# Electricity & charge

1. Jul 29, 2017

### Cici2017

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
I understand I, but struggle to figure out whether rod S is an insulator or conductor. Does this have anything to do with charged object attract uncharged uncharged object?

2. Jul 29, 2017

### Merlin3189

When you bring a charged object near to a conductor, what happens in the conductor?

(Maybe look up "electrostatic induction")

3. Jul 29, 2017

### vela

Staff Emeritus
Seems like none of the answer choices is correct.

4. Jul 29, 2017

### Staff: Mentor

One of the answers offers a correct solution. Merlin3189 is on the right track

5. Jul 29, 2017

### Merlin3189

Actually, I was about to acknowledge Vela's point.
Salutary that one (well, I) can be so easily seduced by the way a question is posed! I suppose this is just what political surveys do.

Certainly section 3 of http://physics.ucsc.edu/~pgraham/spr2009_7b/phys7b_lab2.pdf agrees with you.

But having accepted what now looks to be a wrong answer before, I didn't want to simply accept an alternative answer, without a bit of support. I did hope that there might be a big difference in the size of the force in the two cases.
After a lot of scribbling, fortunately I was unable to come up with a model that I felt happy with. So I was spared having to exercise my very rusty maths and went and did the experiment instead! (If anyone knows where I might look at a mathematical treatment of the magnitude of attraction for induced charges in insulators and conductors, that'd be interesting.)

With a rudimentary apparatus I couldn't measure much, but sure enough, the charged body attracts most things, whether conductor or insulator. I worried a bit that I couldn't be sure that insulators were uncharged, but the only repulsion I got was with something that was itself demonstrably charged.

6. Jul 30, 2017

### haruspex

... and maybe whether they are perfectly nonconducting, including surface moisture?
But as Vela may have in mind, even perfect insulators may form dipoles at the atomic level.

7. Jul 30, 2017

### ElectricRay

I would guess that statement II doesnt make sense. In the drawing is written that Rod S is insulated but is rod as is a unsulator allready how canone insulate a insulator? Or do I make a wrong assumption here?

8. Jul 30, 2017

### haruspex

But that is the flaw in this question, statement II can be true. See my post #6, last line.
It would be more to the point that the stand is also an insulator, otherwise it could have an induced charge distribution.

9. Jul 30, 2017

### ElectricRay

So theoratically I could charge the rod yes is and for on some place on the Rod S locally dipoles. This can only be localy e.g. left side of the rod on the picture. If the insulated stand is clean and a isolator Rod S will be charged. Do I understand it correct like that?

10. Jul 30, 2017

### haruspex

The charge on rod R will induce dipoles at the atomic level all along rod S.
See https://en.wikipedia.org/wiki/Electrostatic_induction#Induction_in_dielectric_objects

11. Jul 30, 2017

### ElectricRay

Im going to have a read on electrostatic induction thanks

12. Jul 30, 2017

### ElectricRay

Ok eureka this is called polarization. I think we use this parameter with meggering electrical machines as well. We measure the Insulation Resistance of 1 min and 10 min and take the ratio. This they call the Polarization Index the number tells something about the quality of the insulation of the windings.