Charged Rod and Neutral Sphere Interaction: Repulsion Confirmed?

[Nikitin]

You have a positively charged rod and you have a neutral metal sphere hanging on a wire.

If the positively charged rod approaches the sphere, the sphere will be repulsed - correct?

 

[Nikitin]

please? There was an important test, and I answered that a neutrally-charged sphere would not be attracted to a positively charged rod. I took this option due to the nature of protons in metal - those things don't move around like electrons.

 

[gneill]

Ah. But electrons are mobile in a conductor. Free electrons will be attracted towards the rod, building up a negative charge excess on the region of the sphere nearest the rod, and leaving a deficit of electrons at the far side (the result canceling the field due to the rod within the conductor, leaving the net field within the conductive sphere at zero). This is what is called an induced charge separation.

 

[Nikitin]

yeh, but there still would be lots of protons at that side.. wouldn't it?

But damn, was I wrong? I gambled that the sphere would be unaffected or even get repulsed, thinking that there would remain an adequate concentration of protons on that side...

 

[gneill]

While the protons remain essentially fixed, there's nothing to stop some of the electrons shifting around quite easily. An excess of electrons makes up a net negative charge. A deficit of electrons (relative to the number of protons) a net positive charge.

So I'm afraid that the sphere should be attracted to the positive rod. Note that if the rod had been negatively charged, it the sphere would *still* be attracted to it, as the negative rod would repel electrons from the near side leaving a net positive charge near the rod which would be attracted to it.

 

[Nikitin]

yes, that i know. but oh well

i hate being wrong ._.