Dielectric in capacitor question....

[elec123]

Hi,

I've been given this question, and i can't get to the answer and it's driving me mad (prob bein very slow!)

So here it is...

A parallel plate, air filled capacitor is connected to a voltage source of constant voltage. The electric field strength between the plates is then 50kV/m. The plate area is 25cm^2. The space between the plates is now filled with a dielectric of relative permittivity 4.0.

Find the charge on the plates of the capacitor

Here's how i have tackled it...

The resultant electric field in the dielectric is 50kV/m.
Get the area in SI units so A = (25*10-2)^2 = 0.0625m^2

Used the formula E= Q / (Er.E0.A )

Rearranged : Q = E.Er.E0.A

with E= 50,000 Er = 4 E0 = 8.85*10^-12

with these values i get Q = 1.1*10^-7

when the correct answer is Q = 4.4*10^-9

Any help on this question would be much appreciated.....

Thanks

 

[Roff]

You are misinterpreting the area. If you use A=25e-4 (25 sq. cm.), it works out. In other words, the plates are not a square, 25 cm on a side. They are a square, 5 cm on a side, or a rectangle, 2.5 cm X 10 cm, or...

 

[Oznog]

I remember asking an instructor about a problem where the capacitance of a IC region was stated in terms of pF/cm^2, but the actual way it was written was a bit more ambiguous.

I wanted to be sure if the parameter was pF per sq cm, or if this was pF per 1/100th of a sq meter (like I say, there was something odd about the particular way he'd written it that I don't recall).

He insisted they were the same thing and couldn't understand why I would ask such a nonsense question...