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The energy of a photon

  1. Oct 12, 2013 #1

    bobie

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    Hi,

    Energy is expressed in J or eV, but E= hf
    As planck constant h is J.s is it possible to express the energy of a photon in h/s? If not, why?
    Can we say that the energy of a photon is 2.41 x 10^14 h/s?
     
  2. jcsd
  3. Oct 12, 2013 #2

    Drakkith

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    If h = Js, then J = h/s, and you can express E in joules, so it appears to me that you are already using h/s.
     
  4. Oct 12, 2013 #3
    E = hf shows photon energy [E] varies with frequency [f].

    Your proposal does not.

    Your proposal is missing a unit of measure.



    For some unit relationships, try

    http://en.wikipedia.org/wiki/Planck_constant
     
  5. Oct 12, 2013 #4

    bobie

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    What unit is missing?
    A photon with frequency 2.418 x 10^14 Hz has E = 1 eV, is that correct?
    h = 4.135 x 10 ^-15 eV.s , h/s = 4,135 x 10^-15 eV (from your link)
    ( 2.318x 10^14 x) h/s = (2.428x10^14 x) 4.135 x 10^-15 eV
    2.318x 10^14 x h/s = 1
    A photon with frequency 2.418 x 10^14 Hz has E = 2.418 x 10^14 h/s

    Where do I go wrong?
     
  6. Oct 12, 2013 #5
    'h' is a constant, right?

    What does 's' mean in your equation?? Please explain.
     
  7. Oct 12, 2013 #6
    Be careful here, you are mixing a constant (h) and a unit (s) in the same unit expression, which is very unusual and should be avoided IMO.

    You obviously mean

    h: Planck's constant ≈ 6.626×10−34 J·s (joules times seconds)
    s: seconds

    but h as a unit usually mean hour. The usual unit - the SI unit - of energy is the joule (J), not any h/s. There are also some other units of energy like the electronvolt (eV).

    Hmm... I'm wondering if someone will give me a prize for using the most wikipedia links in one and the same short post...?
     
    Last edited: Oct 12, 2013
  8. Oct 13, 2013 #7

    bobie

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    Thank you all,
    If I got it right, h = J.s cannot be modified to h/s =J.
    But I read that the Hz is equivalent to 1/s, then in the formula E= h.f => E = h. 1[]/s = aren't we mixing a constant with a unit?
     
  9. Oct 13, 2013 #8

    Bandersnatch

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    For clarity's sake let's use brackets when we mean units, and remember to not mix the bracketed bits with unbracketed ones.

    f=1/T not [1/s]
    [1/s] is the unit of f

    So, E=hf and the units are h=[J*s] and f=[Hz]=[1/s]

    Then E=[J*s*Hz]=[J*s/s]=[J]
     
  10. Oct 13, 2013 #9

    bobie

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    Thanks, if h/s is forbidden, how can we express that the scalar of the frequency is always equal to the scalar of the energy of a photon?
     
  11. Oct 13, 2013 #10

    Bandersnatch

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    Not sure what you mean. Energy equals frequency of the wave times the planck constant E=hf. These are all scalar quantities.
    You can substitute 1/T for f if you like, to get E=h/T, where T is the period of the wave.
     
  12. Oct 13, 2013 #11

    bobie

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    So , what is the E of a photon with frequency 2.418x 10^14 wxpressed in h/T?
     
  13. Oct 13, 2013 #12

    Bandersnatch

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    It's the same as with the energy expressed as hf. The energy of the photon does not change just because you use some arithmetic to rearrange the equation. h/T is, after all, equal to hf in both value and units.

    f=2.418*10^14 [Hz]

    E=fh=2.418*10^14 [Hz]*6.626×10^-34 [J*s]=~16*10^-20 [J]

    T=1/f
    T=1/(2.418*10^14) [1/Hz]
    T=~0.41*10^-14

    E=h/T=6.626×10^-34 [J*s]/0.41*10-14 =~16*10^-20 [J]
     
  14. Oct 13, 2013 #13
    Just came back...
    bobie,
    my post #5 was an attempt to get you to see for yourself the difference between a measure, such as a frequency, f, and the unit utilized to express it, such a cycles per second.
    I think that's what the prior posters are explaining,too.

    If you write out the UNITS associated with any measure, any formula, you will see whether you have consistency:

    for example distance equals velocity times time, right?? :
    a distance = ft/sec x seconds = ft and that makes 'sense'.....the seconds cancel.....if you have instead a measure of speed such as [50] miles/hour x [25] seconds your units become mile-second/ hours.....not a standard set of units....so much better to convert the right hand side to either hours, seconds, or whatever....


    Note also the unit 'Hertz' [and seconds] has some confusion associated with it:

    http://en.wikipedia.org/wiki/Cycle_per_second

    [I wasn't even aware of this supposed confusion til I just checked. ]
     
  15. Oct 13, 2013 #14

    jtbell

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    We could define a new unit of energy: 1 bobie = 4.135 x 10-15 eV = 6.626 x 10-34 J. Give it the symbol 'B'. (Or is there another unit with that symbol? I forget...)

    Then Planck's constant would be 1 B.s, and a photon with frequency 2.418 x 1014 Hz would have an energy of 2.418 x 1014 B.
     
  16. Oct 14, 2013 #15

    bobie

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    Congrats, jtbell, you hit the nail on the very head!, thanks.
    That's exactly what I meant without aspiring to have a unit named after me.:redface:, that would make patent the relation of energy to a single oscillation (I posed a similar problem here :https://www.physicsforums.com/showthread.php?t=712514 , it seemed to me that dimensions prevent seeing the forest because of the trees. There I discovered geometrized units and that dimensions are not indispensable).
    I have little (or no) experience and I still cannot see the subtle difference between 1B and 1 h/s.

    Actually I cannot even grasp what a unit of energy can actually mean when multiplied by time.
    I understand that power (J/s ) is energy absorbed every second but J.s would correspond to distance ( as compared to velocity, right)?
     
    Last edited: Oct 14, 2013
  17. Oct 14, 2013 #16
    J.s is the unit for action (and also angular momentum).
     
  18. Oct 15, 2013 #17

    bobie

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    Can 2 different entities have the same dimensions?, what has h in common with angular momentum?

    ...if h indeed is a unit then it can be mixed with other units and we can use h/s?
     
    Last edited: Oct 15, 2013
  19. Oct 15, 2013 #18

    Bandersnatch

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    Stop it right now, mister! Stop saying that h is a unit, or I'm going to reach through the screen and give you a good spanking! :grumpy:
    It is most emphatically not. Nobody has ever said that. It's a constant and it's units are J*s.

    h/s is mixing constants with units.
    Write J*s/s if you must, although it's obviously just J, if you want to write units. Or write h/T if you want equations. Don't write h/s. It's like writing a=V/s or F=kg*a.
     
  20. Oct 15, 2013 #19

    bobie

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    Why do you blame, me, bandersnatch,did you see that I was simply quoting Mr dauto:confused:, ?
    how can I possibly know who is right or wrong?
     
    Last edited: Oct 15, 2013
  21. Oct 15, 2013 #20

    Bandersnatch

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    Where, oh, where did he say h is a unit? All he said is that J*s is a unit.
     
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