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The electrostatic potential in an - plane for an infinite line charge in the direction with linear density is given by

x

y

z

λ

ϕ(x,y)=-2λlog+

2

x

2

y

We use Gaussian units for compactness. The zero of potential is evidently the value on the circle +=1.

2

x

2

y

For two parallel line charges, with linear densities and , intersecting the plane at and , respectively, the potential function generalizes to

λ

1

λ

2

(-R,0)

(R,0)

ϕ(x,y)=-2log+-2log+

λ

1

2

(x+R)

2

y

λ

2

2

(x-R)

2

y

For selected values of , and , selecting "contour plot" shows the equipotentials of . For =, the equipotentials have the form of Cassini ovals. Also shown as green contours are the orthogonal trajectories , which represent the electrostatic lines of force. These are given by

R

λ

1

λ

2

ϕ(x,y)

λ

1

λ

2

ψ(x,y)

ψ(x,y)=-4arctan-4arctan

λ

1

y

x+R

λ

2

y

x-R

as derived in the Details below.

A 3D plot of the potential contours is also available. Click the checkbox to display, for purposes of comparison, the analogous equipotentials and lines of force for two point charges and replacing the line charges.

q

1

q

2