Percent change in mass when gaining charge.

AI Thread Summary
The discussion revolves around calculating the percentage change in mass of a charged plastic comb. The charge of the comb is given as 3.6*10^-6 C, and the mass change due to the gained electrons is calculated to be 2.05*10^-17 g. However, the initial calculations are questioned due to potential unit errors, particularly regarding the mass of an electron, which should be in kilograms rather than grams. Participants emphasize the importance of using consistent units throughout the calculations to arrive at the correct percentage change. The conversation highlights the need for clarity in unit conversions when dealing with physical quantities.
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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.
 
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Check comb mass. And don't forget to give answer with correct number of significant figures.
 
Check also the electron mass.

ehild
 
More precisely - check your units...
 
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.
 
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
 
I thought it was 9.11*10^-31Kg... If that is not it, what is? Is that where my calculations are going wrong?
 
g or kg? If kg - why do you list mass in grams?

39 g - how many kg?
 
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