# Electric charge question

1. Apr 27, 2014

### cuongbui1702

Electric charge question[Updating...]

1)
I tried to do this homework, but my answer is all , of course it is not as same as this question's answer. That is my procedure:
+) First, 2 charged have a same chagre Q, after these also have a same charge q=(Q-Q').
+) F'/F=[(Q-Q')/Q]^2.
+) I substituting all answer, and it is correct all, i can have 1 Q'.

2)
F=Q.q/4piE(a^2+d^2)
F max when a^2+d^2 min=> d^2 min when d=0 why the answer is E, i think a true answer is A

3)
Following my textbook:

it is also existing an electric field inside a insulator. So the answer E is not correct, because it still have Electric force. I think A is true. Am I right?

Last edited: Apr 27, 2014
2. Apr 27, 2014

### Staff: Mentor

Let Q be the initial charge on A and B. (Express the initial force F in terms of Q.) When C touches A, what are the resulting charges on A and C? Then when C touches B, what are the resulting charges on C and B? Once you have the final charges on A and B expressed in terms of Q, then find an expression for the new force between A and B.

3. Apr 27, 2014

### cuongbui1702

To get a resulting charges on A and C or B and C i think it is quite hard, or i dont know to calculate it

4. Apr 27, 2014

### Staff: Mentor

No, it's easy. And unless you know how it works, you cannot solve the problem.

Here's how it works: When two identical conducting spheres are touched, the total charge distributes uniformly between the two of them.

5. Apr 27, 2014

### cuongbui1702

Oh thank you, that is amazing thing i have learnt today, i got the answer, it is as same as with this( sorry about my terrible English). Why i cant see this in my text book University Physics with Modern Physics, 13th Edition of Young and Freeman

6. Apr 27, 2014

### cuongbui1702

Who can help me Question2 and 3, please

7. Apr 27, 2014

### Staff: Mentor

You need an expression for x-component of the force, not the total force.

(Don't keep adding problems to the original post. That makes things confusing to follow and respond to. Best to solve one problem before starting another; usually best to have one problem per thread, unless they are very related.)

8. Apr 27, 2014

### Staff: Mentor

No, you are not right. The illustration from the book describes a uniformly charged sphere, but the problem is about a spherical shell (a balloon).

9. Apr 27, 2014

### cuongbui1702

- Oh, I read wrong problem :(. But, when i did it again, my process is long(i use derivatives), maybe in this question, i substituting all answers is faster than my process.
- I am scared, admin delete my post when i make a lot of question :(

10. Apr 27, 2014

### cuongbui1702

What is the difference between them??

11. Apr 27, 2014

### Staff: Mentor

A balloon is hollow shell with nothing inside. The text from your book describes a solid sphere of charge, not just a shell.

12. Apr 27, 2014

### Staff: Mentor

I suggest you actually work it out with derivatives; it's not that bad and it simplifies quickly.

13. Apr 27, 2014

### cuongbui1702

Because English is my second language, so that it is hard for me to recognize what it is, if i dont have pictures :D

14. Apr 28, 2014

### cuongbui1702

Doc AI, i have a same problem but my answer is not as same as question's answer. Following you, it must be A.

15. Apr 28, 2014

### Staff: Mentor

In this example, the conducting spheres are not identical. (The more general principle is that the potential will be the same after they touch.)