Can Photon Energy Be Equated to Kinetic and Potential Energy?

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Discussion Overview

The discussion revolves around the relationship between photon energy and kinetic and potential energy, particularly in the context of relativistic mass and the properties of photons. Participants explore the implications of defining photon energy through its wavelength and the conditions under which kinetic energy might be considered non-zero for massless particles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the energy of a photon, defined by its wavelength, can be equated to its potential and kinetic energy, suggesting a possible misunderstanding of the concepts involved.
  • Another participant provides the relativistic energy-momentum relation, indicating that a photon has an invariant mass of zero, which complicates the comparison with kinetic energy.
  • A participant expresses confusion regarding the concept of relativistic mass and its implications for kinetic energy, particularly as velocity approaches the speed of light.
  • There is a discussion about the definition of momentum, with a participant noting that momentum for photons is given by de Broglie's formula, which applies to both massless and massive particles.
  • Participants discuss the term "relativistic mass," with one suggesting it is synonymous with total energy, while another mentions that the term is generally deprecated in favor of clearer terminology.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of mass and energy concepts to photons, with no consensus reached on whether a mass equivalent for a photon exists or how to interpret kinetic energy in this context.

Contextual Notes

There are unresolved assumptions regarding the definitions of mass and energy, particularly in the context of relativistic physics and the behavior of massless particles. The discussion highlights the complexity of relating these concepts without reaching definitive conclusions.

manifested
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If the energy of single photon of electromagnetic radiation is defined by its wavelength, could it also be set equal to its potential and kinetics?(I’m probably missing something)
Please check this statement:
attachment.php?attachmentid=23675&stc=1&d=1265947307.jpg

Where :
h is planks constant
f is frequency.
m0 is rest mass
M is the relativistic mass
v is velocity


If rest mass of electron is zero then mc2 is zero therefore the total energy must be all kinetic(right?).
The only way I could see the kinetic energy not being zero is if somehow the relativistic mass is not zero…or maybe I got it completely wrong…?

Thx in advance.
 

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The correct relativistic expression is:

E^2 = m_0^2 c^4 + p^2 c^2 = h^2 f^2
Where m_0 is the invariant mass, p is the momentum, and the rest are as you have them above.

The expression is general and applies for both massless and massive particles, but a photon has an invariant mass of 0.
 
thx for response.

even so. if p is mass times the velocity, again it comes down the mass or at least the variant mass not being equal to zero.
maybe I'm not looking at it the right way but the variant mass of a particle being defined as
attachment.php?attachmentid=23680&stc=1&d=1265993338.jpg

gives the value 0 devided by zero as v\rightarrow c and as m_0 \rightarrow 0.

i guess the reason for my confusion is that if mass and energy are the same thing in different forms there must be a mass equivalent for a photon(right?)
 

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manifested said:
even so. if p is mass times the velocity, again it comes down the mass or at least the variant mass not being equal to zero.
maybe I'm not looking at it the right way but the variant mass of a particle being defined as
attachment.php?attachmentid=23680&stc=1&d=1265993338.jpg

gives the value 0 devided by zero as v\rightarrow c and as m_0 \rightarrow 0.
Momentum is not mass times velocity in general. As you mention the formula you provided is not valid for photons. The general formula for momentum is de Broglie's formula:
p=\frac{h}{\lambda}
where \lambda is the wavelength.

Which applies for both massive and massless particles.


manifested said:
i guess the reason for my confusion is that if mass and energy are the same thing in different forms there must be a mass equivalent for a photon(right?)
The term you are looking for is "relativistic mass". As you note, it is nothing more than another name for the total energy. Because we already have a perfectly good name for energy this term is generally deprecated. Usually the unqualified word "mass" refers to the "invariant mass" or "rest mass", m_0, but I tend to always explicitly say which definition of mass I am using just to avoid confusion.
 

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