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Homework Help: F=ma prob with electrons (They don't even act like particles )

  1. Oct 6, 2007 #1
    F=ma prob with electrons (They don't even act like particles!!!)

    Well here's the problem:

    1. Electrons in an oscilloscope are accelerated from rest by a constant force of 8.0*10^16 N over a distance of 15mm. Find (a) the acceleration and (b) the final velocity.

    Relevant equations:
    2. F=ma, v^2+u^2=2as, v=ut+1/2at^2

    At any rate it's easy to find the acceleration of 1 electron, but I don't know how to obtain the mass of all of them. I tried to find the force needed to displace a mass of 1 electron (9.1093897*10^-31 kg) by 15mm and then simply cross multiplying the forces and accelerations, but since I don't know the time (since I don't know the speed of the electron) I'm flumoxed!

    3. The attempt at a solution:
    F=ma => a= (8*10^16 N)/m(total)

    a= (8*10^16 N)/(9.1093897*10^-31)x ;where is x =total no. of electrons

    ax = 8.782147641^46 m/s^2

    Again the final velocity is easy to get once I've gotten the acceleration. Any help would be appreciated.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 6, 2007 #2

    Doc Al

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    Staff: Mentor

    Treat each electron separately. Presumably, you are given the force on each electron, so find their acceleration.
  4. Oct 6, 2007 #3
  5. Oct 6, 2007 #4
    Unfortunately, you're only given the force acting on a group of electrons of unknown size.
    Sure if I knew the force acting on each electron it'd be easy!
  6. Oct 6, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor

    You are over-interpreting the phrasing of the problem. It is easy! When they say: "Electons are..." they are talking about each electron. (Just like saying "Electrons have a mass of ....". How many electrons? One!)

    Otherwise you would be correct: The problem would be impossible. A given force is exerted on an unknown mass. Find the acceleration. Sorry, no can do! :wink:
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