Electric fields and metal tube

[ghetto_bird25]

hey guys i was wondering if anyone could help me with this question, kinda got stuck

Homework Statement

Figure 24-31 shows a section of a long, thin-walled metal tube of radius R = 5.56 cm, with a charge per unit length λ = 7.34 x 10-8 C/m. What is the magnitude E of the electric field at radial distance (a)r = 3.15 cm and (b)r = 10.5 cm.
http://edugen.wiley.com/edugen/courses/crs1141/art/qb/qu/c24/Fig23_34.gif

Homework Equations

E=[tex]\lambda[/tex]/2[tex]\pi[/tex][tex]\epsilon[/tex]r

The Attempt at a Solution

well i tried using that equation but got stuck with the radius and where to plug it in since you get 5.56 cm, but wasn't too sure how to exactly plug in the other radius. also i was wondering if that was how you do the question and if you had to plug in the radius in that equation and not R=5.56cm

 

[Dick]

You are given the charge density per meter. Think about Gauss' Law. The only role that R = 5.56 cm plays in the problem is to determine whether r is inside or outside of the cylinder.

 

[ghetto_bird25]

ic so would [tex]\epsilon[/tex][tex]_{O}[/tex] be 8.85e-12 C^2/N m^2

 

[Dick]

If that's the usual value for the permittivity of free space, yes.

 

[ghetto_bird25]

ok thanks that realli helped, btw the answer for this is a) 0 N/C b)12569 N/C
thanks again