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Homework Help: Compare and contrast a 2.2 eV photon with a 2.2 eV electron

  1. Mar 19, 2017 #1
    1. The problem statement, all variables and given/known data
    Hey everyone,
    So my teacher assigned a mini project just before our finals and I was wondering if someone could look over my work and my calculations. I just want to make sure I understood everything since my final exams are coming.

    Here's the assignment.

    Compare and contrast a 2.2 eV photon with a 2.2 eV electron in terms of
    Rest mass (Kg)
    Speed (m/s)
    Wavelength (m)
    Momentum (kgm/s)

    For each, write a brief sentence on the comparison and anything you feel that should be noted.

    2. Relevant equations

    3. The attempt at a solution

    As always, please check the attachment to see my calculations. The brief comparison sentences might be a bit to small to see on the attachment so I'll type it here.

    Photo/electron, Energy: Since both the photon and electron have the same value of energy, in terms of electron volts, they will have the same value in joules.
    Photo/electron, Rest Mass: One can see that an electron at rest has a constant mass value. But, photons do not have a rest mass since they are never at rest.
    Photo/electron, Speed: Here, the photon is much faster in comparison to the electron, traveling at the speed of light. Electrons need more energy in order to travel faster. However, it becomes extremely difficult since as it approaches the speed of light, the electron gets larger and so it becomes even harder to make it go faster.
    Photo/electron, Wavelength: Both the electron and the photon have equally the same amount of energy. Nonetheless, the wavelength of the electron is much smaller in comparison to the photon’s wavelength.
    Photo/electron, Momentum:
    The electron has more momentum than the photon. Conversely, it was thought that since a photon had no mass that it would not have any momentum. This was proven wrong as Arthur Compton instituted a way around this by discovering a comparable mass for a photon. With the help of Einstein’s equation, E=mc^2, Compton solved for the mass, m=E/c^2 and used this expression for mass and incorporated into the momentum equation.


    Attached Files:

  2. jcsd
  3. Mar 19, 2017 #2


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    Your work looks very good to me. However, the assignment is not very clear on what is meant by the "energy of the electron". The 2.2 eV is the kinetic energy of the electron. Does the electron also have energy associated with its rest mass?
  4. Mar 19, 2017 #3

    Thank you for taking your time to review my work!

    As for the assignment, my teacher did mention that the "Mini Project" was unplanned. But she felt it was a good way of distinguishing the difference between the electron and the photon (And to help students with low marks bring their marks up a bit.). She also mentioned that we are to assume the electron is not at rest, except for the "rest mass" portion. She didn't want to give us too much work as our finals are just around the corner.

    Again, thank you for your generosity and help !
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