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class code: SPS21 teacher: Mr. Elert
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Physics A: Problem Set 25: Quantum electrodynamics

recommended reading

High Marks: n/a
Barron's Let's Review: n/a
physics.info: Quantum electrodynamics, X-rays, Antimatter
Wikipedia: n/a
HyperPhysics: n/a
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classwork

  1. A photon has just enough energy to form an electron-positron pair.
    1. Write this reaction out in symbolic form.
    2. What is the energy of this photon?
    3. What is its frequency?
    4. What is its wavelength?
    5. What type of electromagnetic radiation is this?

    1. Here's the reaction in symbolic form.

      00γ → 0−1e + 0+1e

    2. A positron has the same mass as an electron. Only the charge is different. Calculate the energy equivalent to the mass of two electrons using Einstein's famous equation.

      E = mc2
      E = 2(9.11 × 10−31 kg)(3.00 × 108 m/s)2
      E = 1.64 × 10−13 J

    3. Use Planck's equation to determine the frequency of the photon.

      f = 
      E
      h
      f =  1.64 × 10−13 J
      6.63 × 10−34 J s
      f =  2.47 × 1020 Hz  
       

    4. Planck's equation also comes in a version that uses wavelength instead of frequency. That's one way to solve this part of the problem.

      λ = 
      hc
      E
      λ =  (6.63 × 10−34 J s)(3.00 × 108 m/s)
      1.64 × 10−13 J
      λ =  1.21 × 10−12 m  
       

    5. Most reference sources call this a gamma ray photon.

homework

  1. none