# I Elementary question on Electric flux units

1. Feb 23, 2017

### jonjacson

The net flux over a closed surface is:

Flux = The surface integral of the field E = charge / permitivity in space (Gauss law)

The permitivity in space is:

8.85 10-12 Coulombs2/N m 2

So I understand the units of Flux are:

Newton meter2/Coulombs

Is this correct? I don't understand why in a book they say that the net flux is simply the charge, ignoring the permittivity units.

ALternatively the permittivity could be given in Farad/meter so our flux would be:

Is this correct?

Thanks!

2. Feb 24, 2017

### jonjacson

Nobody?

3. Feb 24, 2017

### BvU

a book, eh ? Are they talking about $\vec E$ or about $\vec D$ ?

4. Feb 24, 2017

### Khashishi

Does your book use SI units or Gaussian? You should really identify the book.
In SI, electric flux is volts meters. (Because electric field is volts/meter, and flux is an integral over area)

5. Feb 24, 2017

### Staff: Mentor

As a wild guess, perhaps it uses Gaussian units, in which the vacuum permittivity constant $\varepsilon_0$ does not exist.

https://en.wikipedia.org/wiki/Gaussian_units

[ah, now I see Khashishi beat me to it!]

6. Feb 25, 2017

### jonjacson

Ok, so if it is gaussian units it is just the charge, if it is SI units we have what I said at the start.
I hope this is right, if not let me know.

7. Feb 25, 2017

### BvU

Did you miss

8. Feb 25, 2017

### jonjacson

It is Purcell's classic. It is a fantastic book, it says in gaussian units it is just the charge, but in SI doesn't explicitly say "these are the units", I wanted to check, since I consider that very basic.

9. Feb 25, 2017

### BvU

Purcell Ch1 (probably the rest too) is CGS. He touches upon SI which to me is potentially confusing. Check out appendix E where he says 'in our CGS system'.