Some questions

19.  Use the fact that the stored energy is (1/2)*Q²/C  where capacitance  C = εA/d

 and ε is permittivity, A is area and d is distance between plates.

You can work out Q from the fact that you are given the number of Coulombs per unit area (specified in terms of microCoulombs per unit area)) and you know the radius of a plate so you can find its area. 

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Any help? 

Alan?  

Melody?

Sorry 315, my knowledge is not adequate for your questions :)

Thank you Alan .

Thank you Melody .