Gaussian surface - which charges contribute to the electric field

[alex21]

Homework Statement

in the following figure, the dashed line denotes a Gaussian surface
enclosing part of a distribution of four positive charges.

a) which charges contribute to the electric field at P?
b) is the value of the flux though the surface, calculated using
only the electric field due to q1 and q2, greater than, equal to,
or less than that obtained using the field due to all four charges?

Homework Equations

&Phi;=&int;E*A=Q<sub>enclosed/&epsilon;
Φ=∫E*A=Qenclosed/ε

The Attempt at a Solution

a)what I tried to do is to draw the field line of each charge since all of them are positive they will repel each other and at P the electric field will be zero so none of them contributes to the field
b) I don't fully understan question B but I would say that since all charges are equal and for the flux through the surface I will only consider q1 and q2 the flux must be equal

hel please!

 

[member 392791]

I think you should review a gaussian surface and how the electric field and charges contained within relate. Your equation for the flux should be enough to let you know. The subscript for Q_enclosed is precisely that, the charge enclosed in the gaussian surface. Also, this question belongs in the introductory physics homework subforum, not engineering.

 

[alex21]

ok second chance I was wrong about a) I can say now that all of them contribute to field at P because
The electric fiedl in the expression for Gauss´s law refers to the total electric field, not just the electric field due to any enclosed charge.
about question b) the charge enclosed in gaussian surface is q1+q2, if we use the field due to all four charges then the charge enclosed will be q1+q2+q3+q4 therefore the flux through the surface using only q1 and q2 it would be less if we use all four charges.

 

[alex21]

:( no body help me with me question though