Value of Charge Q: Why Use Distance not Radius?

[random_user13]

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?

 

[Nathanael]

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?

 

[Luke Cohen]

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).

 

[random_user13]

That makes sense now. Definitely mixed up how that was being used.