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Homework Help: Electric fields and parallel plates

  1. Dec 13, 2005 #1


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    I need help on a few questions.

    1.An alpha particle has a positive charge of 2e and a mass of 6.6 x 10^-27 kg. With what velocity would the particle reach the negative plate of a parallel plate apparatus with a potential difference of 2.0 x 10^3 V.
    a)if it started from rest at the positive plate.
    b)if it started from rest at a point halfway between the plates.

    I got the answer for a, it 4.4 x 10^5 m/s. But i dont know how to do b.

    2. An electron is accelerated through a uniform electric field of magnitude 2.5 x 10^2 N/C with an initial speed of 1.2 x 10^6 m/s parallel to the electric field.
    a)Calculate the speed of the electron after it has travelled 2.5 cm in the field.

    I dont know how to do this one at all.

    3. Two electrons are fired at 3.5 x 10^6 m/s directly at each other.
    a)Calculate the smallest possible between the two electrons.

    I tried do Ee=Ek but that didnt work i kept getting the wrong answer. So can anyone please help me.
  2. jcsd
  3. Dec 13, 2005 #2


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    Staff Emeritus
    Science Advisor

    Well how did you do part 'a' of question 1? In part b, the acceleration takes place over half the distance. Hint: The electric field is contant between paralle plates.

    2. What is the force on a charge in an electrical field? Get F, and then solve for a. Then what is the equation for final velocity, initial velocity, acceleration and distance.


    3. Each electron has a kinetic energy and approach each other until they stop at some distance, at which point the energy is all electrical potential energy.

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