I need some help with these problems:(adsbygoogle = window.adsbygoogle || []).push({});

1) A parallel-plate vacuum capacitor has 7.36 J of energy stored in it. The separation between the plates is 3.64 mm. If the separation is decreased to 1.40 mm, what is the energy stored if the following events occur?

a) the capacitor is disconnected from the potential source so the charge on the plates remains constant

b) the capacitor remains connected to the potential source so the potential difference between the plates remains constant

I have the equations U=.5CV^2=.5QV=(Q^2)/(2C)

.5CV^2/Ad

C=Epsilon_0(A)/d, V=Ed, u=.5Epsilon_0*E^2

How do I apply them to this problem?

2) A capacitor has parallel plates of area 20 square centimeters separated by 1.4 mm. The space between the plates is filled with polycarbonate (dielectric constant = 2.8, dielectric strength = 3*10^7)

I correctly calculted the permittivity (2.479E-11) and maximum permissible voltage (42000). I need help finding the surface-charge density on each plate and the induced surface charge density on the surface of the dielectric. I really don't know where to start with this, so any help would be appreciated!

Thanks for help with one or both questions,

eku_girl83

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# Homework Help: Capacitors and Dielectrics

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