was that that had somehow gone bad.. even re sanded the wire hanger part to make sure everything is making a new connection .. did that just now while he is at school.. hoping to surprise him... but it still won't work.... and when he first built it it all worked perfectly


    if it means anything the tinsel does go down like it is supposed to when you touch the can and it can sense when the light or my hand holding the light or whatever is closer to the can as the tinsel will move a bit up and down and you move close and far

    any help please?
     
  8. Nov 29, 2016 #7
    ok i think maybe the plastic bag and the ebonite was just putting way to much charge in to it all.. i stood there with both uv lights for a solid minute and no change at all in the tinsel... then i saw on a video you could use a pvc pipe and a brown paper bag.. so i tried that puts way less charge in to it and the old light was enough to remove it..what did my son say that ancient guy said.. eureka.. yeah that's it eureka ..so i hand washed his wool cloth waiting for it to dry.. hopefully it will also work as they was what he used last time ,, and it will look better if he doesn't change materials
     
  9. Nov 29, 2016 #8

    ZapperZ

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    Staff Emeritus
    Science Advisor
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    2016 Award

    Again, check first if the light source is putting out UV. Use a business card. That usually has a coating that fluoresces with UV.

    My concern here is in terms of safety. If your son ever had to show this at a place that has tight safety standards (such as a US National Lab), then having stray UV light source like that can be a problem. Are you guys wearing some form of a safety glasses, or a polycarbonite glasses at the very least when you have this on? It is way too early to give your son a head start on getting cataracts.

    Zz.
     
  10. Nov 29, 2016 #9
    thank you.. yes we are wearing safety glasses that i ordered when i ordered the first light
    plus both lights have a built in switch that turns them off if they are not pointing straight down .. even a little tilt shuts them off
     
  11. Nov 30, 2016 #10
    there are some really expensive uv lights.. well even the really cheap ones are not cheap from what i found but i bet the really expensive heavy duty ones would have no problem with more static.. probably the ones i can afford just aren't strong enough to handle it
     
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