Photoelectric Effect and energy

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Discussion Overview

The discussion revolves around the photoelectric effect, specifically addressing the relationship between the energy of incident photons and the kinetic energy of emitted photoelectrons. Participants explore the conditions under which electrons are emitted from a metal surface and the factors influencing their maximum kinetic energy.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the relationship between the original kinetic energy of an electron and the energy of the photon, suggesting that the electron's kinetic energy should increase upon photon impact.
  • Another participant clarifies that the maximum energy of emitted photoelectrons is determined by the energy of the photon minus the work function of the metal, challenging the initial claim.
  • A follow-up response reiterates that the maximum energy is not simply the sum of the photon energy and the electron's original energy, but rather depends on the work function and the Fermi energy of the electrons in the metal.
  • It is noted that if a photoelectron originates from below the Fermi level, its emitted energy will be less than the maximum energy calculated.
  • Participants discuss the potential energy barrier that must be overcome for photoelectrons to be emitted, emphasizing that the maximum kinetic energy is influenced by both the photon energy and the work function.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the original energy of the electron and the energy of the photon. There is no consensus on the initial claim regarding how these energies interact, and the discussion remains unresolved regarding the interpretation of the maximum kinetic energy of photoelectrons.

Contextual Notes

Participants reference various energy levels, including the Fermi energy and work function, which may depend on specific definitions and conditions not fully explored in the discussion.

MHD93
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Hello

Before a photon hits an electron, the electron originally had a kinetic energy, if f = f0 we know that
Kmax = hf -hf0 = zero

shouldn't the electron's kinetic energy increase the photon hits it? not the converse?
 
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Kmax = hf -hf0 = zero

Sorry Mohammad, have you misquoted this? Photoelectrons have a range of energies from zero up to Emax = hν - φ, where φ is the work function of the metal. In other words it's just the opposite of what has been said - the maximum energy is determined by the energy of the photon, and not by the original energy of the electron.
 
Bill_K said:
Sorry Mohammad, have you misquoted this? Photoelectrons have a range of energies from zero up to Emax = hν - φ, where φ is the work function of the metal. In other words it's just the opposite of what has been said - the maximum energy is determined by the energy of the photon, and not by the original energy of the electron.

It's what I ask..
Why isn't the maximum energy is the energy of the photon added to the original electron's energy
 
Oh OK, that's because this is the maximum energy. Electrons in the metal have a maximum energy, EF the Fermi energy, which is an amount φ below the vacuum outside. So hν - φ is the maximum energy the emitted photoelectron can have. If the photoelectron came from below the Fermi level, then of course its energy will be less.
 
Mohammad_93 said:
It's what I ask..
Why isn't the maximum energy is the energy of the photon added to the original electron's energy
It is, but that's not the whole picture. As explained by Bill, there's an additional potential energy barrier that needs to be overcome. The maximum KE of the Photoelectron is therefore equal to its original energy (EF) + the photon's energy (hv) - the final potential energy (Evac).

KE(max) = hv - (Evac - EF) = hv - φ

φ is called the Work Function, and if you define, φ = hv0, that gives you: KE(max) = hv - hv0
 
THanks.. that's given me the whole picture, helpful.

##
 
THanks.. that's given me the whole picture, helpful.

##
 

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