Gauss's Law and Finding the electric field

[musicmar]

1.
Two long charged, concentric cylinders have radii of 3.0 and 6.0 cm. The charge per unit length is 5.0 x 10^-6 C/m on the inner cylinder and -7.0 x 10^-6 on the outer cylinder. Find the electric field at (a) r = 3.0 cm and (b) r = 8.0 cm, where r is the radial distance from the common central axis.

Homework Equations

ε₀ ∮EdA = q_enc
λ= Q/L

The Attempt at a Solution

EA = q_enc/ε₀


E = λL/ π r² ε₀

From here, I don't know what to do with L. The height of the cylinder should be irrelevant, right?
I have another similar question using ρ and volume, leaving me with a height in the formula for the volume that I don't know what to do with.

Thanks.

 

[ehild]

EA = qenc/ε0

this is the right formula, just make it clear, what A is.

ehild

 

[musicmar]

I figured out what I was doing wrong. The area is the surface area of the cylinder, not the area of the base. So, L cancels when (2 pi r)h is in the denominator.

 

[ehild]

I figured out what I was doing wrong. The area is the surface area of the cylinder, not the area of the base. So, L cancels when (2 pi r)h is in the denominator.

Well done!

ehild