Find the potential - Long hollow cylinder

In summary: The potential inside the cylinder is -V_0 . The equipotentials are as follows: The potential at the center is 0 . The potential at the top is +V_0 . The potential at the bottom is -V_0 .
  • #1
DieCommie
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0

Homework Statement


Consider a long hollow cylinder of radius b that is divided into equal quarters with alternate segments being held at potential [tex] +V_0 [/tex] and [tex] -V_0 [/tex].

a) Find the potential inside the cylinder.

b) Sketch the equipotentials.



Homework Equations


Is this an application of Laplace's equation?


The Attempt at a Solution



Dont know where to start!


Thx.
 
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  • #2
Yes, it is an application of Laplace's equation. Do you know the solution of the LE in cylindrical co-ordinates? Have you seen Bessel functions before? If not, you need to first study this. You will find it covered in most any standard E&M text (eg: Jackson, and probably Griffiths too).
 
  • #3
Yes I have seen those before, but am not very good at them. From digging around texts I come to the conclusion that the solutions are modified bessel functions. This is because of the cylindrical shape. Does this seem correct?

Now I am running into problems applying the boundary conditions. The potential goes from [tex]+V_0[/tex] to [tex]-V_0[/tex] at [tex]\phi = 0[/tex]. Does that mean the potential at [tex] \phi = 0 [/tex] is [tex] 0 [/tex]? (And likewise at [tex] \phi = \frac{\pi}{2}, \pi, \frac{3\pi}{2} [/tex]?) When many of my constants go to zero and I lose my [tex] \phi [/tex] dependence. I know this is not correct because the potential must be dependent on [tex] \phi [/tex]...

Very confusing for me! and I am sure its confusing for you trying to help me, but if you have any suggestions (especially pertaining to boundary conditions) please tell! Thank you.
 
Last edited:
  • #4
Yes, I'm confused. What are theta and phi? In cylindrical co-ordinates you have only one of them.

Here's the general approach:

Write down the general solution of the LE in cylindrical co-ordinates. Look at the radial part - you can eliminate one of the coefficients by inspection. Then look at the angular part. Again, you can eleiminate one coefficient by noticing the symmetry of the boundary potential (for a particular choice of axes, it will be either even or odd). Now finally, set the potential equal to its boundary values at the boundaries and use this to evaluate the Fourier and Bessel coefficients.
 
  • #5
Sorry about that. I change them all to [tex]\phi[/tex]. I am going to look it over one last time before I ask any other questions. Thanks for your time so far!
 

Related to Find the potential - Long hollow cylinder

1. What is the potential of a long hollow cylinder?

The potential of a long hollow cylinder is the measure of the electric potential energy per unit charge at any point inside or outside the cylinder. It is affected by the charge distribution and the distance from the center of the cylinder.

2. How do you calculate the potential of a long hollow cylinder?

The potential of a long hollow cylinder can be calculated using the formula V = k * λ * ln(r2/r1), where V is the potential, k is the Coulomb constant, λ is the charge per unit length of the cylinder, and r2 and r1 are the outer and inner radii of the cylinder respectively.

3. What is the relationship between the potential and electric field of a long hollow cylinder?

The electric field of a long hollow cylinder is directly proportional to the potential gradient. This means that the electric field increases as the potential increases and decreases as the potential decreases. The electric field is also inversely proportional to the distance from the center of the cylinder.

4. How does the potential vary inside and outside a long hollow cylinder?

Inside a long hollow cylinder, the potential is constant and equal to the potential at the surface of the cylinder. Outside the cylinder, the potential decreases as the distance from the center of the cylinder increases. This is because the electric field decreases with distance from the cylinder, causing a decrease in potential.

5. What are the practical applications of finding the potential of a long hollow cylinder?

One practical application is in the design of capacitors, where a long hollow cylinder can act as one of the plates. The potential of the cylinder is important in determining the capacitance and overall performance of the capacitor. It is also useful in understanding the behavior of charged particles inside and outside the cylinder, which has implications in areas such as plasma physics and particle accelerators.

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