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Photoelectric effect and LEDs

  1. May 3, 2009 #1
    So what has to be right for a metal to release electrons when shot with a light? if you use more than whats needed will it still work? can this be used to power something? what else should i know about it? I wanna buy some 5mm LED bulbs to mess around with. from my understanding you can make them any color you want by changing the current that goes through it, is that correct? and does it have to be some special LED or is it any? thanks for all the help and sorry if my questions are dumb, i just learned about this stuff a few days ago.
     
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  3. May 3, 2009 #2

    mgb_phys

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    The energy of the photon has to be less than the work function of the metal - that means the light must be bluer (more energetic) than some cut off. For most metals this is in the UV.
    If you use more energy, the electron is kicked off with more energy (speed)

    Not really - it's a fairly inneficent way of charging something or even of generating free electrons.

    No - the color of an LED is set by the chemistry. There are some devices that have multiple LEDs in a single package with different wires to turn on different lights, some have built in electronics to turn on different mixes of color at different voltages/currents.
    The individual LEDs are only the color they were built as.
     
  4. May 3, 2009 #3
    so is there any use of it? theres a certain speed at which increasing the energy wont increase the speed of the electrons, right? but it COULD power something right, it just wouldnt be efficient? what do you mean by free electrons?? and what do you mean by different wires, whats different about them? so there is no LED bulb i can buy and control what color i want it to be by adjusting the voltage? thanks for all the help. oh yeah one last thing is that how solar panels work? by absorbing light from the sun and releasing electrons? again thanks for the help
     
  5. May 3, 2009 #4

    mgb_phys

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    The only real use of the photoelectric effect is to prove Quantum theory - the important point is that the energy of one photon kicks out one electron. The intensity (number of photons doesn't matter) you can't use a bright red flood light in place of a weak UV lamp.
    Some photomultipliers (night vision goggles) use the photoelectic effect.

    Solar panels are a sort of photoelectric effect, except that the electron is kicked accross the junction of a semiconductor. An LED is just the same thing backward, in fact you can shine light on an LED and get electricity out!

    Multicolor LEDs are really a red/green/blue LED in one package, sometime they have 4 wires, one for each color and a ground, or sometimes the color depends on which way you supply the power.
     
  6. May 3, 2009 #5
    thanks for all the info!!! Would you mind explaining how shining a light onto an led will produce a current? or give me a link? does it have to be any special light of led? so if i take an led and hook it up to something and shine light on it, it will run ?? thanks again for the help!
     
  7. May 3, 2009 #6

    mgb_phys

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    An LED and a photodiode are the same thing, it's just a question of what they are optomised for.
    You can put a voltmeter on a LED point it at the sky and you will measure a voltage (but not much current).
    You can also connect a voltage across a solar panel and it will glow (although you might damage it)

    As far as te semiconductor is concerned, electric field moves electron across barrier and kicks a photon out or a photon in kicks the electron across barrier - same thing.
     
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