Question

Exercise 11This exercise explores the concept of photon momentum, but of course if all I did was ask you to find values for various photon energies, you would have a stack of numbers but no idea about what they meant. A photon’s momentum is small, say, compared to a freight train. But what about compared to other subatomic particles? Let’s compare the momentum of a photon to an electron whose KE is the same as the photon’s.Fake Part A) Calculate the momentum for a photon and electron of energy 10.0eV. Solution (yes, I am doing this for you – which is why it’s Fake Part A): Using the more convenient form for momentum of a photon p = E/c, and converting eV to Joules, I get 5.34E-27 kgm/s. For an electron of KE 10.0eV (do you need gamma or not?) I get 1.71E-24kgm/s.B) Repeat (fake) Part A for 1000.eV (100. times more, and 3 sig figs is fine.) NOTE which of these momentum values increased MORE. You might see how, as energy increases, they might eventually cross, or at least agree. (Partial ans: 1.71E-23kgm/s or 1.708E-23kgm/s)C) Now repeat part A for a KE of 5.11E6eV. Note that is 10× the electron’s rest energy. Can’t use ½mv2 for this. And that is the KE of the electron, not the total energy. To find the electron momentum you can either use γmv OR use E2 = p2c2 + m2c4 (in which E is the total energy IN JOULES) and solve for p. The photon, of course, uses the same old energy.D) In graduate school I worked on a detector for a high energy experiment in which electrons collided with positrons. The energy of each was 14.5E9 eV. That is a gamma factor of above 28,000. These electrons were moving at 6 cm/s LESS than the speed of light. Why would you expect in this case that these particles should have essentially the same momentum value as a photon of the same energy?

Answer 1