# Need some hints for a Coulomb's Law question!

## Homework Statement

Suppose that resting on your non-conducting desktop is a tiny piece of paper(m=0.08g) that carries a charge of +4 x 10-10C. You comb your hair and then slowly lower your comb until when it is 3 cm away from the paper, the paper jumps up to the comb.
a) Give an approximate value for the charge on the comb.
b) Does the comb carry an excess of electrons or protons? How many?

## Homework Equations

Coulomb's Law:
F=(1/4*pi*E0)*(|Q1Q2|/r2)
Magnitude of charge of electron and proton: e=1.60217 x 10-19
Q=ne

## The Attempt at a Solution

I know that we have to consider w=mg since there is mass involved for the paper. I tried to use Coulomb's Law to find the charge, but I don't know what the Force is between the two, so how can I find charge for the comb? I've converted the given numbers to the right units which are 0.08g = 0.8 x 10-4 and 3cm = 0.03m.
Now, my question is, can anyone give me a hint on how to start this question?

gneill
Mentor
What force is required to lift a piece of paper that has mass 0.08 grams?

What force is required to lift a piece of paper that has mass 0.08 grams?

That would be w=mg! But can I use this force as the force between the two objects? If so, I will then be able to solve for the unknown charge.

Am I on the right track?

gneill
Mentor
That would be w=mg! But can I use this force as the force between the two objects? If so, I will then be able to solve for the unknown charge.

Am I on the right track?

Yes, yes you are!

Yes, yes you are!

Thanks so much for the hint! Just one small question, for calculating the number of electrons(excess electrons of comb);I do Q=ne which is:

1.96x10-7C = n*1.60x10-19C
and I will get e = 1.23x1012. $$\leftarrow$$This is the number of electrons right?

Thanks again!

gneill
Mentor
Your unknown charge looks to be a bit large (by several orders of magnitude). Maybe you can check or show your calculation.

Your method for finding the number of electrons transferred looks fine.

Hey, just had to come back here to say thanks again. I asked my teacher the same question, and it turned out right!

Thanks a bunch.