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Homework Help: Planck constant V in E=qV question

  1. Mar 8, 2012 #1
    Planck constant "V" in E=qV question

    1. The problem statement, all variables and given/known data

    I am trying to prove Planck's constant using LEDs. So far I have measured several LED's wavelengths using a spectrometer and from this their frequency. This will be f in the equation h = E/f. For E I put the LEDs in a forward biased circuit and am trying to use the equation E = qV to find E so I can therefore find planck's constant. My question is which value do I use for V? The minimum voltage needed to produce a current in the circuit or the voltage the circuit eventually tends to.

    2. Relevant equations


    Thanks very much in advance.

    Also, do you know of any related experiments that I could do concerning this sort of area. I was going to find the wavelength of a UV LED and find its energy by a different method (not going to look at a UV LED through a spectrometer) but I wasn't sure exactly how to do that.
  2. jcsd
  3. Mar 8, 2012 #2
    Re: Planck constant "V" in E=qV question

    I think you should use the voltage when the diode starts to emit light.
    Once the led is conducting there should not be much variation in voltage for different brightness (current)
    This is a standard experiment in England for the IOP based physics course
  4. Mar 8, 2012 #3
    Re: Planck constant "V" in E=qV question

    well i tried it today and a blue LED started at 2.50V but the voltage went well above 5V and the LED just became brighter.

    I'm in Scotland by the way. Part of an investigation
  5. Mar 8, 2012 #4


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    Staff: Mentor

    Re: Planck constant "V" in E=qV question

    Not sure, but maybe a look at the diode current equation would offer some insight.
    If I recall correctly, over the years Scientific American's Amateur Scientist column has reported a method or two for measuring h using ingenious setups. A search there might turn up something useful.
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