# Why does the electric field of an infinite line depend on the distance, but not on an infinite plane?

I understand that with an infinite plane, as you get closer, the infinitesimal contributions to the electric field become greater in module. The direction of the vectors become less perpendicular to the plane as you get closer, hence reducing the overall electric field in the perpendicular direction. Both effects offset each other such that the electric field is the same at whatever distance.

However, wouldn't this same argument apply to an infinite line? Gauss's law shows that the electric field of an infinite line depends on the distance, but supposedly the same would happen as with the infinite plane, and yet it doesn't.

Does anyone have an explanation for this?

Well, the explanation really is held in Gauss's Law. That shows that the field of an infinite line is distance dependent, while an infinite plane is not.

But I expect you're looking for a more intuitive answer. So I'll give my best shot at one. Keep in mind, that dealing with an infinite anything tends to be non-intuitive, so this may be slightly "hand-wavy".

Electric field works on an inverse-square law, meaning that the when you view a charge from twice as far away, the field strength is four times weaker. It turns out that visual perception is also (approximately) an inverse square law. The amount of area in your field of view an object takes up follows the same proportionality as electric field does. So a way to get a rough estimate for how an electric field changes strength, is to see how the sources size changes with distance.

If you consider looking at an infinite plane, it would take up your entire field of view, (assuming your view is only 180 degrees). As you got further and further from the plane, it would still take up your entire field of view. So the strength of the field doesn't change.

However, the infinite wire does change size. While it's long dimension stays the same in your view, it get's thinner. So the field strength gets smaller and approaches zero at infinity, since the wire would appear infinitesimally thin as you approach infinity.

Hopefully that made it a little more intuitive!

• Good answer. A point charge (zero dimensions) generates a field that is proportional to 1/r^2. An infinite line (one dimension) generates a field that is proportional to 1/r. An infinite cylinder does the same because really it is just a superposition of infinite lines forming a cylindrical shape. An infinite plane (two dimensions) generates a field that is proportional to 1/1=constant. An infinite plate with depth does the same because it is a superposition of infinite plates. – Steve May 25 '17 at 20:48
• It's not a perfect analogy, e.g. a point charge always takes up zero area in a field of view. But I think it's helpful in at least the line and plane case. – Eeko May 26 '17 at 20:25
• If anyone asks you about intuition, this is the answer to give him. Great answer! – J. Manuel Aug 1 '19 at 19:05

Think of electric field lines and how they diverge from a charged wire. Also, think about how field strength is related to the "density" of field lines. Having an infinite plane counteracts this diverging effect as adjacent lines keep the field directed perpendicular to the plane at a constant distance apart.

You have the correct understanding of why the electric field is constant for any distance from the plane.

With an infinite line, if you look at the infinite line from the top, it looks like a point charge, and point charges do not have constant electric fields. So even if we flipp back to the side view of the infinite line, the "ends" of the infinite line will still behave like point charges ie. if you have a point r distance above the end of the line, as the distance increases the electric field would decrease because the "ends" behave like point charges. Which is not the case for an infite plane.

For a point charge field will be inversely proportional to $$r^2$$ because, both dimensions are finite. In an infinite plane, it is independent of both r's as both the dimensions are infinite. In the case of an infinite line charge, only one dimension is infinite. So the field is inversely proportional to only r.