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Electron and positron

  1. Jul 28, 2009 #1
    A positron is an elementary particle identical to an electron except that its charge is . An electron and a positron can rotate about their center of mass as if they were a dumbbell connected by a massless rod. What is the orbital frequency for an electron and a positron 2.00nm apart?

    My Approach:
    I treated the electron and positron as a system with a torque.
    I found that the torque = pEsintheta = qsEsintheta = (1.60*10^-19)(2*10^-9)(1.94*10^9)
    but I don't know theta
  2. jcsd
  3. Jul 28, 2009 #2


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    Homework Helper

    Hi imafam,

    This is the formula for the torque on a dipole in an external field E, which is not what you have here.

    Instead, think in terms of Newton's law. Draw a force diagram for one of the particles. Also, the particles are going in a circular path; what do you know that is true about circular motion, that you can use here in Newton's law Fnet=ma?
  4. Jul 28, 2009 #3
    That Fnet = mv^2/r
  5. Jul 28, 2009 #4


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    That's right; once you plug in what Fnet is for this case, you'll have an equation with r and v in it.

    From that, you can determine the period of the orbit, which is just how long it takes for the particle to go in a complete circle. And then the frequency is just the reciprocal of the period.
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