# Electric field inside an electret

• rockbreaker
In summary, the conversation discusses the calculation of the electric field inside an electret, which is a dielectric with a quasi-permanent polarization. The problem involves a uniformly polarized cylindrical electret with a given radius and length, and the goal is to calculate the electric field at the center and on the midplane of the cylinder at a certain distance. The equations and steps for solving the problem are discussed, including the use of the potential expansion and the approximation of a point dipole.
rockbreaker

## Homework Statement

I would like to refer to this problem about the electric field inside an electret. https://www.physicsforums.com/showthread.php?t=444106
An elecret is a dielectric that has a quasi-permanent polarization when there is no applied field. Consider a uniformly polarized cylindrical electret. The cylinder has a radius of 10h and a length h. The polarization is Pk where k is a unit vector along the axis.

a. Draw the electric field lines. (Done)
b. Calculate the electric field at the center of the cylinder. Ignore edge effects.
c. Calculate the electric field on the midplane of the cylinder at a distance
100h from the center. The approximation of a point dipole can be used since the distance is very large.

D=ε0*E+P
D=εE

## The Attempt at a Solution

I want to calculate the electric field inside the electret. P is given, but I miss D to be able to calculate E. Can someone give me a hint how to advance? I guess I have to start with calculating σb=P*n. Is then E the volume integral over σb?

Last edited:
b) As there are no other charges involved, I think the field inside the electret is
σB=P*n=-P
E=σ/ε0=σb/ε0=-P/ε0

For c), I would like to use the expansion of the potential, V=1/4∏ε0*(Q/r+r*p/r^2), where the dipole term dominates because we are far away from the electret. But isn't r*p=0 (scalar product) although the field mustn't be 0 there?

## 1. What is an electret?

An electret is a dielectric material that has a permanent electric charge, either positive or negative, on its surface. This charge is created through a process called polarization, where the dipoles (molecular or atomic level electric fields) in the material become aligned in a specific direction.

## 2. How is the electric field inside an electret different from that of a regular dielectric material?

The electric field inside an electret is constant and does not depend on external factors, such as the presence of other charges or applied electric fields. In contrast, the electric field inside a regular dielectric material can be altered by these external factors.

## 3. How is the electric field inside an electret calculated?

The electric field inside an electret can be calculated using the equation E = σ/ε, where E is the electric field, σ is the surface charge density of the electret, and ε is the permittivity of the material. This equation is derived from Gauss's law.

## 4. Can the electric field inside an electret be manipulated?

Yes, the electric field inside an electret can be manipulated by changing the polarization of the material. This can be done through various methods, such as applying a high voltage electric field or heating the material above its Curie temperature.

## 5. What are some real-world applications of electrets?

Electrets are used in various technologies, including microphones, speakers, and sensors. They are also used in air filters to attract and capture particles, such as dust and pollen. Electrets are also used in medical devices, such as pacemakers, to provide a constant electric field for proper functioning.

Replies
4
Views
5K
Replies
4
Views
2K
Replies
4
Views
3K
Replies
7
Views
4K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Electromagnetism
Replies
5
Views
2K
Replies
3
Views
3K
Replies
5
Views
5K