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Basic Electric Charge Help Needed

  • 05-07-2013 10:47am
    #1
    Username99
    Closed Accounts Posts: 235 ✭✭


    Hi,

    I'm starting to learn about electric charge, I thought I had a basic idea of how it works but I got stumped on the first question ( See attachment ).

    The answer at the back of the book is: 3,1,2,4 (zero).

    I would have said that the magnitude of 1 and 2 are equal and the magnitude of 3 and 4 are equal.

    My reasoning behind this was that in number 1, for example, you have two negative charges to the left of the central particle and to the right you have a positive and a negative charge.

    So on the left you have -2e and on the right you have 0e. This is obviously the incorrect way of thinking about it though.

    Can anyone help me with this please.


Comments

  • #2
    FISMA
    Closed Accounts Posts: 2,616 ✭✭✭FISMA


    Username99 wrote: »
    So on the left you have -2e and on the right you have 0e. This is obviously the incorrect way of thinking about it though.

    This is a fundamental problem. You're going to use Coulomb's Law which tells us that
    F=kQ1Q2/r²

    Like Newton's Second Law, Coulomb's Law may only be used when you have point particles. It can be used on objects that are small by comparison to the separation distance.

    Since, the Force is inversely proportional to the square of the separation distance, you cannot combine the charges on the left, which is what you did, and what they are trying to test.

    Let's label all of the charges from left to right, 1, 2, 3, and 4. Consider our test charge to be positive.

    I'll do #4, then you try the other three.

    Note that in #4, the outer most charges (1 and 4) are the same. #1 pulls on the test charge at the center to the left and #4 pulls with the same amount of Force to the right. Since they are the same distance from the test charge, the Forces are balanced and cancel out.

    Note that in #4, the inner two charges (2 and 3) are both positive and can only push our positive test charge at the center. #2 pushes to the right, just as hard as #3 pushes to the left. Hence, they cancel out.

    Now go one by one and determine: whether the charge pushes or pulls and if it is a large Force (closer to the center) or a small Force (farther from the center).


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