Two coaxial conducting tubes are 30 cm long. The outer diameter of the inner tub

Question

Two coaxial conducting tubes are 30 cm long. The outer diameter of the inner tube is 3 cm, the inner diameter of the outer tube is 4 cm. When these are connected to 45 volt battery, how much energy is stored in the electric field between the tubes?

I found the answer here : http://www.chegg.com/homework-help/questions-and-answers/two-coaxial-aluminum-tubes-30-cm-long-outer-diameter-inner-tube-3-cm-inner-diameter-outer--q3806543

But in that page there are 4 different answers I'm not sure which one is correct. Can someone do this problem with steps so I know how to do it?

Explanation / Answer

energy stored is U = 0.5*eo*E^2*volume of tube

Electric field is E = V/d = 45/(0.01) = 4500 N/C

then U = 0.5*eo*E^2*pi*(R^2-r^2)*l

U = 0.5*8.85*10^-12*4500^2*3.142*(0.04^2-0.03^2)*0.3 = 5.91*10^-8 J

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