TBR: Charging by Induction

[justadream]

TBR Physics Book II page 145



I apologize in advance for how ignorant this question may be but here goes:



TBR does an experiment in which a charged glass rod is brought near a metal sphere.



Apparently, electrons are transferred from the rod to the sphere (or the other way, it doesn’t matter). TBR calls this “induction”. But how exactly are the electrons transferred?



I understand that in the initial “charging” of the glass rod (in which the rod is rubbed against something), electrons can be directly transferred through physical contact. But electrons can also just be transferred through the air? (or in a vacuum)?

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[Cawolf]

The one with two spheres?

The negatively charged rod is near the neutral sphere, so the sphere becomes "polarized" with electrons moving away from the rod. They then connect the spheres with a wire that allows the electrons to move further from the negatively charged rod - into the farther sphere. When the wire and rod are removed simultaneously, charge can no longer flow. This leaves the right-most (farthest from rod) sphere with a negative charge and the left (close) sphere positive.

Is this the experiment you refer to? It requires two conducting objects so that when the conductors are separated the unequal charge distribution remains.

The electrons are not leaving the rod, they are just "inducing" an unequal charge distribution in the spheres.

 

[justadream]

@Cawolf

Yes, that is the passage but electrons apparently ARE leaving the rod (that is what confused me).

If you look at #15 on that passage

"How do the masses of the spheres in Experiment 2 compare before and after"

The answer is that one increases and one decreases (and the reason TBR gives is that electrons are gained/lost).

 

[Cawolf]

The spheres change mass because one transfers electrons to the other.

The electrons are not leaving the rod.

 

[justadream]

@Cawolf

Ohhh! Makes much more sense now. Thanks!

 

[Cawolf]

No prob - it is good feeling when something clicks.