Help understanding the units in Gauss's law

  • Thread starter goldenwest
  • Start date
  • #1

Homework Statement



Well, here's the homework question:

A charge Q is located inside a rectangular box. The electric flux through each of the six surfaces of the box is: phi1=+1500Nm^2/C, phi2=+2200Nm^2/C, phi3=+4600Nm^2/C, phi4=-1800Nm^2/C, phi5=-3500Nm^2/C, and phi6=-3400Nm^2/C. What is Q?

My guess is that I will have to use Gauss's law (electric flux = enclosed charge over electric constant) to find out the charge for each number and then find the mean of all the charges? But what's really confusing me is that Gauss's law's units don't seem to cancel out properly. Please show me why I'm an idiot.

Homework Equations


confusion1.gif

seems like it should be
confusion2.gif

...why isn't it?

The Attempt at a Solution


My attempt at a solution would be
+1500 * 8.99E9 = 1.35E13
+2200 * 8.99E9 = 1.98E13
+4600 * 8.99E9 = 4.14E13
-1800 * 8.99E9 =-1.62E13
-3500 * 8.99E9 =-3.14E13
-3400 * 8.99E9 =-3.06E13

The mean of those numbers = -5.83E11 = Q?
 

Answers and Replies

  • #3
Okay so [tex](\sum\phi)\epsilon = Q[/tex]? Q = -2.16E13?
 
  • #4
LowlyPion
Homework Helper
3,090
4
Okay so [tex](\sum\phi)\epsilon = Q[/tex]? Q = -2.16E13?
Are you sure that's the sum of the Φ ?
 
  • #5
Oops, -3.60E12.... but am I doing it correctly now?
 
  • #6
LowlyPion
Homework Helper
3,090
4
Oops, -3.60E12.... but am I doing it correctly now?
So correctly that it looks like you got it right.
 
  • #7
Okay, thank you very very very very much. I'm still confused about why the units work out that way - could you explain that to me?
 
  • #8
LowlyPion
Homework Helper
3,090
4
ε is in units of C²/Nm²

Φ is in units of Nm²/C

Q is in units of C

Edit: Sorry. I posted the units of Coulombs Constant which is the inverse.
 
Last edited:
  • #9
Yes, I know. I know this is probably a stupid question... please bear with me.

[tex]\stackrel{Q}{\overline{\epsilon}} = \stackrel{C}{\overline{Nm^{2}C^{-2}}} = \stackrel{C^{3}}{\overline{Nm^{2}}}[/tex]

Why is that wrong?
 
  • #10
LowlyPion
Homework Helper
3,090
4
Sorry. I posted the units of Coulombs Constant which is the reciprocal of the units of ε.

I corrected my error.

Electrical flux has SI units of volt metres (V m), or, equivalently, newton metres squared per coulomb (N m² C−1).
http://en.wikipedia.org/wiki/Electric_flux
 
  • #11
Edit: Sorry. I posted the units of Coulombs Constant which is the inverse.
Oooooooooook, now I understand. Thank you very much for all your help!
 
  • #12
So was I using the wrong constant when solving for the problem.... meaning I got the answer wrong? I was using 8.99E9. Should I be using 8.85E-12?
 
  • #13
LowlyPion
Homework Helper
3,090
4
So was I using the wrong constant when solving for the problem.... meaning I got the answer wrong? I was using 8.99E9. Should I be using 8.85E-12?
Yes. I missed that too I guess. I should have checked it.

εo is what you want, and not k.
 
  • #14
Thanks again, you have been extremely helpful.
 

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