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Electron launched at an angle in a parallel plate field.

  1. Jan 12, 2014 #1
    1. The problem statement, all variables and given/known data

    http://puu.sh/6i6HY.png [Broken]

    2. Relevant equations

    Fe = q*Ee

    3. The attempt at a solution

    a)
    dx = vx*t
    0.0682 = 4.92*106 * cos(68) * t
    t = 0.0682 / [4.92*106 * cos(68)]
    = 3.70036*10-8 seconds

    dy = dyi + vy*t + at2
    0 = 0 + 4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) + a * [0.0682 / [4.92*106 * cos(68)]]2
    a = -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2
    = -1.23278 * 1014 m/s2

    mass of electron = 9.10938291 * 10-31 kg
    charge of electron = 1.60217657 * 10-19 C

    Fe = ma
    = 9.10938291 * 10-31 * [ -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2 ]

    Ee * q = Fe
    Ee * 1.60217657 * 10-19 = 9.10938291 * 10-31 * [ -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2 ]
    Ee = 9.10938291 * 10-31 * [ -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2 ] / (1.60217657 * 10-19)


    I got part b correct, so I must have the correct acceleration - now I can't figure out where I'm going wrong.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 12, 2014 #2

    Simon Bridge

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    Check your reasoning ...
    Also: Remember your ballistics?

    If you have the acceleration right, then E=a(m/q)
    If that gives the wrong E then (a) you have used a bad value for q/m for an electron - maybe a rounding error? or (b) you didn;t get the right acceleration and the correctness of the answer to part b is a fluke.

    Best practice is to do all the algebra before putting the numbers in.
     
    Last edited: Jan 12, 2014
  4. Jan 12, 2014 #3

    ehild

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    The time of flight is correct if you meant in in seconds.


    Is it the same d as before, when you wrote that d=vt?


    The electric field is normal to the capacitor plates. What is the direction of the acceleration? Does the electron accelerate horizontally?

    ehild
     
    Last edited by a moderator: May 6, 2017
  5. Jan 12, 2014 #4
    No I used horizontal velocity and distance to find time, and then used vertical velocity, distance and time to find acceleration. I'll add subscripts to the OP to make that more clear

    I took the electron values to many more decimals than the homework system checks (3).

    I changed all my calculations to not use any previously calculated values and reformatted the exponents. Do you see any specific incorrect logic or calculation now?
     
  6. Jan 12, 2014 #5

    ehild

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    The equation

    dy = dyi + vy*t + at2

    is wrong. Check.

    ehild
     
  7. Jan 12, 2014 #6
    Oh man, messed up that equation after using it hundreds of times. Thank you very much, that fixed it.

    (and I forget the 1/2 both times and the mistakes cancelled out which is why I got the right answer for part b :rolleyes:)
     
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