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Homework Help: Help with this problem due tonight

  1. Jun 16, 2004 #1
    Help with this problem due tonight!!!

    Find the direction and magnitude of the net electrostatic for exerted on the point charge q2. There are 4 points of a square. q1 has a charge of -1.6E-6, q2 has a charge of -3.2E-6, q3 has a charge of -4.8E-6, and q4 has a charge of -6.4E-6. The distance between each of the four points around the square is 0.45m. Can anyone show me how to do this, I need to turn it in tonight, I would greatly appreciate it. Thank you.
  2. jcsd
  3. Jun 16, 2004 #2


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    [tex]\vec{F}_2 = \vec{F}_{12} + \vec{F}_{32} + \vec{F}_{42}[/tex]

    [tex]\vec{F}_2 = \frac{q_2}{4 \pi \epsilon _o} \sum _{k=1,3,4}\frac{q_k}{|\vec{r}_{k2}|^2}\hat{r}_{k2}[/tex]

    Where [itex]\vec{r}_{k2}[/itex] is the vector from the point where [itex]q_k[/itex] is located to the point where [itex]q_2[/itex] is located.
  4. Jun 16, 2004 #3
    could you explain that a little more, that would really be helpful
  5. Jun 16, 2004 #4


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    Is it the vector notation that you're having difficulty with?

    A vector has a magnitude and direction; the little arrow says 'this thing is a vector quantity'; the subscripts tell you something about the vector (per AKG's definition). The first equation is an addition of vectors, which general topic you will have learned when they were introduced in class.

    Or maybe you haven't been taught vectors yet?
  6. Jun 16, 2004 #5
    supermenscher, a question of this type has undoubtably been presented as an example. Look to that example for help. Additionally, I'm sure a number of questions just like this have already been asked, if you'd look through some older threads.

    If part of the question is still giving you trouble, ask about that part and show your work.

  7. Jun 16, 2004 #6


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    Perhaps it would be best if you asked about the parts you don't understand. [itex]\frac{1}{4 \pi \epsilon _o} \approx 9.0 \times 10^9[/tex]. You may be used to the number on the right, so if that's what's confusing you, hopefully it's clear now. You can find out more on [itex]\epsilon _o[/itex] here. Now, you should know something like this:

    [tex]F = (9.0 \times 10^9)\frac{q_1q_2}{d^2}[/tex]

    That's the magnitude of the electrical force between two charges, [itex]q_1[/itex] and [itex]q_2[/itex] separated by a distance of [itex]d[/itex]. And you can tell if the force is attractive or repulsive depending on the signs of the charges. Do you understand that? If not, then I'm stumped as to why they've given you this question. If so, then please tell us where you're confused.
  8. Jun 16, 2004 #7
    What I would really like is for someone to show me step by step how to do the problem...that would really help me out and allow me to learn the material better
  9. Jun 17, 2004 #8
    I'm not trying to be mean, but that's what your textbook is for.

    Which part of the question is confusing you? You've been given three explanations of how to approach the problem, but you've apparently made no further attempt at it. You'll find that if you're unwilling to put some effort into the problem, others won't be terribly willing to put effort into helping you either.

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