Time it takes for an electron and proton to collide

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Homework Help Overview

The problem involves calculating the time it takes for an electron and a positron, initially at rest and separated by a distance R, to collide. The context is rooted in electrostatics and dynamics, particularly focusing on the forces acting between charged particles.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the implications of the changing force due to the inverse square law and consider integrating the force to find the time of collision. Questions arise about the applicability of using F=ma given the changing acceleration and the symmetry of the problem.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem and the implications of mass differences on the motion of the particles. Some guidance has been provided regarding the use of reduced mass and integration techniques, but no consensus has been reached on a definitive approach.

Contextual Notes

There is a noted confusion between the particles involved, initially referring to a proton instead of a positron, which has implications for the symmetry of the problem. Participants are also grappling with the effects of changing forces and acceleration on their calculations.

gsingh2011
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Homework Statement


An electron and positron have mass me and separation R and are initially at rest. Find the time T it takes for them to collide.


Homework Equations


F=kq1q2/R2


The Attempt at a Solution


My problem is that the force is not constant. Do I integrate the force then? And from what bounds?
 
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What do you get when you use F=ma?
 
vela said:
What do you get when you use F=ma?

I don't know, acceleration is changing. Let say we have F=kq1q2/r2=ma. Since this problem is symmetrical, I was thinking the two particles would meet in the middle, so each would travel a distance of R/2. So I could divide by m and integrate twice to find the position function, and set that equal to R/2. The only problem with that idea is the force is changing with respect to r, so I can't integrate with respect to t. So I guess that method won't work.
 
gsingh2011 said:
I don't know, acceleration is changing. Let say we have F=kq1q2/r2=ma. Since this problem is symmetrical, I was thinking the two particles would meet in the middle, so each would travel a distance of R/2.
The difference in masses breaks the symmetry. The proton won't move as far as the electron because it's so massive in comparison.
So I could divide by m and integrate twice to find the position function, and set that equal to R/2. The only problem with that idea is the force is changing with respect to r, so I can't integrate with respect to t. So I guess that method won't work.
The standard trick is to multiply by [itex]\dot{r}[/itex]. You'll find it makes both sides of the equation integrable, but first you need to find the correct equation.

Are you familiar with the concept of the reduced mass?
 
Last edited:
The proton won't move as far as the electron because it's so massive in comparison.
It's a positron not a proton, so the mass is the same.
 
Oops, not sure why I thought it was a proton. Yes, you're right, they'll meet in the middle.
 

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