4 ms·

In the spirit of pointing out inconsistencies...I am not a physicist, but something I've been confused about lately is why the action constant has a unit of sec

by rpz 7y ago

In the spirit of pointing out inconsistencies...I am not a physicist, but something I've been confused about lately is why the action constant has a unit of seconds baked into it. Feels like E = hf is an experiment with a hard coded 1 second measure time. Why not express the relationship in terms of power? P=uf where u is the action constant without the seconds unit hard coded. And E=utf has a variable time parameter

rpz 7y ago
Down votes? Care to explain? I'm just trying to learn here :)
- plmu 7y ago
  Agree, I'm wondering too.
- akubera 7y ago
  Didn't downvote, but they probably came because you asked a very tangential question to the subject. I'll try to answer it: I think you just want to move units around so h (Planck's constant) has units of Joules instead of Joule-seconds. While this works mathematically, it doesn't really make sense physically, where we care about the energy of a photon because that's the value that is conserved (along with momentum, also relating to h) during any interaction (e.g. the quantity gained by an electron if the photon is absorbed). It really doesn't make sense to want to quantify the "power" of a photon, which doesn't have any physical meaning, in favor of a simpler constant. You are right about the constant 'h' having a "hard-coded" 1-second built in, but so do ALL derived physical constants . If we, say, changed it to two seconds, the number would be cut in half to reflect the scale change and preserve the energy of the photon (or any action) because that does not depend on scale. As to "why is it Joule-seconds?": that's the discovered law of nature. If the energy was proportional to frequency squared, h would be in units of J*s².
  - rpz 7y ago
    Consider it this way. Suppose we all agreed to never use numerical values in the units section of an equation... then in this case we'd need a symbol for cycles in cycles per second aka Hz. If we leaves Planck's constant as is in E=h(cycle/second), then the units would end up as Joulecycle... odd right? That's what brought me to writing the formula as E=utf I guess it should really be Delta E=u(delta t)f where u is in joules t is (seconds per cycle) and f is (cycles per second)
    - rpz 7y ago
      The upshot of what I'm saying here is that I believe that each wavelength of light is delivering the same quanta of energy regardless of frequency
tlb 7y ago
h (Planck's constant) has units J*s, and f (frequency) has units 1/s. Therefore E=hf has units of Joules. E is the energy of a single photon at that frequency. It's not a measurement of power.

by rpz 7y ago

rpz 7y ago

Down votes? Care to explain? I'm just trying to learn here :)

plmu 7y ago
Agree, I'm wondering too.
akubera 7y ago
Didn't downvote, but they probably came because you asked a very tangential question to the subject. I'll try to answer it: I think you just want to move units around so h (Planck's constant) has units of Joules instead of Joule-seconds. While this works mathematically, it doesn't really make sense physically, where we care about the energy of a photon because that's the value that is conserved (along with momentum, also relating to h) during any interaction (e.g. the quantity gained by an electron if the photon is absorbed). It really doesn't make sense to want to quantify the "power" of a photon, which doesn't have any physical meaning, in favor of a simpler constant. You are right about the constant 'h' having a "hard-coded" 1-second built in, but so do ALL derived physical constants . If we, say, changed it to two seconds, the number would be cut in half to reflect the scale change and preserve the energy of the photon (or any action) because that does not depend on scale. As to "why is it Joule-seconds?": that's the discovered law of nature. If the energy was proportional to frequency squared, h would be in units of J*s².
- rpz 7y ago
  Consider it this way. Suppose we all agreed to never use numerical values in the units section of an equation... then in this case we'd need a symbol for cycles in cycles per second aka Hz. If we leaves Planck's constant as is in E=h(cycle/second), then the units would end up as Joulecycle... odd right? That's what brought me to writing the formula as E=utf I guess it should really be Delta E=u(delta t)f where u is in joules t is (seconds per cycle) and f is (cycles per second)
  - rpz 7y ago
    The upshot of what I'm saying here is that I believe that each wavelength of light is delivering the same quanta of energy regardless of frequency

tlb 7y ago

h (Planck's constant) has units J*s, and f (frequency) has units 1/s. Therefore E=hf has units of Joules. E is the energy of a single photon at that frequency. It's not a measurement of power.