Why Does the Force Between Two Charged Objects Change After Contact?

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The discussion centers on the behavior of two charged objects when they come into contact and then are separated. Initially, one object has a charge of +8 and the other -4, leading to an attractive force. Upon contact, the charges redistribute, resulting in both objects having equal charges, which can be less than their initial values. This redistribution can lead to a scenario where the force after separation (F2) is smaller than the initial force (F1), depending on the specific charges involved. The overall conclusion emphasizes that the change in force depends on the initial conditions and the final distribution of charges after contact.
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I was wondering if you could help me explain a physics problem. The topic is Electrostatics: We have two objects that attract each other and the distance between them is r and the force is F1. Then we put those two objects together and then put them back to the same distance as before (r).

Why is the new force F2 smaller than F1 and why do the two object repel each other now?

Well I think that as you move the objects together, the positively charged particles move to one object and the negatively ones go to the other one. But I do not understand why F2 is smaller than F1? Shouldn't it be the same because you don't change the distance, you don't take any charge away?

Thank youu!
 
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Hello chick, :welcome:

Maybe it helps to try a numerical example:

Suppose one is charged +8 and the other (number 2) is charged -4 before contact.
What, do you think, happens when they are brought into contact ?
As you say, charge is exchanged; until ... ? When does the charge stop moving ?
 
BvU said:
Hello chick, :welcome:

Maybe it helps to try a numerical example:

Suppose one is charged +8 and the other (number 2) is charged -4 before contact.
What, do you think, happens when they are brought into contact ?
As you say, charge is exchanged; until ... ? When does the charge stop moving ?
Until the charge is the same on both objects?
 
Correct (we assume the objects have approximately the same capacity to hold charge).

What are the respective charges then ?
 
BvU said:
Correct (we assume the objects have approximately the same capacity to hold charge).

What are the respective charges then ?
Neutral I suppose?
 
In post #1 you concluded
xxchickapooxx said:
you don't take any charge away
So if you start with +8 and -4 = +4 in total, and
  1. you don't take any charge away, and on top of that
  2. both objects have equal charge,
then what are the respective charges ?

(and subsequently: what does that mean for the force F2?)

PS have to run now (sports :smile:)
 
BvU said:
In post #1 you concluded
So if you start with +8 and -4 = +4 in total, and
  1. you don't take any charge away, and on top of that
  2. both objects have equal charge,
then what are the respective charges ?

(and subsequently: what does that mean for the force F2?)

PS have to run now (sports :smile:)

Are they +6 and -6... Or I think I got really confused
 
xxchickapooxx said:
Neutral I suppose?

You start with charges of +8 and -4 before contact. What is the total (that is, NET) charge? What will you see after contact? The answer follows logically from material you have probably already seen (or I hope you have seen).
 
xxchickapooxx said:
Neutral I suppose?

Not neutral, but rather equal to each other. (This assumes of course that both objects are conductors having symmetrical geometries. If the objects are of different sizes and shapes it gets more complicated. Let's just assume for this exercise that both objects are identical, conducting spheres). You had it right in post #3.

xxchickapooxx said:
Are they +6 and -6... Or I think I got really confused

Not equal and opposite to each other. Just equal to each other (again this assumes symmetrical geometries and such).

So what's the net charge left over after the objects come in contact with each other? Divide that leftover charge equally between the two. :wink:
 
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xxchickapooxx said:
But I do not understand why F2 is smaller than F1?

Oh, and by the way, I wanted to point out that in general, F_2 is not always smaller than F_1. It depends upon the specific initial conditions which force will be larger. Sometimes F_2 will be smaller than F_1, but it can be larger, depending on how the numbers work out.

I hate to bring up new examples before the previous example was finished, but it might help to first consider even simpler examples than the one @BvU introduced. I wholeheartedly apologize now if this ends up adding confusion.

Example 2:
One object has a charge of 8 units and the other has a charge of -8 units. What are the forces before and after the balls come in contact with each other and are separated again? You may answer qualitatively rather than plugging the numbers into formulas.

Example 3:
One object has a charge of 8 units and the other has a charge of -8.001 units. What are the initial and final forces in this case? (Again, qualitative answers are fine.)

Example 4:
One ball initially has a charge of 8 units and the other has a charge of 0 units (i.e., the other ball is initially neutral). What happens this time?
 
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