Talking Physics

Photoelectric Effect

This topic contains 7 replies, has 3 voices, and was last updated by  Alessio Bernardelli 4 days, 4 hours ago.

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  • #47019
    Profile photo of Andrew Normand

    We recently changed our photoelectric effect demonstration.

    IMAG0337
    Above is the first attempt at the new set up. The glass plate can be used to show that it is the UV from the lamp. When the glass plate is in between the lamp and the metal can, we get no effect.

    new build

    In the above picture you can see we took an aluminium can and sanded off the paint. We then stuck this on polystyrene cups to insulate. The part used to indicate if charged on the right hand side of the picture is just standard cooking foil.

    Here it is in action.

    I also still show the electroscope with zinc plate.

    What I have not shown in the videos is how we show shining the light through a glass plate has no effect as the UV is blocked. I also use a filament bulb and this has no effect.

    While I have the electroscope out I usually show them why the modern mini was recalled.IMG_6507

    A great story when teaching static electricity. You can show students how the Driver and Vehicle Standards Agency issues a recall.

    The fault developed because conductive paint on the fuel filler neck was wearing off too quickly. Apparently at worse only the fuel at the top of the filler pipe could flash. A few newspapers covered the story a good article is found here

    http://www.esdjournal.com/archives/press_releases/2001/bmw/mini.htm

    Here is a video of an electroscope with a plastic cup on it. I am pouring semolina down a plastic tube into the cup.

  • #47026
    Profile photo of Andrew Normand

    Lamp

    The above is the lamp holder we use. I really like it as we can lay it flat and it directs all the UV light downwards. We can also put in our UVb bulb and do all our fluorescent demonstrations. The link is now dead but you may be able to search for a new one of a similar type with the information.

    http://www.ebay.co.uk/itm/13W-SUPER-FLAT-UNDER-CABINET-LIGHT-T5-BULB-/380327466121?pt=UK_Light_Fittings&hash=item588d489489

    This will bring up similar things. However when I purchased our one it was only £14. There is a good thread here with my lamp in action for fluorescent materials.

    https://www.talkphysics.org/groups/news-and-comment/forum/topic/fluorescence-and-phosphorescence/#post-39107

  • #47039
    Profile photo of Andrew Normand

    I like the can demo! It would be a nice and simple make and take workshop. How expensive are the UV lamps you use?

  • #47047
    Profile photo of Andrew Normand

    I find that the biggest problem with the traditional approach is the zinc plate must be cleaned each time both sides (I use wire wool) or it won’t work.

    The PAG in OCR only uses the LEDs for this now, I made a few more kits up so we have a class set. The ones I made using photo diode tubes don’t tend to get used any more which I think is a shame because they give you a much better view of what is going on and much more like the GLE demo.

     

  • #47493
    Profile photo of Andrew Normand

    I agree Nick and we keep a piece of wire wool in the box of kit. I did like your photo diode set up you once shared on here. Another good reason to show a photo diode is to show how early television cameras worked. I sometimes like to discus older technology. Everything is done with solid state devices these days. I know of loads of good examples using magnetic phenomenon. However I was pleased to find an application of the photoelectric effect.

    I found this gem in “New Elementary Physics” by Millikan, Gayle and Coyle.

    It is the relationship between intensity of the light and number of photons being emitted that made the photoelectric cell one of the first ways to turn light into an electric signal for the sound motion picture industry.

    sound

    Incredible to see how the sound was added and synced with the video. Variations in the original electrical signal from the microphone being turned into variations in light intensity. These captured variations in light intensity could then be directed at a photoelectric cell which could then turn them back to an electrical signal that would be sent to a loud speaker.

    In old TV cameras we have a scanning system that takes the intensity of light from a region of say the face in the image below. This is then directed onto a photoelectric cell to make an electrical signal carrying the light intensity information.

    Photelectric TV sender

     

    Then when the signal is played back through a neon lamp we see (there is a little persistence of vision involved) the image. The rotating disk has a spiral of holes that scan the different regions of the face.

    Photelectric TV receiver

    Another interesting tale to tell with the photoelectric effect is the apparent atmosphere of the Moon. Astronauts noticed a glow from dust suspended above the lunar horizon at sunset. This is charged dust particles from a photoelectric effect of Moon dust repelling and floating above the surface. This fine charged dust also caused problems for the Apollo missions,

    “Fine as flour and rough as sandpaper, Moon dust caused ‘lunar hay fever,’ problems with space suits, and dust storms in the crew cabin upon returning to space.”

    You can read a good article about it here
    https://www.sciencedaily.com/releases/2008/09/080924191552.htm

     

     

  • #47494
    Profile photo of Andrew Normand

    The photo diodes I used for my photo electric kit were originally designed for things like film projectors I believe, certainly the smaller one I made.

  • #57153
    Profile photo of Andrew Normand

    Alom Shaha’s videos are very informative and full of practical hints and tips for all science teachers. I know many teachers like myself who also find their own students gain much from watching them.

    Here is his photoelectric effect video.


    There are many more to watch here
    http://alomshaha.com/demo-films/

     

  • #57181
    Profile photo of Andrew Normand

    Thanks for these ideas Dave. Great stuff. I like using the Tesco’s shots cups to demo photoelectric effect. They come in different colours, so I mark a line on each representing the energy shifted by a photon as it gets absorbed by and electron in the metal plate. I also use two small beakers, one marked Work Function with a line up to a certain level and another marked Ek for the kinetic store associated to the photoelectron. Then I fill each shot cup with water up to the level marked and see if that energy (water) is enough to fill the work function. The red and green cup will not quite fill it, so there is no energy left for the kinetic store and the electron cannot escape. The bluish cup I pretend to be a UV photon and just makes it (so it is at the threshold frequency), but again no energy left for the kinetic store. But the pink/purple cup is a higher energy UV photon and there is energy left to fill the kinetic store a bit… Students find this quite easy to understand and you can use the cups and beakers as direct substitutes to build the equation, and show that pouring the water from the kinetic store and work function back into the photon energy cup gets you back to the original energy of the photon.

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