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Planck's Constant, what does it mean?

  1. Mar 10, 2012 #1

    I've been doing some research on the internet about Max Planck and his Achievements. One of them being planck's constant (6.64 x 10-34)... but there is nowhere on the internet that actually tells me what that numbe actually represents? Sure, there are a whole heap of formula which are associated with "h" but when I find the gradient of frequency vs Kmax what exactly am I looking at?


    *also, could I ask whoever is explaining this to keep the complex terminology to a minimum. I've just started learning about Quantum Physics and I'm not familiar with all the lingo just yet ;)*

    Cheers :smile:
  2. jcsd
  3. Mar 10, 2012 #2
    It's important to include the units: h = 6.63e-34 kg m^2 /s

    You can think of the units as being units of position (meters) times momentum (kilogram-meters per second). The value of h might be most easily understood based on its fundamental role in the uncertainty principle. Loosely stated, the uncertainty principle says that if we measure a particle's position to some accuracy A (measured in meters) and simultaneously its momentum to some accuracy B (measured in kilogram-meters per second), then we must have A*B > h. Thus, given A or B there is a minimum value of the other. If I measure the position of a particle to within 1 millimeter, there is a minimum uncertainty in its momentum of h/(1 millimeter) or about 6.6e-31 kg*m/s. This is of course a tiny uncertainty, because h is such a tiny number, at least when expressed in familiar units like kg, m, and s.

    If we don't care about knowing any momenta to this precision (e.g. in analyzing the flight of a baseball) we can use classical mechanics and forget quantum effects entirely. But once we start looking at very small scales and investigating things like the position and momenta typical of electrons in atoms, the uncertainty principle starts placing important bounds, and thus quantum effects become important. So the value of h tells you when quantum mechanics is important, or when it isn't and you can use classical mechanics.
    Last edited: Mar 10, 2012
  4. Mar 10, 2012 #3
    So planck's constant is the Uncertainty?
    I still don't understand, sorry :uhh:
  5. Mar 10, 2012 #4


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  6. Mar 10, 2012 #5
    Planck's constant is not an uncertainty itself. But if, for example, you measure position to some fixed accuracy, the size of Planck's constant determines the minimum uncertainty in momentum.
  7. Mar 11, 2012 #6
    Oh dear... I think my brain just imploded

    I undertstand how the energy of confinement related to plank's constant. But I don't see the connection with Blackbody Radiation and The Photoelectric Effect.:frown:
    Last edited: Mar 11, 2012
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