# B Permittivity of free space

1. Dec 7, 2015

2. Dec 7, 2015

### Staff: Mentor

It is simply a SI unit conversion factor between Newtons, meters, and Coulombs. It does not even exist in other unit systems, like Gaussian units.

3. Dec 7, 2015

### gracy

what it does?

4. Dec 7, 2015

### Staff: Mentor

It converts units of Coulombs to units of Newton's and meters.

5. Dec 7, 2015

how?

6. Dec 7, 2015

### Staff: Mentor

$1\;C^2=4\pi \epsilon_0 \;N m^2$

7. Dec 7, 2015

### gracy

But I thought it is something abut allowing electric field to enter in a region.

8. Dec 7, 2015

### Staff: Mentor

It is just a unit conversion factor, and only in SI units. In Gaussian units charge is defined in terms of the dyne and cm such that there is no similar conversion factor.

You may be thinking about the permittivity of a material, but free space is not a material.

9. Dec 7, 2015

### gracy

Ok what is permittivity of material?

10. Dec 7, 2015

### Staff: Mentor

I think you already have a separate thread on that. It is not a good idea to have two open threads discussing the same topic. Let's keep this thread for vacuum questions and the other thread for material questions.

11. Dec 7, 2015

### gracy

But that thread was about susceptibility not permittivity.

12. Dec 7, 2015

### Staff: Mentor

$\epsilon = (1+X_e) \epsilon_0$

13. Dec 7, 2015

### gracy

Ok, susceptibility and permittivity are related but not the same thing.

14. Dec 7, 2015

### Staff: Mentor

More importantly, vacuum and material are not the same thing.

15. Dec 7, 2015

### gracy

yes.

16. Dec 7, 2015

### gracy

What each term indicates in your equation?
$ε$= permittivity of a material
εo=permittivity of free space/vaccum

χe =susceptibility of ?

17. Dec 7, 2015

### cnh1995

According to my understanding, permittivity is a measure of charge required to create an electric field in a dielectric. It is not the physical definition but certainly helped me understand its applications.

18. Dec 7, 2015

### cnh1995

The material..

19. Dec 7, 2015

### cnh1995

In this video example, a slab of conducting material is placed in vacuum and subjected to an electric field E. Due to this field, there will be rearrangement of the charges on the slab surfaces to make the field inside the slab 0. Due to rearrangement, there will be charge densities created on opposite faces. Now, this charge density will be of the lowest possible value in case of vacuum(εo is the lowest). If this slab were placed in water(higher ε), the charge density required(or allowed) to create the same electric field E would be higher (εr times).
This is the principle used in the capacitors.

Last edited: Dec 7, 2015
20. Dec 7, 2015

### gleem

Susceptibility is a measure of the ease with which a material may be polarized defined by the equation

P = ε0χE

with χ = K -1 K being the dielectric constant of the material and the permittivity ε = (1 + χ)ε0

Thus permittivity is a measure of how a material affects an electric field by its presence. since

D = εE since D is the electric field due to free charges and E is the net field in the presence of a dielectric.

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