The Math Forum

Ask Dr. Math - Questions and Answers from our Archives
Associated Topics || Dr. Math Home || Search Dr. Math

Coulomb's Law and Particle Repulsion

Date: 09/26/2004 at 16:43:42
From: Carlos
Subject: Calculate the force of repulsion between two electrons

Two electrons are located 0.08 picometers from each other.  Calculate
the force of repulsion between them.  I do not know the equation and
do not know how to do this problem.

Date: 09/28/2004 at 10:21:00
From: Doctor Ash
Subject: Re: Calculate the force of repulsion between two electrons

Hi Carlos. 

In order to solve this problem, you have to use Coulomb's Law:
  F = (K*q*q')/(d^2)


  F   is the force of attraction or repulsion
  q   is the charge on the one particle
  q'  is the charge on the other particle
  d   is the distance between the charged particles
  K   is a constant, approximately equal to 9*10^9 (N*m^2/C^2)

Note that it doesn't matter which electron has charge q and which has
charge q', since multiplication is commutative. 

The units should be as follows in order to comply with the unit of K:

  q and q'   should be measured in C (Coulumbs)
     d       should be measured in m (meters)
     F       should be measured in N (Newtons)

So, to solve the above question, first convert the units into the 
appropriate ones, i.e., 

  1 picometers (pm) = 10^-12 m

Then substitute in the equation to find the magnitude of the electric
force between the electrons. 

You now have K and d. What are q and q'? The charge on one electron is 
constant and is equal to -1.6 x 10^-19 C.  Thus, 

  q = q' = -1.6 x 10^-19 C

Now you have all the values on the right side, so you can compute the
force.  The force will have a positive value showing that it is a
repulsive force.  (Attractive force is negative by convention).  I 
hope this helps, and if you have any further questions, don't hesitate 
to ask.

- Doctor Ash, The Math Forum 
Associated Topics:
High School Physics/Chemistry

Search the Dr. Math Library:

Find items containing (put spaces between keywords):
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

Math Forum Home || Math Library || Quick Reference || Math Forum Search

Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.