Percent change in mass when gaining charge.

[Cllzzrd]

Homework Statement

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?

Homework Equations

n/a?

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.

 

[Borek]

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

 

[ehild]

Check also the electron mass.

ehild

 

[Borek]

More precisely - check your units...

 

[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.

 

[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

 

[Cllzzrd]

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

 

[Borek]

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

39 g - how many kg?