# Electric Fields

1. Aug 6, 2008

### steph124355

I'm a little stuck on the theory behind this question:

(in relation to a line of infinite charge: a wire)
A hollow cylindrical metal tube with inner radius 2.5mm is now placed around the wire, to form a coaxil cable. What will be the charge per unit length on the inner surface of the tube. Explain.

I am thinking it is 0, how or why i got that I dont know!!

2. Aug 6, 2008

### Staff: Mentor

Hint: Make use of Gauss's law.

3. Aug 6, 2008

### steph124355

Use Gauss' law as though the cylinder is not there?
In which case to calculate using E= λ/(2πε_0 r) and use the 2.5mm as the radial distance?

Is there any different concepts behind this scenario compared to using a gaussian surface?

4. Aug 6, 2008

### Gear300

Infinite charge...wow...wonder how that happens

5. Aug 6, 2008

### Gear300

Its not 0. Considering that metals are good conductors, what happens is that when placed in an electric field, a polar accumulation of charges occur (meaning the positive charges go in one direction and the negative in the other). Therefore, charges accumulate on the surfaces of the cylinder. The inner surface will have a charge density.

6. Aug 6, 2008

### steph124355

haha thanks!
physics does my head in but i enjoy it!!
thanks for your help

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