Spherical surface surrounding charges find flux

[phys62]

A spherical surface completely surrounds a collection of charges. Find the electric flux (with its sign) through the surface if the collection consists of (a) a single +5.5 x 10-6 C charge, (b) a single -4.8 x 10-6 C charge, and (c) both of the charges in (a) and (b).

Ok, so this is what I've been able to figure out:
Ea=kq1/r^2=49445/r^2

Eb=kq2/r^2=-43152/r^2

and for part c: Ea + Eb = 6293/r^2

I'm just so confused as to how to get rid of the r^2.. Have I used the wrong equation entirely? Thanks so much to anyone who can help!

 

[LowlyPion]

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gaulaw.html#c2

They want to know the flux across the surface.

You don't need r, because they aren't asking for the field strength.

 

[phys62]

The total of the electric flux out of a closed surface is equal to the charge enclosed divided by the permittivity.

so then for example on part a, would I not just take:

q/k = (5.5x10^-6)/(8.99x10^9) = 6.117x10^-16

?? I know that's not right but I don't understand why. Do I have the sign backwards?

 

[LowlyPion]

The total of the electric flux out of a closed surface is equal to the charge enclosed divided by the permittivity.

so then for example on part a, would I not just take:

q/k = (5.5x10^-6)/(8.99x10^9) = 6.117x10^-16

?? I know that's not right but I don't understand why. Do I have the sign backwards?

Φ = Q/ε_o

But you used the right value.

If the charge is - then the flux is -, meaning that it points towards the charge. If + it would be pointing outward through the closed surface.

 

[phys62]

so you're saying my answer is right then, right? I submitted it on my online homework thing, and it says I'm incorrect :/

 

[LowlyPion]

so you're saying my answer is right then, right? I submitted it on my online homework thing, and it says I'm incorrect :/

Do they want 6.1 instead of 6.117, since they only gave you 5.5 for the charge in the first place?

I ask in case this is something your instructor likes to do.

 

[phys62]

No, our answers can be to whatever decimal point we want, just as long as it's within +/- 2% of the correct answer

 

[LowlyPion]

Sorry. I just realized you were using k after all and not ε_o .

ε_o = 8.85*10^-12

I get them confused. I thought it was right a moment ago when I quickly checked.

http://en.wikipedia.org/wiki/Electric_constant

 

[phys62]

Ohhh. lol. okay, now if only I knew what eo was..

 

[phys62]

oh wait... ok so the equation is Φ = Q/εo

do I just need to solve for Eo?

 

[phys62]

I see! lol thanks so much for your help! :]

 

[LowlyPion]

ε_o = 8.85*10^-12

I get them confused. I thought it was right a moment ago when I quickly checked.

http://en.wikipedia.org/wiki/Electric_constant

I edited it in a moment ago below.