Electric Flux due to Two Point Charges: Analysis and Calculations

[Aserap]

A point charge q_1 = 3.40 {\rm nC} is located on the x-axis at x = 1.80 {\rm m}, and a second point charge q_2 = -5.80 {\rm nC} is on the y-axis at y = 1.10 {\rm m}

What is the total electric flux due to these two point charges through a spherical surface centered at the origin and with radius r_1 = 0.710 {\rm m}?
What is the total electric flux due to these two point charges through a spherical surface centered at the origin and with radius r_2 = 1.65 {\rm m}?
What is the total electric flux due to these two point charges through a spherical surface centered at the origin and with radius r_3 = 2.95 {\rm m}?

i am so confused at where to start! i keep getting the wrong answer

 

[Doc Al]

Show what you've done so far.

Hint: Use Gauss's law.

 

[Aserap]

There are so many ways to use gauss's law.
i used the equation $$\phi$$ = q/ E0 for each point and then added them together to get the total. the total i found for the first part was -2.7 x10^11. But it is wrong. i am not sure if i need to take into account the location of each of the points or not.

 

[Doc Al]

i am not sure if i need to take into account the location of each of the points or not.

Of course you do. Gauss's law tells you the total flux through a closed surface in terms of the total charge within that surface. You have to know if the charges are inside or outside of your spherical surface in each case.

 

[Aserap]

So would i use the electric field equation to find the value of each point, where r is the radius of the sphere plus the distance of each point. then i would find the flux for both and add them together? that the only possible answer i can think about doing! thanks for helping by the way!

 

[Doc Al]

So would i use the electric field equation to find the value of each point, where r is the radius of the sphere plus the distance of each point.

No. The only equation you need is Gauss's law. All you have to do, for each of the three cases in your problem, is figure out the total charge contained inside your spherical surface. Then use Gauss's law to find the total flux through that surface.

It's easier than you think.

 

[Aserap]

okay, so would the answer for each section of the problem be the same answer?

 

[Doc Al]

okay, so would the answer for each section of the problem be the same answer?

Nope.

Ask yourself: Are the charges inside or outside the spherical surface? That depends on how big the spherical surface is, right?