Finding Final Flux in a Flat Coil-Ring System

[zorro]

Homework Statement

A flat coil of area A and n turns is placed at the centre of a ring of radius r(r²>>A) and resistance R. The two are coplanar. When current in the coil increases from 0 to i, the total charge circulating in the ring is?

The Attempt at a Solution

dQ=dΦ/R

Initial flux = 0
How do I find out the final flux through the ring?

 

[hikaru1221]

To find flux, find B and the effective area: $$\Phi = \oint \vec{B}d\vec{S}$$
To find B, find current of the B source and its geometry: B~i

 

[zorro]

B at the centre of the coil is μnI/2a, where a is the radius of the coil.
Flux through the bigger ring is B*A.
The problem I am facing is that the radius of the ring is given, not that of the coil.

Edit: Is something wrong with the question?

 

[hikaru1221]

So what?

 

[zorro]

lol...so what?
The final answer contains 'a' (not given in the question) not 'r'.

 

[hikaru1221]

We have the area of the coil A, right?
r is redundant here I guess.

 

[zorro]

Even your 'guesses' are always 100% correct
Thanks!