# Homework Help: Electric fields and metal tube

1. Jan 21, 2008

### ghetto_bird25

hey guys i was wondering if anyone could help me with this question, kinda got stuck
1. The problem statement, all variables and given/known data
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

2. Relevant equations
E=$$\lambda$$/2$$\pi$$$$\epsilon$$r

3. 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

2. Jan 21, 2008

### 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.

3. Jan 21, 2008

### ghetto_bird25

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

4. Jan 21, 2008

### Dick

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

5. Jan 21, 2008

### ghetto_bird25

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