# Cant seem to get the answer right (E&M HW)

#### cronxeh

Gold Member
The charge per unit length on a long, straight filament is -95.0 µC/m.

(a) Find the electric field 10.0 cm from the filament. Distances are measured perpendicular to the length of the filament. (Take radially inward toward the filament as the positive direction.)

(b) Find the electric field 29.0 cm from the filament.

(c) Find the electric field 120 cm from the filament.

Formula: 4*pi*8.99*10^9*distance*(-95*10^-6)

On the side of caution I've used both + and - aforementioned answers and both seem wrong. Am I missing something here?
I thought http://physics.villanova.edu/faculty/shaw/2402/extprob/m1224.gif [Broken] ?

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#### cronxeh

Gold Member
Also one more problem

A conducting spherical shell of inner radius of a and outer radius b carries a net charge Q. A point charge q is placed at the center of this shell.

(a) Determine the surface charge density on the inner surface of the shell. (Use Q, q, a, and b as necessary.)

(This is correct)

(b) Determine the surface charge density on the outer surface of the shell.
(This is marked wrong!)

#### mantito

i don't understand how you get your formula. i get
E=(-95*10^-6)/2*pi*R*ε0, as in formula in the image you gave link to.

#### OlderDan

Homework Helper
cronxeh said:
A conducting spherical shell of inner radius of a and outer radius b carries a net charge Q. A point charge q is placed at the center of this shell.

(b) Determine the surface charge density on the outer surface of the shell.
(This is marked wrong!)

The radius of the outer surface is ?????.

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