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Homework Help: What is the electric potential

  1. Feb 4, 2008 #1
    i have 2 questions for electromagnitism and i just cant do them. my lecturer is just not up to the job of teaching and everyone is struggling and its probably something easy. anyway rant over.

    1) you have an equlateral trinagle with chare q in 2 corners. what charge would you have to put into the third corner so the centre has
    a)0 electric field
    b)0 electric potential

    in part a) what is the electric potential
    in part b) what is the electic field

    for this i have just been plating with trigonomatry and it just seems to longwinded for an answer that just doesnt seem right.

    2) state how electric field can be calculated by electric potential.
    the electric potential at derpendicular distance r from a long straight wire with cross section a is

    v(r)=-k ln (r/a)

    k is constant. calculate electric field as a function of r.

    i know this is to do with intergrating or differentiating but not sure which.

    if someone would be so kind to answer these and also give some details on elecromagnitism as a whole that would be amazing.

  2. jcsd
  3. Feb 5, 2008 #2

    1. The problem statement, all variables and given/known data
    there are 2 questions to this.
    1) you have a equlateral triangle with 2 charged particles q in the corners. by choseing the third particle make the centre of the triangle have 0
    in case a what is the potential in case b what is the field.

    2. Relevant equations
    my lecturer isnt the best in the world but i think these are the formulae
    some trig?

    3. The attempt at a solution

    i have just been playing around with trig to get the r value then doubling it as there are 2 charge q then thring to exuate it to a mystery charge.

    thanks for any insite im going off to get a book to try and help me along with this as my lecturer is just not worth while
  4. Feb 5, 2008 #3


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    Let's start with question (a). Can you write an expression for the electric field at the centre of the triangle in terms of their charges and position?
  5. Feb 5, 2008 #4


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    The electric field is a vector field and the total vector field is found by vector addition. Find the fields magnitude and direction due to the first two charges and work out what charge you will need to cancel it out. Electric potential due to a load of point charges is just the sum due to the individual point charges.

    For part 2, remember that a conservative vector field is the gradient of a scalar potential function.
  6. Feb 5, 2008 #5


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    I merged the two threads, based on thread titles and problem statment.
  7. Feb 5, 2008 #6
    ok i think for a

    the charge will be q due to symetry
    the distance r between to corner and centre is the same so if the extra one has the same charge it will just all cancal out.


    rearanged and simplifed

    now with the extra charges the rest should fall into place.
  8. Feb 5, 2008 #7


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    Yes, looks ok to me.
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