# Air-filled parallel-plate capacitor question

1. Feb 24, 2004

### Aerospace

An air-filled parallel-plate capacitor is to have a capacitance of 0.4 F.
If the distance between the plates is 0.7mm, calculate the required surface area of each plate. Answer in mi^2.

Okay, so I used the Capacitance equation where C=(EoA)/d where Eo is a constant 8.85e-12 C^2/N.m^2
and A is the Area of the plates and d is the plate seperation.
So I plugged in the numbers, found the area and divided it by two to get the area of each plate. I'm getting the wrong answer and I don't know what I'm doing wrong.

2. Feb 24, 2004

### NINHARDCOREFAN

Why did you divide it by 2? The A is your final answer

3. Feb 25, 2004

### turin

Probably because the text books are quite pathetic when it comes to introducing capacitors (among other things). They say things like, "the area of the plates," and it is only natural for some students (IMO the more rigorous ones) to interpret that as the total area of both plates.

Aerospace,
The A in the equation is the area of each plate, not the total area of both plates. Don't divide by 2.