1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Current in an Ionic Solution

  1. Feb 1, 2008 #1
    1. The problem statement, all variables and given/known data

    Current passes through a solution of sodium chloride. In 1.00 second, 2.68 x 10^16 Na+ ions arrive at the negative electrode and 3.92 x 10^16 Cl- ions arrive at the positive electrode.

    What is the current passing between the electrodes?
    Express your answer in milliamperes to three significant figures.

    Part B

    What is the direction of the current?
    ~away from the negative electrode or
    ~toward the negative electrode

    2. Relevant equations

    I = Q/t

    3. The attempt at a solution

    the difference between 2.68x10^16 and 3.92x10^16 as a Na+ ion and Cl- ion carry precisely one electric charge each, and the charge of an electron is 1.60 x 10^-19 C

    I = Q/t
    I = (1.60 x 10^-19 )/1
    I = 1.60 x 10^-19 A = 1.60 x 10^-16 mA

    am I right?

    Part B

    Conventional current was defined early in the history of electrical science as a flow of positive charge, so it would be towards the negative electrode?
  2. jcsd
  3. Feb 1, 2008 #2
    So for part A, the net positive charge would be q = Ne = (Cl - Na)e = (1.24x10^16)(1.6x10^-19) = 1.98 x 10^-3 and when I convert it to mA, I get 1.984, but it says it's wrong. Any ideas?
  4. Feb 2, 2008 #3
    how did u do dat ???? im really confused with dis question
  5. Feb 2, 2008 #4


    User Avatar
    Homework Helper
    Gold Member

    You are correct with the direction of the current. Conventional current would flow toward the negative electrode.

    For part a) I also get 1.984mA. You have said you want the answer to be in three significant figures, but 1.984mA has 4 significant figures. Could this be why it is being labeled as wrong? (I assume this is a computer based question?)
  6. Feb 2, 2008 #5
    I figured out the problem. I wasnt supposed to subtract the number of Na+ ions from the Cl- ions, I was supposed to add them together, and I would get 6.6 x 10^16 which I would multiply it by the charge of an electron, and once I convert it to mA, I get 10.6mA.

    If the problem was too many significant digits, masteringphysics (the computer based website problem) would still give me the marks saying I just rounded it differently.

    Thanks for everyone's help.
  7. Feb 2, 2008 #6


    User Avatar
    Homework Helper
    Gold Member

    AHHH!! Yes, I made the same mistake, don't feel too bad about it! Sorry if my mistake mislead you in any way.
  8. Feb 2, 2008 #7
    What quality must the charge density on the surface of a conducting wire possess if an electric field is to act on the negatively charged electrons inside the wire?

    The charge density must be

    can anybody plz help with dis..i think its uniform but im not sure
  9. Feb 2, 2008 #8
    It's okay; I got the answer before you posted, and I'll prefer to do this on an assigment worth about 1% than a test any day

    and ravi1611, the answer is nonuniform
    Last edited: Feb 2, 2008
  10. Feb 3, 2008 #9
    thx alot
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook