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.....
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...
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...