What is the correct answer for the electric charge question?

[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'.
Why the answer is C.

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?

 

[Doc Al]

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'.
Why the answer is C.

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.

 

[cuongbui1702]

To get a resulting charges on A and C or B and C i think it is quite hard, or i don't know to calculate it

 

[Doc Al]

To get a resulting charges on A and C or B and C i think it is quite hard, or i don't know to calculate it

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.

 

[cuongbui1702]

Oh thank you, that is amazing thing i have learned today, i got the answer, it is as same as with this( sorry about my terrible English). Why i can't see this in my textbook University Physics with Modern Physics, 13th Edition of Young and Freeman

 

[cuongbui1702]

Who can help me Question2 and 3, please

 

[Doc Al]

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

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.)

 

[Doc Al]

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?

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).

 

[cuongbui1702]

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.)

- 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 :(

 

[cuongbui1702]

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).

What is the difference between them??

 

[Doc Al]

What is the difference between them??

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

 

[Doc Al]

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

 

[cuongbui1702]

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

Because English is my second language, so that it is hard for me to recognize what it is, if i don't have pictures :D

 

[cuongbui1702]

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.

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

 

[Doc Al]

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

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