# Infinite dielectric permittivity

1. Jul 22, 2010

### Stanley514

I read that in some ferroelectrics,such as Barium Titanate,
dielectric permittivity approaches infinity near Curie point.
For example,for Barium Titanate its closer to 120 centigrads.
Do those effect allow as (in theory) create capacitor with
almost unlimited energy density?Or there are even theoretical
obstacles to do it?

2. Jul 25, 2010

### Antiphon

It really means that the dielectric deformation of the matererial happens very easily with the imposed field and that the electric field in the material is nearly completely eliminated by the deformation. It is almost as if there are unbound charges as in a conductor but there aren't and the material is still an insulator. (this is how an infinite permittivity acts like a conductor when looking at wave reflections at a surface.)

What will happen is that the dielectric strength of the material will go way down as the permittivity goes up and it will only be able to store a reasonable amount of energy which is probably even less than when it is not near it's Curie temperature.

3. Jul 25, 2010

### Curl

what happens if you have a conductor wrapped in an insulator for a dialectic? Isn't it a similar effect?

4. Jul 26, 2010

### Stanley514

What if we use combination of material with high dielectric strenghts at 120 centigrads and Barium Titanate with dielectric permittivity appraching infinity at that point?

5. Jul 26, 2010

### htg

If you use layers, one of barium titanate, the other of your high strength dielectric, then it will give you the same result as connecting two capacitors in series - the capacitance will be smaller than the capacitance of the smaller capacitor.
If you use mixture of dielectrics, things get complicated, but you will not have very high capacitance without significant decrease of dielectric strength.

6. Jul 26, 2010

### Stanley514

I read mind of some student on scientific forum who stated that energy storage similar to
capacitors,flywheels,superconductors, (which he generalized as mechanical energy storage); principaly cannot exide energy density of fuel (such as gasoline).He clamed energy storage of fuel is based on brokening of intermolecular forces while capacitors and other mechanical storage on their deformations.I dont know, but scientists still didnt find the best dielectric in the world.I would be glad to know if there is some physical theoretical limits to dielectric strengts of matter.Did I understood you right that for capacitor with unbelievebly huge energy density we need almost infinite dielectric strenghts,not only permittivity?

7. Jul 26, 2010

### Antiphon

E = 1/2 C * V^2.

Either C or V can be big but for practical energy storage you usually have a very large C and moderate to high V.

8. Jul 27, 2010

### Stanley514

Is it possible to use triboelectric effect for practical energy storage?
For example ebonite could store negative electric charge, I guess it depends on surface
area.What is material that could have the largest known triboelectric charge potential?

Last edited: Jul 27, 2010
9. Jul 27, 2010

### Dickfore

There are different definitions of dielectric susceptibility. One is a "static" (I don't know if this is the correct term), defined as:

$$P_{i} = \epsilon_{0} \, \sum_{k}{\chi_{i k} \, E_{k}}$$

for linear media, i.e. media where the polarization is always proportional to the existing electric field; and the other is "dynamic" (agaim, terminology), defined as the tensor:

$$\chi_{i k} = \frac{1}{\epsilon_{0}} \, \frac{\partial D_{i}}{\partial E_{k}}$$

For isotropic materials, $\chi_{i k} = \chi \, \delta_{i k}$.

The dielectric permittivity tensor is defined through the susceptibility tensor as:

$$\epsilon_{i k} = \epsilon_{0} \, \left(\delta_{i k} + \chi_{i k}\right)$$

or for isotropic media ($\epsilon_{i k} = \epsilon \, \delta_{i k}$):

$$\epsilon = \epsilon_{0} \, (1 + \chi)$$

so, in infinite permittivity would correspond to infinite susceptibility. I would suspect that this refers to the second definition, i.e. a susceptibility of the "dynamical" kind and corresponds to a vertical tangent on the P - E plot (or D - E plot).

If you know thermodynamics, you will see that the susceptibility corresponds to a second derivative of the Helmholtz free energy with respect to the field, and according to the Ehrenfest classification of phase transitions, this would indicate a second order phase transition (or a critical point).

10. Jul 27, 2010

### htg

Van de Graaf generators practically use triboelectric effect for generation of high voltages. But not for energy storage.

11. Jul 27, 2010

### Stanley514

I have question regarding electric hazard.For example a person touches live electric wire on fifth floor of a building made of concrete.To get shock, charge should flow through person.But where will it flow,into the ground? Charges usually flow to opposite,but what is charge of the Earth? And how charge could penetrate tens of meters of air and concrete which seems to be a dielectric? Or there is some different principle?

2)Is it possible to make energy storage (capacitor) with carges of only one kind,for example negative?Could you discharge it to the grownd through electric motor and make it work?

12. Jul 28, 2010

### htg

1) A human body can be viewed as a conductor. It has its capacitance and when connected to a non-zero potential it will get charged. In the process of charging current will flow.
2) You can make a unipolar capacitor, but to use it to power a motor you would have to make it huge and discharge it through e.g. a voltage regulator.

13. Jul 28, 2010

### Stanley514

What do you meen as potential?I read that if person will hang on one hand on a live wire and
will not touch ground he will not be killed.Birds could freely seat on high power wires without being killed,because of low difference in potentials.Why person doesnt get serious shock when getting charged from carpet,but only when discharges this charge?You said current should flow into person?
Do you think it will allow to store more energy than usual capacitor?There is a greater distance between capacitor and Earth than between two plates of capacitor.

3)Is it possible to make capacitor which would store energy solely in polarisation of material but would not have electrons displaced?
Isnt ferroelectric RAM working in a similar way?
If material is electracaly polarized but doesnt have electrons difference between plates,could electric breakdown still occur?

4)What to you think about digital quantum battery?
http://www.physorg.com/news180704455.html" [Broken]
Will that thing really work?

Last edited by a moderator: May 4, 2017
14. Jul 28, 2010

### htg

Bird's body capacitance is quite small. That's why they can sit on high voltage lines.
If you get charged slowly (e.g. from a carpet) then small current flows and does not cause shock. During sudden discharge relatively large current flows.

15. Jul 28, 2010

### Stanley514

If I no make mistake, dry skin is rather prone to charge positevly - e.g. to give up electrons,not to accept them.What about other parts of human body?How could human body charge negatively?

Last edited: Jul 28, 2010
16. Jul 28, 2010

### Stanley514

100.000 Volts at 1 Amp=100.000 Wats?
Will 100.000 Volts at 1 Amp Direct current kill a person?

Last edited: Jul 28, 2010
17. Jul 29, 2010

### htg

About 10 mA may kill a person. 1 A certainly will (unless it is very high frequency, so that skin effect has a protective effect).

18. Jul 29, 2010

### Dickfore

Electric current is a flux measurement, i.e. it measures the total stream of current carriers from the source of emf to its other end. What is more important is how this current is distributed. Did you mean the whole of 1 A passes through the human body? Because, as the previous poster had suggested, even 10 mA (although I think the correct value is 100 mA through vital organs) is sufficient to kill you. Then, it also depends on the frequency.

19. Jul 30, 2010

### Antiphon

Yes.

20. Aug 3, 2010

### Stanley514

What is sense to use high voltage motors in electric vehicles such as Toyota Prius which works from few hundreds of volts?
Could we use low voltage and high current motor similar to starter in gasoline car?
I mean could we compensate reduced length of wires by making them thicker and obtain the same power?Because you will need some licensed electrician to work on high voltage equipment.