# Parallel plate air capacitor

1. Feb 11, 2012

### thunderjolt

A parallel-plate air capacitor is made by using two plates 16 cm square, spaced 3.7 mm apart. It is connected to a 12-V battery. What is the capacitance? What is the charge on each plate? What is the electric field between the plates? What is the energy stored in the capacitor?

What I believe to be relevant equations:
C=KC_0=Kε_0*A/d
Q=CV
E=Q/(ε_0*A)
U=.5QV=.5CV^2
K for air is1.00059

3. The attempt at a solution:
Really I'm just stuck on the first part. I tried C=KC_0=Kε_0*A/d, and put in C=(1.00059*8.854*10^-12*.0016/.0037)

2. Feb 11, 2012

### ehild

Yes, evaluate.

ehild

3. Feb 11, 2012

### Staff: Mentor

Be careful with the area of the plates, particularly in converting to square meters.

Is "16 cm square" the same as 16 cm2, or does it mean (16 cm) x (16 cm)?

4. Feb 11, 2012

### thunderjolt

That's the exact question word-for-word, so I can't tell either. The answer I got from doing the calculation was 3.83pF, which is wrong.

5. Feb 11, 2012

### Staff: Mentor

Suggest you try the other interpretation.

6. Feb 11, 2012

### thunderjolt

The other option also resulted in a wrong answer. 6.13*10^-11F using an area of (.16m)^2. I've 3 tries left.

7. Feb 11, 2012

### Staff: Mentor

Does the system want the answer in any particular units? Significant figures?

8. Feb 11, 2012

### thunderjolt

It asks for Farads, and to put everything as XXX*10^Z. I have done so. I've used Mastering Physics all last year, so I know how to deal with the inputs.

Is there some special integral or another equation that I could try?

P.S. The way I'm doing the problem is exactly like the example in the book.

9. Feb 11, 2012

### Staff: Mentor

The equation is correct, and it appears to me that you've obtained a correct answer. If the method of data entry is also correct then it is possible that the machine's answer key is wrong. This has been known to happen from time to time. You should report it to your course instructor.

You might want to see if the capacitance value obtained returns correct results for other parts of the problem.