A small doubt about electric flux

Tags:
1. Dec 7, 2014

Joe Da Bro

Hey there...
As far as I know, electric flux is a scalar quantity which means that negative values are smaller than zero. This concept really confuses me since most of the exercises I dealt with suggest that negative flux somehow is larger than zero flux

Example 1

the uniform field is directed to the right, which surface has the lower electric flux?
is it S1 which is negative or S2/S4/S5/S6 ( that's the correct answer according to my book)?

Example 2
https://lh6.ggpht.com/f3KBE_uxMhA2zCSgcxfdIu26-n0OOGrOHNwK7lQxsSYGp__YJLIWuP7uSkZUBVLUFpwJQw=s170
Which surface has the lower electric flux?
is it S2 which is negative or S1 which is zero ( that's the correct answer according to my book) ?

So is the problem with me or with the book itself or what exactly?
Thanks in advance.

Last edited: Dec 7, 2014
2. Dec 7, 2014

Simon Bridge

Welcome to PF;
The amount of flux passing through a surface is a scalar - not the flux itself.
The flux may be positive or negative depending on which way it flows through the surface.

3. Dec 7, 2014

Joe Da Bro

Isn't the flux itself a dot product? which also means it's scalar too. care to elaborate please?
Thanks for your reply...

4. Dec 7, 2014

Simon Bridge

A dot product of what with what?
Think of the flux as the amount of flow.

5. Dec 7, 2014

Joe Da Bro

the electric field and the vector area

6. Dec 7, 2014

Simon Bridge

Backtrack - I think I can see a way through for you.
This is just like the component of the velocity vector in the x direction of interest is given by a dot product ($\vec v \cdot \hat i$), which will give a scalar, which may be positive or negative; but a negative speed is no slower than a positive speed. Similarly a negative flux is the same amount of flux as a positive flux, it's just headed into the volume rather than out of it ... and that is an arbitrary choice: it is just as good to define positive in and negative out.

7. Dec 7, 2014

Joe Da Bro

that's what I wanted .. I am really thankful, just to make sure, negative flux>zero flux in both examples? if so then, all the doubt will vanish.. Thanks again!!!

8. Dec 7, 2014

Staff: Mentor

I think when your book asks about "higher" or "lower" they are referring to the magnitude (absolute value) of the flux. As a native English speaker, I personally would say "larger" or "smaller", instead, because that implies magnitude. Is this an English-language textbook, or are you trying to translate from some other language? Or maybe the book was written by a non-native English speaker.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook