In: Physics
Two infinitely long parallel rods carrying uniform charge density lambda are at a distance d from each other. Find the electric force on a particle of charge q* located a distance d directly left of one of the rods. (Derive the formula using Gauss' Law)

Consider a Gaussian cylindrical surface of radius "d" and length "L" for rod closer to the particle and of radius "2d" and length "L" for rod farther to the particle :
For the closer rod :
A = Area of the Gaussian surface = 2
dL
Qenclosed = amount of charge enclosed =
L
Ecloser = Electric field by closer rod on the particle
Using Gauss's law
Ecloser A = Qenclosed /
Ecloser (2
dL) =
L/
Ecloser = 2
/(4
d)
eq-1
For the farther rod :
A = Area of the Gaussian surface = 2
(2d)L
Qenclosed = amount of charge enclosed =
L
Efarther = Electric field by farther rod on the particle
Using Gauss's law
Efarther A = Qenclosed /
Efarther (4
dL) =
L/
Efarther =
/(4
d)
eq-2
Using eq-1 and eq-2
E = Total electric field at the location of charge = Ecloser + Efarther
E = 2
/(4
d) +
/(4
d)
E = 3
/(4
d)
Force on the particle is given as
F = q E
F = 3
q/(4
d)