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: Applying Lorentz transform to current 4 vector

  1. Jan 1, 2013 #1
    1. The problem statement, all variables and given/known data

    Consider an infinite line of charge with density λ per unit length lying along the z-axis. If the line of charge is stationary in frame S, use the Lorentz transform to find the current and charge densities in a frame S' which is moving with velocity v parallel to the z-axis of S.

    2. Relevant equations

    4 current defined as

    [tex] j_\mu = (J_1,J_2,J_3,c\rho) [/tex]

    3. The attempt at a solution

    In S I find

    [tex] j_\mu = (0,0,0,c\lambda) [/tex]

    then applying the Lorentz transform to find

    [tex] j'_\mu = (-\gamma\beta c \lambda,0,0,\gamma c \lambda) [/tex]

    in S'

    Is this correct?
    Last edited: Jan 1, 2013
  2. jcsd
  3. Jan 3, 2013 #2


    User Avatar
    Homework Helper

    Hey, welcome to physicsforums!
    It's almost correct. You have the correct answer if the charge was uniform over all space. (since you have put the constant lambda, the charge will spread the same over all space). What the question asked for was a charge distribution that is concentrated on the z axis. So which mathematical object can you use to represent this charge distribution?

    Also, usually the charge is written as the zeroth component, and the current as the other 3, but I guess it is only convention, so it doesn't really matter. It just might be confusing when you look at other people's work.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook