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← 08ps-07 Cathode Ray Tube

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Showing Revision 1 created 10/27/2012 by Amara Bot.

  1. For this problem, we're going to be talking about cathode ray tubes.
  2. And it may or may not be a name that means something to you depending on how old you are.
  3. Cathode ray tubes used to be main way we got televisions to work
  4. before we had LED, LCD, and plasma televisions.
  5. And let's investigate how exactly these tubes work.
  6. So on one end of the tube we have an electron here, and we give it some really large
  7. initial velocity V₀ such that it shoots across this tube towards this green screen over here.
  8. And when this electron hits the screen over here,
  9. it temporary causes it to light up producing a dot on the screen.
  10. Now you can imagine if we were able to draw lots of these dots on the screen
  11. we could use it to draw images like this one here.
  12. Well in order for us to be able to make dots on other parts of the screen,
  13. we have to bend the path of the electron so it doesn't just go on a straight line.
  14. And how can we bend the path of an electron? Well one easy way is with an electric field.
  15. And we saw on the previous two questions that a great way to produce a nice
  16. constant magnetic field is to have two parallel plates like these two.
  17. When this negatively charged electron moves into this electric field,
  18. it'll get pulled towards the positive plate directing it to this part of the screen.
  19. And you could imagine if we wanted the electron to go down here,
  20. we just switch the charges so the negative charges were up here
  21. and the positive charges were down here.
  22. This means we can control what part of the screen the electron hits and therefore
  23. lights up just by changing the strength and direction of this electric field here.
  24. For this problem you're going to tell me how strong I should make this
  25. electric field such that this electron gets bent a distance away from the center
  26. from its straight line path of Δy.
  27. You should assume that the electron is in the magnetic field
  28. for a total distance of 0.5 m and we want the deviation away
  29. from the center of Δy to be equal to 0.1 m.
  30. We'll also assume that the electron enters the field with an initial velocity of 1x10⁷ m/s.
  31. You will also need the charge and mass of the electron, which I have provided here.
  32. Now a few quick notes. You'll notice that the speed here is really large
  33. and the super student among you might notice that it's actually getting close
  34. to being the speed of light.
  35. If you heard anything about the theory of relativity, you might know that as we get
  36. close to the speed of light strange things start to happen.
  37. We won't worry about any of that in this problem now.
  38. Just assume that everything behaves as we're used to.
  39. Also note that even though I've drawn my field line so the arrows are pointing down
  40. the electron is going to bend up because it has a negative charge
  41. and we always draw the field lines in the direction a positive charge will move.
  42. All right--so you can put your answer for the strength of the electric field
  43. right here and put it in units of N/C--all right, good luck.