## ganeshie8 Group Title in 3D, why theta = l/r^2. in 2D theta = l/r. im not good with non-euclidean geometries.. can somebody pls show a proof or give a good link pls 2 years ago 2 years ago

1. mukushla

*

2. experimentX

due to the nature of conservation of flux

3. ganeshie8

its related to 1/r^2 relationship ?

4. experimentX

yep

5. ganeshie8

yeah i believe everything that propagates in 3D follows that

6. experimentX

if universe were flat ... the law of gravity would be F = Gm1m2/r instead of r^2 <--- at least that's how i'm thinking these days

7. ganeshie8

yes i get that.. im looking for a proof. i got stuck while refreshing electrostatics when i come across flux around point charge

8. experimentX

i think Gauss law is fine ...

9. ganeshie8

gauss law is next topic .... ill see if it gets addressed .. thank you :)

10. ganeshie8

there maynot be any derivation. its just a fact this 1/r^2 relation for 3D model

11. experimentX

this is very easy to visualize ... in terms of light. the intensity of light decreases by /r^2 ... because light diverges in all 3d directions. |dw:1344027074813:dw|

12. experimentX

|dw:1344027132708:dw| the total light must be same in both surface.

13. experimentX

let's say ... this is due to the geometry of the sphere (surface) in 3D ... and the conservation of theorem.

14. experimentX

*conservation law

15. ganeshie8

i see that... total light has to escape throuh both the spherical surfaces. its same light that goes out

16. experimentX

yep ... or i must be stupid.

17. ganeshie8

this is a law is it.. we cannot derive 1/r^2 relationship... like we derive 1/r in circle for theta

18. ganeshie8

lol

19. ganeshie8

everything seems new.. now when i open my books again

20. experimentX

what do you mean by 1/r in circle ... can you elaborate a bit?

21. experimentX

my suggestion ... never open a book :D

22. ganeshie8

il get back... read some more. actually im looking for some derivation of relation between angle and area for a sphere

23. experimentX

you need solid angle

24. ganeshie8

yeah looks like we need to consider surface area of sphere... i think im getting it now.. il try... thank you :D

25. experimentX

yw!!