# Homework Help: Percent change in mass when gaining charge.

1. Aug 28, 2010

### Cllzzrd

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

When an object such as a plastic comb is charged by rubbing it with a cloth, the net charge is typically a few microcoulombs.

If that charge is 3.6*10^-6 C, by what percentage does the mass of a 39g comb change during charging?

2. Relevant equations

n/a?

3. The attempt at a solution

Ok here it goes...

total charge/electron charge=number of electrons
3.6*10^-6 / 1.60*10^-19 =2.47*10^13 Electrons

number of electrons* mass of electron at rest= change in mass
2.47*10^13 * 9.11*10^-31=2.05*10^-17g

change in mass/mass of comb= %change
2.05*10^-17 / .39 = 5.25*10^-17%

It says this is wrong, but I don't know of any other way to work this problem.

2. Aug 28, 2010

### Staff: Mentor

Check comb mass. And don't forget to give answer with correct number of significant figures.

3. Aug 28, 2010

### ehild

Check also the electron mass.

ehild

4. Aug 28, 2010

### Staff: Mentor

More precisely - check your units...

5. Aug 29, 2010

### Cllzzrd

Ok... checking units now...

I will just do unit calculations without the numbers... then substitute the numbers back in later if I am correct.

C*(e/C)*(kg/e)= kilograms gained... right?

When I calculate this using the numbers, I still get the same, incorrect answer.

6. Aug 30, 2010

### ehild

You have written for the mass of the electrons:

2.47*10^13 * 9.11*10^-31=2.05*10^-17g

Is the mass of one electron 9.11*10^-31 g?

ehild

7. Aug 30, 2010

### Cllzzrd

I thought it was 9.11*10^-31Kg... If that is not it, what is? Is that where my calculations are going wrong?

8. Aug 30, 2010

### Staff: Mentor

g or kg? If kg - why do you list mass in grams?

39 g - how many kg?

Last edited by a moderator: Aug 13, 2013