so u considr ur arrangement to b  an isolated metal conductor ........we know that isolated capacitors exist and the other positive plate serves the purpose of a missing plate.....

but still  there is contradiction here coz  in an isolated conducter which is assigned a capacitance by considering a  considering sphere of infinite radius where the electric field lines wud end but if they gotta end then it means they r negatively charged isn't it??

It IS a capacitor. The capacitance of an object dpends only upon its geometrical construction and not the charge on it. Even a single plate can be a capacitor, with the other plate at infinity... At first, this may not seem so, as c = q / v. But keep in mind that this V is not the absolute potential, it is the FALL in potential across the capacitor, which is 0 in this case. Hence, q=0 and v (or rather, ) = 0, so c = 0/0 which is some finite value...

Hope this clears your doubt

EDIT: it is NOT necessary that the opposite plates have opposite charge, even unequal charges of same sign will do. However, having the same charge does not mean the system ceases to be a capacitor. It is merely the same state as an uncharged capacitor

Capacitor- A system of 2 conductors embedded in a homogenous dielectric.

Capacitance- Ratio of the magnitude of the total charge on either conductor to the potential difference between the conductors.

The V used in the formula, is the potential difference between the 2 plates.

Now in your question, If both the plates have equal positive charge, then potential difference = 0. Means capacitance is infinite.

P.S.- The above definitions and explanations are self written but written after consulting the following books- H.C.Verma, Resnick Halliday Walker, Engineering electromagnetics by William H. Hayt.

Now, as the formula says capacitance is infinite.

Logical explanation - If the 2 plates have opposite charges, we say capacitance is not infinite but some finite value. TRUE!!! Becoz capacitance means capacity to store charge. Now when the 2 plates have opposite charges, there is electric field between them and if u keep increasing charge on them, at some point, the electric field will increase so much that electric breakdown of air/insulator takes place and now there is no insulator between the plates. Now it cant store any more charge.

For a capacitor having same charge on both plates, there is no electric field between the plates, so u can keep storing as much charge as u want. Hence, its capacity becomes infinite.

Hope it helps.

ya, it is not a capaitor at all

ok, x4 sent me the diagram with the ACTUAL qn; here it is:

Now, his original qn was: what is the capacitance of the system of plates II and III. He feels that it should be infinite, whereas i feel that now, IN THIS CASE since the two plates are conected, they do NOT form a capaticitive system by themselves. Plz comment

Well, this question is easy I guess.

First of all in such questions, we dont talk of plates as capacitors. The sides of plates act as capacitors and not the plates themselves.

For example, in the figure given by you, there will be 2 capacitors[ (1,2) & (5,6) ] in parellel and hence the equivalent capacitance between A and B wud be 2C.

Now as far as (3,4) is concerned, I think its a controversial question becoz in solving such circuits, we dont take them to be a capacitor. And the reason- as given by astatine19, that it, the 2 plates are connected to each other, so its a single conductor actually. How can it act as a capacitor??

Plz comment. It can be a gud question for discussions and arguements.