# Gauss's law problem.

Consider two point charges +q,+q. Separated by distance d.
now there exist a point P on the line joining these two charges where electric field cancels out at distance d/2 from the charge.
If we make a Gaussian surface at this point and work out the surface integral it wont be zero.
since two field come in at this point to cancel each other.
My argument for why those field line coming in at point P has to meet is symmetry they cannot go right or left doesnt make any sense.
But there is no charge there!
how is it possible?
or is there any mistake in my question .

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rude man
Homework Helper
Gold Member
Depends on the Gaussian surface you have chosen.

If the surface excludes either charge, the surface integral of E (or D) is zero. If both charges ar included within the surface the surface integral of D = 2q. Etc.
Not sure what your question really is.

BvU
Homework Helper
work out the surface integral it wont be zero
It will be, as rudy says. To prove otherwise, show us and do the math !

Depends on the Gaussian surface you have chosen.

If the surface excludes either charge, the surface integral of E (or D) is zero. If both charges ar included within the surface the surface integral of D = 2q. Etc.
Not sure what your question really is.
the image shows the gaussian surface around which integral need to be taken and am talking just about that single field line (along the line joining the two charges )has to come in and cancel each other because of the symmetry and the surface in the image has lines coming in.
we have a point where the field is zero if we go a little to the left (along the line joining the two charges)than we have a field coming same goes for the right side.
Flux through a closed surface without any charge inside.
(I may be wrong some where but where?)

BvU
Homework Helper
Do the math. Sphere with radius ##>0## will have field lines coming in from left & right, but going out top & bottom

Delta2
Orodruin
Staff Emeritus
Homework Helper
Gold Member
Sorry, but you are just simply wrong. You cannot compute surface integrals by looking at field lines - that is just a heuristic. If you want to draw the lone you are talking about, then you should also be drawing the vertical line in the middle of the charges. This is where the lines you are talking about ”come out” if you wsnt to use the heuristic.

vela
Staff Emeritus
Homework Helper
the image shows the gaussian surface around which integral need to be taken and am talking just about that single field line (along the line joining the two charges )has to come in and cancel each other because of the symmetry and the surface in the image has lines coming in. we have a point where the field is zero if we go a little to the left (along the line joining the two charges)than we have a field coming same goes for the right side. Flux through a closed surface without any charge inside. (I may be wrong some where but where?)
Since there is cylindrical symmetry in this problem, consider a cylindrical Gaussian surface. You're only looking at the flux through the two ends of the cylinder. What about the flux through the cylindrical part of the surface?

BvU
Homework Helper
View attachment 228858

My argument for why those field line coming in at point P has to meet is symmetry they cannot go right or left doesnt make any sense.
But there is no charge there!
how is it possible?
or is there any mistake in my question .