# Homework Help: Electron and positron

1. Jul 28, 2009

### imafam

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
HELP!!

2. Jul 28, 2009

### alphysicist

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?

3. Jul 28, 2009

### imafam

That Fnet = mv^2/r

4. Jul 28, 2009

### alphysicist

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.