# Charged straight wire Electrostatics

1. Mar 31, 2014

### Lucky7

1. The problem statement, all variables and given/known data
We have a charged straight wire with radius ra = 0.2 cm and with charge ta = 3nanoC/m. Around the wire there are two hollow concentric cylinders, so that the wire is in their axis. Let the charge on the first cylinder be of radius rb = 3cm and its charge tb = -1.5nC/m and on the second cylinder with rc = 5cm with charge tc = 2 NC / m What is the force on the charge Q =-5NC in the distances
R = 0.8 cm,
R = 2.5 cm,
R = 100 cm?

I do not have the slightest idea what to do. I do not want you to compute it for me but give samo advice, please.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 31, 2014

### BvU

What you want to do is collect some relevant equations and list them under 2.)
Hint: They might have to do with electric field calculations and with forces on a charge in an electric field.

Then you attempt a solution, e.g. at calculating the field in different areas. If that's too difficult, start with leaving out the outer cylinder.

3. Mar 31, 2014

### Simon Bridge

What he said:
Do you know the relationship between the force on a point charge and the electric field?
How would you normally calculate the electric fields around charged objects?

4. Apr 1, 2014

### Lucky7

The attempt at a solution
In the wire there should be electrical intensity E = 0. From the Gauss Law for cylindric symmetry the electric intensity E1 between the wire and the first cylinder should be E1=ta/(2pi*r), where 0.2< r< 3 thus E1(R)=ta/(2pi*R). Analogically for E2 = (ta+tb)/(2pi*r), whre 3<r<5 and E3=(ta+tb+tc)/(2pi*r), where r>5.
Does it make sense? Thank you in advance!

5. Apr 1, 2014

### BvU

Yes. There might be a factor $1/\epsilon_0$ somewhere as well.
And then there still is the relationship between Force, test charge and E field.