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Name That Charge - help7

There are some fundamental principles that one will have to understand and use in order to successfully analyze all seven Name That Charge situations. Those fundamental principles are ...

Electrons are negatively charged and protons are positively charged.
Only electrons are able to move during electrostatic charging processes.
Opposites attract and likes repel. Thus, electrons are attracted to positively-charged objects and repelled by negatively-charged objects and by each other.
If a neutral object gains electrons, then it becomes negatively charged. If a neutral object loses electrons, then it becomes positively charged.

There are a few nearly-identical versions of this question. Two of the versions include:

Version 1:

Three neutral, metal cans - mounted on insulating stands - are touching. A negatively-charged balloon is brought near Can A. Can B is then removed. What is the charge on Can A? What is the charge on Can C?

Polarization Step	Charging Step
A charged balloon is brought near to a neutral object (Can A) which is touched to a third object (Can B) and to a fourth object (Can C).	When Can B is removed, there are two charged cans.

Version 2:

Three neutral, metal cans - mounted on insulating stands - are touching. A positively-charged balloon is brought near Can A. Can B is then removed. What is the charge on Can A? What is the charge on Can C?

Polarization Step	Charging Step
A charged balloon is brought near to a neutral object (Can A) which is touched to a third object (Can B) and to a fourth object (Can C)	When Can B is removed, there are two charged cans.

This question is an example of charging by induction. Charging by induction typically involves a two-step process - a polarization step and a charging step. In the polarization step, a neutral object is brought near to a charged object . This induces a movement of electrons within the neutral object. In this situation, Cans A, B and C act as a system of objects with electrons flowing out of one of the cans through the middle can and to the opposite can. Think about fundamental principle #3 (above) to determine which direction electrons will move. Will electrons in the can furthest from the balloon be attracted towards the balloon or will electrons in the can closest to the balloon be repelled by the balloon into the can that is furthest away?

As electrons are attracted towards the balloon or repelled by the balloon, the system of three cans is polarized. Charge within the cans is separated into opposites. The charging step simply involves the removal of the middle can so that there are two distinctly charged cans.

In this process, there is NEVER any transfer of electrons between the charged object and the neutral object that is being charged. Thus the charged object will keep the same amount and type of charge that it originally had.

Charging by Induction