# Electric potential of cylinder problem

• azone
In summary: Thanks, but what you mean by U, is it the electric potential ?I'm convuming between the notations !

## Homework Statement

A solid cylinder with radius R and length L has uniform charge density ro. Its base is in the x-y plane and it's axis is coincident with the z-axis (symmetrical about the z-axis)

## Homework Equations

Find the electric potential at point P outside the cylinder at a distance z from the origin (P is on the z-axis)

## The Attempt at a Solution

So I compared this to finding the potential at a distance z above a uniformly charged disk. In the case of a disk, I divided the disk into concentric rings, found the potential contribution due to one ring, and then integrated over all rings with the limits 0 to R.
I tried to use a similar argument and divide the cylinder into equal sized disks with radius R stacked on top of one another (so the only difference is the distance from the point P). I then want to integrate this over all disks with the limits 0 to z. So can I just use my answer for the potential above a charged disk, and then integrate it with respect to z to cover all the distances?
thanks!

You idea sounds correct.

hi i have the same proplem but for hight 2h and radius (a) and charge density ro inside and outside the cylinder and i tryed to use gauss's law and find fy the electric potential but I'm not sure can some one help.
i use
V(r)=- integrate E.da=E.2pi a (2h) =1/ebslon .Qenc

I would use Gauss's Law to find the E-vectors around and inside the cylinder, when remember that $$U=\int_{\infty}^r \vec{E}\bullet d\vec{l}$$ and $$V=\frac{U}{q}$$.

A hint: Use a cylinder for your gaussian surface, with the same axis as your charged cylinder.

Thanks, but what you mean by U, is it the electric potential ?
I'm convusing between the notations !

Hi eman2009, I think espen180 is confusing electrostatic potential energy and electrostatic potential.

As for your original question, if the cylinder only has a finite length $2h$, does it still have the requisite cylindrical symmetry to use Gauss' Law?

If not, you'll have to find the potential through another means...can you think of any formulas that directly relate potential to charge density?

close INT E . da=1/ebsolon Qenc

as in Griffiths book(Introduction to Electrodynamic) p.68, equation 2.13
and
Q=INT RO dt t(tao) is the infinitesimal displacement

dt for cylinder

dt=4 pi R^2 dr

That's just Gauss' Law, and it is only useful in cases where symmetry allows you to pull E outside of the integral...is a cylinder of finite length one of those cases?

Instead, try equation 2.29, it directly relates rho to V.

eman2009 said:
Thanks, but what you mean by U, is it the electric potential ?
I'm convusing between the notations !

Sorry, I mistook your cylinder as one of infinate length. As for my notation, U is the electrostatic potential energy and V is the electric potential.

espen180 said:
Sorry, I mistook your cylinder as one of infinate length. As for my notation, U is the electrostatic potential energy and V is the electric potential.

Sure, but wouldn't you also say

$$U(r)=-\int_{\infty}^r \vec{F}\bullet d\vec{l}\neq\int_{\infty}^r \vec{E}\bullet d\vec{l}$$

? Gah! You're right! Sorry about that. It should be an F, not an E.

## 1. What is the formula for calculating the electric potential of a cylinder?

The formula for calculating the electric potential of a cylinder is V = kλ/r, where V represents the electric potential, k is the Coulomb's constant, λ is the linear charge density, and r is the distance from the center of the cylinder.

## 2. How do you determine the direction of the electric potential for a cylinder?

The direction of the electric potential for a cylinder is determined by the direction of the electric field lines, which point towards the positive end of the cylinder. The direction of the electric field lines can be found using the right-hand rule.

## 3. Can the electric potential of a cylinder be negative?

Yes, the electric potential of a cylinder can be negative. This occurs when the linear charge density is negative, meaning the cylinder has a net negative charge. In this case, the electric potential is negative towards the center of the cylinder.

## 4. How does changing the linear charge density affect the electric potential of a cylinder?

Changing the linear charge density will directly affect the electric potential of a cylinder. As the linear charge density increases, the electric potential will also increase. Similarly, decreasing the linear charge density will result in a decrease in the electric potential.

## 5. Is the electric potential of a cylinder affected by the radius of the cylinder?

Yes, the electric potential of a cylinder is affected by the radius of the cylinder. As the distance from the center of the cylinder increases, the electric potential decreases. This means that a larger radius will result in a lower electric potential, and vice versa.