# Homework Help: Acceleration of 2 charges

1. Sep 17, 2009

### Josh930

1. The problem statement, all variables and given/known data

Two very small 8.55-g spheres, 20.0cm apart from center to center, are charged by adding equal numbers of electrons to each of them.

Disregarding all other forces, how many electrons would you have to add to each sphere so that the two spheres will accelerate at 15.0 g when released?

2. Relevant equations

F=ma
F=k(q1q2/r^2)
q=ne

3. The attempt at a solution

for one.. i don't know why the acceleration is at 15.0 g?

But i did try to set the forces equal and solve q, then plugged q in to the equation q=ne and it wasn't right.

2. Sep 17, 2009

### rl.bhat

3. Sep 17, 2009

### Josh930

F=ma=F=k(q1q2/r^2)

q1=q2; let Q^2=q1q2

sqrt(mar^2/k)=Q

sqrt((8.55g*15.0m/s)((2m)^2)/k)=Q

Q=2.39e-4

Q=ne

n=2.39e-4/1.6e-19

=1.49e15 electrons

4. Sep 17, 2009

### rl.bhat

The acceleration is 15*g i.e. 15*9.8 m/s^2
In the solution you have written (2m)^2. What is that?

5. Sep 17, 2009

### Josh930

they are 20cm apart? so .2m

6. Sep 17, 2009

### Josh930

i messed up and wrote 2m. so 15g is 15x the acceleration of gravity

7. Sep 17, 2009

Yes.