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Name That Charge - help5

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 ...

  1. Electrons are negatively charged and protons are positively charged.
  2. Only electrons are able to move during electrostatic charging processes.
  3. Opposites attract and likes repel. Thus, electrons are attracted to positively-charged objects and repeled by negatively-charged objects and by each other.
  4. 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:

Two neutral, metal cans - mounted on insulating stands - are touching. A negatively-charged balloon is brought near Can A. The cans are then separated and the balloon is moved away. What is the charge on Can A? What is the charge on Can B?

Polarization Step

Charging Step
A charged object is brought near to a neutral object (can A) which is touched to a third object (can B).
When the cans are separated, there are two charged objects.


Version 2:

Two neutral, metal cans - mounted on insulating stands - are touching. A positively-charged balloon is brought near Can A. The cans are then separated and the balloon is moved away. What is the charge on Can A? What is the charge on Can B?

Polarization Step

Charging Step
A charged object is brought near to a neutral object (can A) which is touched to a third object (can B).
When the cans are separated, there are two charged objects.

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 and B act as a system of objects with electrons flowing out of one of the cans and into the other cans. Think about fundamental principle #3 (above) to determine how electrons move. The charging step involves the separation of the two cans so that there are two distinctly charged objects.

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