Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electron in electric field

  1. Jul 30, 2010 #1
    1. The problem statement, all variables and given/known data
    a cathode ray tube contains parallel plates that are 30mm long and have an electric field strenght between them of 2.5x10^4. an electron travelling at 2.9x10^7m/s enters the field at right angles. explain why the electron experiences a change in velocity.


    2. Relevant equations



    3. The attempt at a solution
    im thinking that only the direction of the electron changes but the speed stays the same. so the velocity changes in terms of direction. is that right?
     
    Last edited: Jul 30, 2010
  2. jcsd
  3. Jul 30, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    Do you have any idea of what principles you might need to apply?

    You need to show some attempt before anyone at PF can help you.
     
  4. Jul 30, 2010 #3
    okay i did make an attempt but im not really sure about the principles i need to apply. i think only the direction changes but not the speed. but im not sure why or if its true.
     
  5. Jul 30, 2010 #4

    rock.freak667

    User Avatar
    Homework Helper

    Well firstly what will cause a change in velocity?
     
  6. Jul 30, 2010 #5
    probably the +ve plate attracting the electron
     
  7. Jul 31, 2010 #6

    rock.freak667

    User Avatar
    Homework Helper

    Which causes what type of force to be exerted on the electric charge?
     
  8. Jul 31, 2010 #7
    electric force.i think thats what its called?
     
  9. Jul 31, 2010 #8
    Listen no doubt the direction of velocity changes, but the electron also gathers a certain velocity in the upward direction towards the positive plate.And this velocity increases because of the constant acceleration the electron experiences.So now the net velocity of the electron is the resultant of both the initial horizontal component as well as the perpendicular component both in magnitude and direction.Got it??
     
  10. Jul 31, 2010 #9
    yeah i get your last sentence but i thought the acceleration increased and that it wasn't constant.
     
  11. Aug 1, 2010 #10

    rock.freak667

    User Avatar
    Homework Helper

    Before the electron enters the electric field, it is in a gravitational field. So the effects of gravity are quite small. But as it enters the electric field, it causes an acceleration which affects the vertical component of its velocity.

    F=ma=EQ.

    So its horizontal velocity will stay the same, but the vertical velocity changes with time.
     
  12. Aug 4, 2010 #11
    oh ok yh thanks :)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook