Help with linear charge density and flux

  • #1

Homework Statement

The y-axis carries a uniform linear charge density of -2 nC/m, and there is a 8 nC point charge
at the point (3 cm, 0 cm, 0 cm) as well as a -4 nC point charge at the point (-8 cm, 0 cm, 0 cm).
What is the electric flux through a closed spherical surface of radius 4 cm centered at the origin?

Homework Equations

I know that flux is 4πkQin or Qin0, i don't understand where the linear charge density comes in. I know that the 8nC charge would be inside the sphere.

The Attempt at a Solution

I tried adding the linear charge density and the 8nC charge and putting it through that formula but i don't understand how the -4nC charge affects the flux, or how to factor it in.

Any help would be greatly appreciated
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Answers and Replies

  • #2
The charge outside the sphere doesn't affect the flux, correct? when adding the charges inside the sphere, -2 and 8, which gives 6nC, and putting it through the formula, 4πkQin, i get 678.58, but the correct answer is 886Nm2/C
  • #3
You are correct that the charge outside the sphere has no influence.
You have made a mistake in determining the charges inside. While there is one charge of 8nC, the 2nC/m is a charge density, not a charge itself.
You need to calculate the amount of charge within the sphere given this charge density, and the radius of the sphere. How might that be accomplished?
  • #4
well i have the equation E = 2kλ/r. E = k8nC/(.08)2?? but how does that help??
  • #5
Just think about the charge on the y-axis for a moment.
It has a charge density -2nC/m. The sphere is radius 0.04m, so what is the total charge (on the y-axis) inside the sphere?

Add this to the 8nC charge, and you have your Qin.
  • #6
im not sure. i don't see how its related at all, i mean besides a volume-density relation
  • #7
The charge density means that for every metre on the y-axis, there are -2nC of charge present. There is 0.08m of the y-axis within the sphere.
Since charge density = charge/length, you can rearrange this equation to give the charge on the y-axis contained within the sphere.

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