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Home Concept Builders Static Electricity Getting the Hang of Charge Help Help 15

Getting the Hang of Charge - Questions 15 Help

The type of charge an object possesses can be determined by observing its interactions with other objects of known charge. To make such a determination, you need to combine the observations with three simple rules regarding charge interactions:

  • Objects that possess opposite types of charge will be observed to attract each other.
  • Objects that possess the same type of charge will be observed to repel each other.
  • A neutral object and a charged object will always attract each other.

There are two similar versions of this question. Here are the two versions:

Version 1:

Three balloons are hung from the ceiling by threads. Balloon A is positively-charged. Balloons A and B attract each other. Balloons B and C attract each other. Balloon C is ________.
 
Version 2:
Three balloons are hung from the ceiling by threads. Balloon A is negatively-charged. Balloons A and B attract each other. Balloons B and C attract each other. Balloon C is ________.

It's All About Reasoning
Like a lot of our Concept Builders (but maybe more so with this one), you have some thinking and logical reasoning to do. The physics rules are straight-forward:

  • opposites attract,
  • likes repel, and
  • charged and neutral attract.

But the application of these simple rules can get complicated. Get yourself organized and prepare to reason.


The Game Plan
This question is about Balloon C. The charge of Balloon A is stated. But Balloon C does not interact with Balloon A; it only interacts with Balloon B. So you must first figure out what charge Balloon B has from its interaction with Balloon A. The charge of Balloon A is stated. Observe how it interacts with Balloon B. Ask yourself if the diagram displays attraction or repulsion. Then look up the corresponding rule - they're listed in the paragraph above - It's All About Reasoning. It's important to note that there are two rules for explaining why attraction occurs. There's only one rule for repulsion. There's more on the logical significance of this in the next few paragraphs.


Repulsion Provides Evidence for Clear Conclusions
There is only one reason for why two ballons repel - they are both charged and have the same type of charge. But in this question there is no repulsion occurring - not between Balloons A and B nor between Balloons B and C. So given the lack of repulsion, you could prepare yourself for a multitude of possible conclusions regarding the charge on Balloon C. Read on.
 



Careful about Attraction
There are two reasons for why two objects attract. They could be oppositely charged or one could be charged and the other be neutral. And so if you observe that Balloons A and B attract, that means Balloon B could be charged (opposite of Balloon A) or neutral. You will not know which it is until you have more information about B's interaction with another object.

The only other interaction of Balloon B is its interaction with Balloon C. And that is also an attractive interaction ... which always results in the least narrowing of conclusions. So as you try to figure out the charge type of Balloon C, do two logical "runs" - one for the possibility that Balloon B could be neutral and one for the possibility that Balloon B could be charged (the opposite of Balloon A). As long as you consider both Balloon B possibilities, you will get this question correct.

Try this link to The Physics Classroom Tutorial for more help with understanding charge interactions:

Charge Interactions

Concept Builder Teacher Notes Directions