1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Magnitude, 2D co-ordinates and Coulomb's Law

  1. Oct 20, 2014 #1
    1. The problem statement, all variables and given/known data

    How do you calculate compute the the magnitude of the total force of three charges and also the angle it makes with the x-axis? Knowing the magnitude and also the 2d co ordinates of the charges. (x1,y1) (x2,y2) (x3,y3)

    I know for definite I use the below calculation but what exactly is r hat?
    K0 * Q1.Q2 / d^2 * r hat

    there is apparently 2 thetas aswell.

    2. Relevant equations

    K0 * Q1.Q2 / d^2 * r hat

    r hat = cos(theta)i + sin(theta)j
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^I have no idea how to get this with 2D coordinates

    3. The attempt at a solution

    In my attempt at a solution I subbed in K0=8.9875 * 10^9

    I subbed in Q1Q2 (F12)
    I subbed in Q1Q3 (F13)
    I subbed in Q2Q3 (F23)

    Then for each of them I used the x and y co ordinates of them,.
    Then I got 100 % confused by r hat :(

    I appreciate any help at all

    Thanks in advance :)
  2. jcsd
  3. Oct 20, 2014 #2
    r-hat ([itex]\hat{r}[/itex]) is a unit vector. Unit vectors have a length of 1. In this way you can separate out the "magnitude" of a vector from its direction. So you can think of r-hat as a vector (of length 1) that points in the direction from the particle to where you want to measure the force at.
    Any vector can be expressed as the product of a magnitude and a unit vector, like: [itex]R\hat{r}[/itex] where R is the magnitude (length) of the vector and [itex]\hat{r}[/itex] is the direction.

    So in your equation: K0 * Q1.Q2 / d^2 * r hat
    (K0 * Q1.Q2 / d^2)
    is the magnitude and r hat is the direction.

    In the equation:
    r hat = cos(theta)i + sin(theta)j
    'j' and 'i' are also unit vectors: i=(1,0) j=(0,1)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted