# Homework Help: Gaussian surface

1. Dec 19, 2013

### alex21

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

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?

2. Relevant equations

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

3. 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 dont 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

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Last edited: Dec 19, 2013
2. Dec 19, 2013

### Woopydalan

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.

3. Dec 21, 2013

### 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 GaussÂ´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.

4. Jan 17, 2014

### alex21

:( no body help me with me question though