Capacitance of concentric circles

[adichy]

Homework Statement

i have a graph or r against V and to make it a straight plot i then plotted ln(r) against V,

Homework Equations

The Attempt at a Solution

the problem is i dnt know how to find the capacitance from the graph. it definitely isn't the gradient or the point of interception since the capacitance needs to be in the pF. I've already worked out what the capacitance should be theoretically i.e. using C= 2$$\pi$$$$\epsilon$$/ln (r/a) and it came out to be 40pF

any advice would be appreciated. Thankyou

 

[Mindscrape]

You know that C is defined as Q/V. Nobody can see your graph so nobody will really be able to help you at all.