Value of Charge Q: Why Use Distance not Radius?

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Homework Help Overview

The problem involves two charged volleyballs tethered by strings, with a focus on understanding the appropriate distance to use in calculations related to electrostatic forces. The context is drawn from a specific problem in Purcell's Electricity and Magnetism textbook.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster questions the use of distance between the charges versus radius in the context of calculating electrostatic forces. Some participants seek clarification on what is meant by radius and its relevance to the problem.

Discussion Status

Participants are exploring the definitions and implications of distance and radius in the context of Coulomb's law. There is a recognition of confusion regarding terminology, and some clarification has been provided regarding the relationship between radius and distance in this context.

Contextual Notes

The original poster references a specific problem setup and diagram, which may not be fully clear to all participants. There is an indication that the diagram may have issues that affect understanding.

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Homework Statement



this is problem 1.4 from purcells electricity and magnetism 2nd edition.

Two volleyballs, mass 0.3 kg each, tethered by nylon strings and charged by an electrostatic generator, hang as shown in the diagram. which cam be found here http://www.fysik.su.se/grulab//LabDataBas/2005/2005_9_27_14_21_10/R1_03.pdf
the height is 2.5 m and distance between the charges 0.5 m the diagram is a little messed up.
What is the charge on each in coulombs assuming the charges are equal? When trying to work this problem, I ended up just looking up the solution online here. i was getting close to the answer. I used the radius instead pf the distance between the two charges. Hence my question:

I don't understand why the distance between the two charges is used. Should the radius not be used?
 
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random_user13 said:
I don't understand why the distance between the two charges is used. Should the radius not be used?
Used for what? For finding the electrostatic force between them? And by radius you mean the length of of the string?
 
Radius is the same thing as distance. In Coulomb's force equation, radius refers to the distance between the two point charges, not half the distance (like the definition of radius you are thinking).
 
That makes sense now. Definitely mixed up how that was being used.
 

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