1. Jan 26, 2016

### xxchickapooxx

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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 eachother 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 eachother 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!

2. Jan 26, 2016

### BvU

Hello chick,

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 ?

3. Jan 26, 2016

### xxchickapooxx

Until the charge is the same on both objects?

4. Jan 26, 2016

### BvU

Correct (we assume the objects have approximately the same capacity to hold charge).

What are the respective charges then ?

5. Jan 26, 2016

### xxchickapooxx

Neutral I suppose?

6. Jan 26, 2016

### BvU

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 )

7. Jan 26, 2016

### xxchickapooxx

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

8. Jan 26, 2016

### Ray Vickson

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).

9. Jan 27, 2016

### collinsmark

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.

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.

10. Jan 27, 2016

### collinsmark

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?

Last edited: Jan 27, 2016