Physics Help Forum Planck's blackbody derivation

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 May 1st 2018, 01:44 PM #1 Banned   Join Date: Apr 2018 Posts: 92 Planck's blackbody derivation Why does Planck's energy element (hv) have the units of the kinetic energy yet light is massless?
May 1st 2018, 07:40 PM   #2

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 Originally Posted by lovebunny Why does Planck's energy element (hv) have the units of the kinetic energy yet light is massless?
Lght has a momentum $\displaystyle p = \hbar k$ and from this we can compute $\displaystyle p = \hbar k = E/c$.

E is not a kinetic energy. It is closer, conceptually anyway, to the rest energy of a particle. But as a photon cannot be at rest this is what we have to work with.

-Dan
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 May 4th 2018, 02:02 PM #3 Banned   Join Date: Apr 2018 Posts: 92 You have not explained why Planck's energy element (hv) has the units of the kinetic energy. Why can't you say you do not know.
May 4th 2018, 03:08 PM   #4

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 Originally Posted by lovebunny You have not explained why Planck's energy element (hv) has the units of the kinetic energy. Why can't you say you do not know.
Please look at the last line of my previous post.

In case, somehow, you didn't realize it: work, potential energy, kinetic energy, and rest energy all have the the same units... J. But they are all different concepts.

So just because $\displaystyle E = h \nu$ has units of kinetic energy does not make it a kinetic energy. Like I said above, in the case of a photon $\displaystyle E = h \nu$ acts somewhat like a rest energy.

-Dan
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May 5th 2018, 05:32 AM   #5
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 Originally Posted by lovebunny You have not explained why Planck's energy element (hv) has the units of the kinetic energy. Why can't you say you do not know.
All "Energy, whether "potential energy", "kinetic energy", or "Planck's energy element" has the same units! You are following a red herring.

 May 6th 2018, 10:54 AM #6 Banned   Join Date: Apr 2018 Posts: 92 The kinetic and potential energies represent a mass yet light is massless which negates your argument.
May 6th 2018, 11:42 AM   #7

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 Originally Posted by lovebunny The kinetic and potential energies represent a mass yet light is massless which negates your argument.
No they don't. Learn some basics before you come back.

-Dan
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