# Finding potential difference between two points on a cone

1. Dec 27, 2009

### issacnewton

Hello

I am solving some problems from "Intro to EM" by David Griffiths ( third edition)
Problem 2.26 ( attached file 2.26.jpg) and I have also attached the solution from the solution
manual (griffiths-2.26.jpg). For both part a and b I am getting different answer.

I have chosen vertex as the origin and the axis of the cone as the positive z-axis.
The bottom of the cone is toward positive z axis. Now any differential area element on the cone would be (using cylindrical coordinates) , x*d(phi)*dz , where x is the perpendicular
distance of the area element from the z-axis. But for this geometry, x=z for any point on the
cone , so dz = dx. The distance r of this area element from the origin(vertex) would be
sqrt(2)*x. I am supposed to use the formula (formula.jpg).

Why are my solutions not working ? Or is the instructor's solution manual incorrect ?

Regards
I Newton
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

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• ###### formula.JPG
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2. Dec 27, 2009

### tiny-tim

Hello issacnewton!

I can see the instructor's solution but not your solution.

3. Dec 27, 2009

### issacnewton

hello tiny-tim

i will write my solution though i just explained what i did. i see people here write using LateX. how can i write like that ? is there any 'How to' tutorial for doing that ?

Newton

4. Dec 28, 2009

### tiny-tim

Hello issacnewton!

(just got up :zzz: …)

LaTeX is very easy.

The best way to see how to use it is click the "QUOTE" button on someone else's post and copy what they do.

There's a list of symbols and a guide at http://www.physics.udel.edu/~dubois/lshort2e/node61.html#SECTION008100000000000000000" [Broken]

and there's also a palette of symbols if you click the ∑ tag just above the Reply box (though I never use it).

Last edited by a moderator: May 4, 2017