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Homework Help: Percent change in mass when gaining charge.

  1. Aug 28, 2010 #1
    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. jcsd
  3. Aug 28, 2010 #2

    Borek

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    Staff: Mentor

    Check comb mass. And don't forget to give answer with correct number of significant figures.
     
  4. Aug 28, 2010 #3

    ehild

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    Homework Helper

    Check also the electron mass.

    ehild
     
  5. Aug 28, 2010 #4

    Borek

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    Staff: Mentor

    More precisely - check your units...
     
  6. Aug 29, 2010 #5
    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.
     
  7. Aug 30, 2010 #6

    ehild

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    Homework Helper

    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
     
  8. Aug 30, 2010 #7
    I thought it was 9.11*10^-31Kg... If that is not it, what is? Is that where my calculations are going wrong?
     
  9. Aug 30, 2010 #8

    Borek

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    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
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