# Homework Help: Electrostatics quick question help

1. Jul 28, 2008

1. The problem statement, all variables and given/known data
I have a series of about a dozen questions, my son is taking a course that was outlined by his school " as a great way to get ready for Grade 12" it is mostly multiple choice so if someone could help us choose the right answer with a little bit of an explanation. He has done all the questions himself i just need help marking them.

Question 1.
One of the ways in which Newton's law of universal gravitation differs from Coulomb's law is that gravitational force can only attract, whereas the electric force can only repel.
True Or False

2. Relevant equations

3. The attempt at a solution
I think it is True because no where does it state in electrostatics that the electric force Can repel.

2. Jul 28, 2008

### nicksauce

Well look at Coulomb's law: F = k*q1q2/r^2. Note F ~ q1q2. Charges can be either positive (like protons) or negative (like electrons).
(Edited for clarity). If q1 and q2 are of the opposite sign then F is negative and attractive, whereas if q1 and q2 are of the same sign then F is positive and repulsive.

Now compare to the Gravitational force: F ~ m1m2. Since mass is only positive, F can only have one sign, and so it is always attractive.