Calculating Charge & Electric Energy Above a Plane

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bg646
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Hello, I am stuck with this one. Your help/comments would be most appreciated.

A charge "+Q" lies at a distance "d" above an infinitely large conducting plane. Applying the method of images, i found the right result that the total charge of the infinite plane is "-Q" and that the stored electric energy is:
[tex]W_{E}=-\frac{Q^{2}}{8\pi\epsilon_{0}d}[/tex]
Since the stored electrostatic energy also satisfies:
[tex]W_{E}=CV^{2}/2[/tex]

it seems that the capacitance of the system would be negative which must be non sense. Therefore the charge above the plane is *not* a capacitor.

Is this correct or am i missing something?
Thanks for your help
 
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