A straight, nonconducting plastic wire 9.00cm long carries a charge density of 1

Question

A straight, nonconducting plastic wire 9.00cm long carries a charge density of 130nC/m distributed uniformly along its length. It is lying on a horizontal tabletop

A) Find the magnitude and direction of the electric field this wire produces at a point 4.00

cm directly above its midpoint.

B) Is the electric field upward or downward?

C)If the wire is now bent into a circle lying flat on the table, find the magnitude and direction of the electric field it produces at a point 4.00

cm directly above its center.

D) Is the electric field upward or downward?  

For A I'm getting 3.88*10^4 N/C ,but it keeps telling me it is wrong

Thank you

Explanation / Answer

a)
E = (lamda/(2*pi*epsilon*r) )*sin(theta)

= 2*k*lamda/r)*(4.5/sqrt(4.5^2+4^2))

= (2*9*10^9*130*10^-9/0.04)*

= 4.37*10^4 N/c

B) Upward

C) total charge, Q = lamda*L

= 130*10^-9*0.09

= 11.7 nC

R = L/Pi

= 0.09/3.14

= 0.0287 m

E = k*Q*x/(R^2+x^2)^3/2

= 9*10^9*11.7*10^-9*0.04/(0.0287^2+0.04^2)^1.5

= 3.53*10^4 N/c

D) upward

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