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Electric field problem

  1. Sep 13, 2006 #1
    Determine the point( other than infinity) at which the electric field is zero.

    * ---------- *

    One point particle denoted Q1 = -2.50e-3 C and Q2 = 6.00 e-3 C
    the distance of separation is 1.00 m..
    I am not sure how to determine R, s.t. my Electric field is Zero.
  2. jcsd
  3. Sep 13, 2006 #2
    Do you know what R is?
  4. Sep 13, 2006 #3
    We are suppose to determine a distance which will produce an electric field that is zero. I happen to choose R..maybe that was a bad letter to represent a variable.
  5. Sep 13, 2006 #4
    Oh no. R is fine. I asked you what [itex] R [/itex] is, because [itex] R [/itex] should be a scalar (ie the distance) from something. Now [itex] R [/itex] could be the distance from anything, but would probably be the distance from the one of the charges.

    For example if you had this configuration below


    |<----- 1m -------->|

    Then if you wanted the distance from (-q) called R, you would have:

    R<------- (-q)---------------(q)

    Now the distance from (q) would be R+1m

    do you see why it's important?
  6. Apr 30, 2009 #5
    doin this question now myself, honestly doesnt make much sense to me to have an electrice field equal to zero unless we added another charge partical to cancel out.
  7. Apr 30, 2009 #6


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    Gold Member

    There isn't any reason why you would need a third charge here. The electric field scales as 1/r^2. Ignoring the spatial relationship between the two charges, you only need to find two distances such that the field from charge one is equal and opposite to the field of charge two. Keeping in mind that the E field is a vector, it's not hard to arrange the two charges arbitrarily to have a point of null field.

    Now with this problem, the two charges are fixed in relation to eachother. So the first thing is to setup up an equation for the total field using a single vector to represent the distance instead of two. FrogPad shows one such way to do this. Now, you could have a situation where you can't have any nulls, but I think we can assume that it will not be the case. Don't forget that the electric field is a vector, not just a scalar.
  8. Apr 30, 2009 #7
    been trying to use

    k2.5x10^-6/X^2 = k6x10^-6/(X+1)^2

    but not getting the answer the book gives :( seriously these electrical charges and things just aint making sense to me!
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