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Homework Help: The integral

  1. Sep 25, 2009 #1
    1. The problem statement, all variables and given/known data

    I am soooo lost. I don't even know if this is the right forum...... But where is the bridge between Calculus and Physics? I can Integrate equations, but when it comes to physics, i for one, don't know when to integrate; two, i don't see how you find the constants to remove from the integral; and three, Even given the integral formula for an equation, i still don't know what im doing. ?? finding the electric field of an object?? i thought Electric field was (1/4pi*epsilon naught)(Q/r^2). So how do i find the E field for different shapes?

    another example...

    ex. Va-Vb=SE.dl

    E-Electric Field
    dl-small segments of length

    I don't know how to use the equation;

    Or, electric flux,

    Flux=SE . dA

    What am i not understanding. Please help

    2. Relevant equations

    Flux=SE . dA


    3. The attempt at a solution
  2. jcsd
  3. Sep 27, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Josh930! Welcome to PF! :smile:

    (have an integral: ∫ and a pi: and an epsilon: ε and try using the X2 and X2 tags just above the Reply box :wink:)
    Some physical quantities are A times B, or (vector) A dot B or A cross B …

    for example, work done = force times distance …

    if A and B are constant, then you just multiply, but if one or both is varying, then you have to integrate, eg: ∫A dB or ∫A.dB
    Do you mean the constant of integration? You choose it to fit the initial (or boundary) condition: eg, you might choose potential energy to be zero at infinite distance.
    To find E at position x for different shapes, basically you integrate ∫∫∫ Q(r - x)d3r/4πε0(r - x)3

    Va - Vb = ∫E.dl is the work-energy theorem: the LHS is the increase in PE, and the RHS is the work done … if E varies, then you need to integrate.

    And yes, electric flux = ∫ E.dA … what is worrying you about that? :smile:
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