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Homework Help: Electricity and magnetism confusion!

  1. Dec 29, 2012 #1
    1. The problem statement, all variables and given/known data
    I've been set an assignment and I'm struggling with one question in particular:

    2. Relevant equations

    'Use the following data and plot the graphs for a point charge'

    'Plot graph of E against r^2. Interpret the graph to describe electric field strength and electric potential.'

    The figures provided are r(m)=Distance from a point charge, and, V(V)=Voltage at a distance 'r' from a point charge.

    3. The attempt at a solution
    More than anything I'm just really stuck how to find E from the figures provided? Any help would be greatly appreciated! Thanks
  2. jcsd
  3. Dec 29, 2012 #2
    The filed at r=ri wil be:
    E = -dV/dr ≈ -ΔVi/Δri
    where ΔV = V(ri+1)-V(ri) and Δri = ri+1-ri
  4. Dec 29, 2012 #3


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    Science Advisor

    I don't see how it would be possible to answer this without knowing what data you are given!
  5. Dec 29, 2012 #4
    Apologies, the date given is as follows:

    R(m)=Distance from point charge:


    And corresponding respectively, the V(V)=VOltage at a distance 'r' from a point charge:

  6. Dec 29, 2012 #5


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    Staff: Mentor

    You're told that the given data corresponds to potential U at distances r from a point charge. What expressions do you know that correspond to electric potential U and electric field E at distance r from a point charge?
  7. Dec 30, 2012 #6
    Ok, so from equations to give V and E, I've come up with this as a potential solution for the first E value from the first two values of distance and voltage. Is this correct?
  8. Dec 30, 2012 #7


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    Staff: Mentor

    No need to plug in numerical values yet; you have a whole table full of values to plot later. Just write the expressions for potential and electric field for a point charge. Solve the potential expression for Q and substitute it into the field expression. That'll leave you with an expression for E involving only V and r. You can plot that since you have a table full of values for V and r.

    (as an aside, note that electric field strengths are given in V/m or N/C, not Teslas which are a measure of magnetic field strength)
  9. Dec 30, 2012 #8
    Brilliant, piece of cake. Many thanks to those who have helped!
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