Either I can't do Pythagoreas or this book can't

[flyingpig]

Homework Statement

http://www.students.uidaho.edu/documents/HW02%20with%20solutions.pdf?pid=102870&doc=1

Go to question 19.

How in the world is it R^2 - d^2?

Also, when they say the flux is 0 for same amount that enters and exits, why isn't it the same case for R>d?

[PLAIN]http://img406.imageshack.us/img406/590/68326652.png

Look at picture, is it because the E field is only going out and nothing is really "going in" that for R>d, it isn't 0?

The Attempt at a Solution

 

[cepheid]

Look at picture, is it because the E field is only going out and nothing is really "going in" that for R>d, it isn't 0?

Yeah. For there to be a net flux through the surface, there has to be a "source" or a "sink" for field lines (a place where they diverge from or converge to). That's precisely what electric charges are. In the absence such a source or a sink, there is no NET flux through the surface, because any field line that enters from the outside ends up passing out again.

 

[flyingpig]

Yeah. For there to be a net flux through the surface, there has to be a "source" or a "sink" for field lines (a place where they diverge from or converge to). That's precisely what electric charges are. In the absence such a source or a sink, there is no NET flux through the surface, because any field line that enters from the outside ends up passing out again.

But isn't that what my drawing is? While the charge is inside, the amount that is inside is also the amount that is leaving

 

[cepheid]

But isn't that what my drawing is? While the charge is inside, the amount that is inside is also the amount that is leaving

Yes, I am not disagreeing with you. I was saying that there would be no net flux if there were NO charges enclosed (as would be the case for R < d). But in the case of your diagram, since R > d, there IS charge enclosed, and these sources of field lines within the surface lead to a net outward flux through that surface.

By the way, look at the diagram below: can you do Pythagoras on either one of the two identical right triangles shown in order to solve for x?

http://img829.imageshack.us/img829/3406/linecharge.th.png

 

[flyingpig]

Yes, I am not disagreeing with you. I was saying that there would be no net flux if there were NO charges enclosed (as would be the case for R < d). But in the case of your diagram, since R > d, there IS charge enclosed, and these sources of field lines within the surface lead to a net outward flux through that surface.

By the way, look at the diagram below: can you do Pythagoras on either one of the two identical right triangles shown in order to solve for x?

http://img829.imageshack.us/img829/3406/linecharge.th.png

Ohhh okay, thank you, that was amazing!

 

[flyingpig]

By the way, did you use Paint to draw this? Because that's what I use...

 

[cepheid]

By the way, did you use Paint to draw this? Because that's what I use...

No, I used the drawing program that comes with OpenOffice. OpenOffice is a suite of software that includes word processing, spreadsheet, and presentation-making programs. So, it's just like Microsoft Office, except that it is free and open source.

 

[flyingpig]

No, I used the drawing program that comes with OpenOffice. OpenOffice is a suite of software that includes word processing, spreadsheet, and presentation-making programs. So, it's just like Microsoft Office, except that it is free and open source.

Do you need to purchase an electronic pen to do that?

 

[cepheid]

Do you need to purchase an electronic pen to do that?

No, the drawing program works like Paint. You use the mouse to draw shapes, etc.

 

[flyingpig]

No, the drawing program works like Paint. You use the mouse to draw shapes, etc.

Is it better than Paint...?

 

[cepheid]

Is it better than Paint...?

I would say that it has more features. Once again: it's free. That means that you can download it and try it out. If you don't like it, you can just uninstall it.