Hello everyone, I'm stuck on a mutliple choice question: A parallel-plate capacitor has a plate area of 0.3 m2 and a plate separation of 0.1 mm. If the charge on each plate has a magnitude of 5 10-6 C then the force exerted by one plate on the other has a magnitude of about:(adsbygoogle = window.adsbygoogle || []).push({});

1 E4 N

9 E5 N

5 N

9 N

0

Here is what i did:

I know the E-field right outside a conductor is E = (Q/A)/Eo <--permitivity. So i found E = (5e-6/.3)/8.85e-12 = 1883239 N/C; Then i knew F = QE, so I said, F = (5e-6)(1883239) = 9.4N, which was wrong, then i thought, hm..maybe i messed up a unit somehow, so i tried 9e5N, also wrong, then i thought, well if they are both positive, that means the net force is going to be 0, because ulike charges repel, same charge, 0 net force. also wrong. What did i do wrong?

I also ran into odd problem, i'm wondering if the homework problem is wrong...

A 20 F capacitor is charged to 200 V. Its stored energy is:

4000 J

0.1 J

2000 J

0.4 J

4 J

Easy enough, U = .5*CV^2;

U = .5(20)(200)^2 = 400000J;

The answer is .4J though?

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# Homework Help: Finding the force exerted by one plate on another

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