Insulator charge question

[nns91]

Homework Statement

Expalin how a positively charged insulating rod can be used to give a metal sphere a positive charge. Cam the same rod be used to simultaneously give one sphere a psotive charge and another sphere a negative charge without the rod having to be recharged ??

Homework Equations

None

The Attempt at a Solution

So I said charge a ball with contact by using the rod. Then, that ball will be - charged. Use the ball to charge the other ball by contact then we have a + charged ball ? Is it true ?

How about the second part, I am not really sure.

 

[kuruman]

An insulating rod has no charges that are free to move around, so exactly how can it be used to give a metal sphere a positive charge? What do you know about charging by induction?

 

[tomcue]

Yes, your solution for the first part is correct. By using the positively charged insulating rod to make contact with the metal sphere, some of the positive charge from the rod will transfer to the sphere, giving it a positive charge as well.

For the second part, it is possible to use the same rod to simultaneously charge one sphere positively and another sphere negatively without having to recharge the rod. This can be achieved by using a process called induction.

First, bring the positively charged rod close to both spheres without making contact. This will cause the electrons in the metal spheres to repel away from the rod, leaving a net positive charge on the side of the spheres facing the rod. Then, while still keeping the rod close, touch one of the spheres with your hand. This will allow the electrons to flow from the sphere to your hand, leaving the sphere with a net positive charge. Next, touch the other sphere with your hand, but this time, do not let the electrons flow back to your hand. This will leave the second sphere with a net negative charge.

So to summarize, by using the positively charged insulating rod and the process of induction, it is possible to simultaneously charge one sphere positively and another sphere negatively without having to recharge the rod.