# Coloumbs law question (checking answer)

1. Feb 25, 2016

### romakarol

1. The problem statement, all variables and given/known data
Two spherical objects are separated by a distance of 2.5 x 10-3 m. The objects are initially electrically neutral and are very small compared to the distance between them. Each object acquires the same negative charge due to the addition of electrons. As a result each object experiences an electrostatic force that has a magnitude of 5.75 x 10-3 N. How many electrons did it take to produce the charge on one of the objects.

2. Relevant equations
F=(k)[(q1xq2)/r2] coloumbs law

F=electrostatic force
r=distance
k=8.99x10^9
q1/q1=charge on each object

3. The attempt at a solution

d=2.5x10^-3 metres (given)
F=5.75x10^-3 (given)
q1/q2=to be found

I changed moved the formula to leave q1/q2 on one side, giving me:
r^2(F/k)=(q1)(q2)

with values plugged in it boiled down to:
4X10^-18=q1(q2)

Since q1 and q2 have the same charge, I figured it'd be fair to say
4x10^-18=q^2

therefore
q=square root of (4x10^-18)
q=0.000000002 Coloumbs

then converted coloumbs to number of electrons giving me and answer of:
1.25x10^10 electrons

This is my first attempt of a question on this semester and I'm doing them as I'm going over the lecture notes, did I get it right?

2. Feb 25, 2016

### Staff: Mentor

Your result looks good. One might quibble about the significant figures, since your given distance value has only two and you've specified your result to three, but otherwise you've done very well.