1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find the electric force on a point charge

  1. Feb 8, 2016 #1
    1. The problem statement, all variables and given/known data
    A point charge q1 = 4.40 μC is at the origin and a point charge q2 = 6.00 μC is on the x axis at x = 2.49 m.
    i) Find the electric force on charge q2
    ii) Find the electric force on charge q1

    2. Relevant equations
    Coulomb's law F = k (q1q2)/d^2

    3. The attempt at a solution
    Part i)
    Using k = 8.99 x 10^9, q1 = 4.40 x 10^-6, q2 = 6.00 x 10^-6 and d = 2.49m
    Plug these values into the equation

    F = (8.99 x10^9) x (4.40 x 10^-6) x (6.00 x 10^-6)/ (2.49)^2
    F = 3.83 x 10^-2 N

    Checked this value a couple of times and seems correct.

    Part ii)
    Since we are looking for the force from q2 to q1, it is the same magnitude but opposite in direction.

    F = -3.83 x 10^-2 N

    I'm a little unsure of this, but if feels correct. If I'm wrong, any guidance welcome.
     
  2. jcsd
  3. Feb 8, 2016 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Your work looks correct. But I wouldn't use a minus sign to indicate the direction of the force in part (ii). If you want to specify the directions of each force, then it would be better to use a descriptive phrase such as "toward the other charge" or "away from the other charge".

    [If you are dealing with components of a force, such as the x-component or the y-component, then you could have a negative component. But this would require having a clearly stated coordinate system in which the directions of positive x and positive y are known.]
     
  4. Feb 8, 2016 #3
    Thanks for your reply.

    "A point charge q1 = 4.40 μC is at the origin and a point charge q2 = 6.00 μC is on the x axis at x = 2.49 m. "
    From the question(extract above) it specifies the forces along the x-axis, so I would take these to be the x-components of the forces.

    Therefore should I quote the minus sign for the answer to part ii) ?
     
  5. Feb 8, 2016 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    In this case I would give the direction of the force in (i) as "toward the positive x direction",
    or I would state the answer as Fx = 3.83 x 10-2 N, Fy = 0. (Also, Fz = 0 if you usually work in 3 dimensions of space.)

    Similarly for (ii) I would give the direction of the force as "toward the negative x direction",
    or I would state the answer as Fx = -3.83 x 10-2 N, Fy =0. But I would still not state the answer as F = -3.83 x 10-2 N.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted