Help with school problem

[shorteddiode]

I have been fighting with this problem for hours. If I am thinking right if C3 shorts out it would be like the cap wasn't there and a wire was there instead.
So I figured C2 and C4 as parellel caps and added them together and which makes 0.02uF. The figured C1 and the 0.02 as a series circuit. I keep getting 0.12 as an answer, which as you can see is not an option. What am I doing wrong??
If all the caps are good wouldn't the capacitance be 0.15uF? If not I screwed up the other question too.
I hope someone can explain this to me.
Thanks in advance

[mariushm]

Re: Help with school problem

I would say 0.3 uF - the short would make C2 and C4 useless.

Without short, the three 0.1 uF would be equal to 0.3 uF as they're in parallel. So a fictional C5 = C2 + C3 + C4 = 0.3 uF

Then, C1 and C5 are in series .. which means 1 / C total = 1/C1 + 1 / C2 or 1/Ct = (C1 + C2 )/ C1xC2 => Ct = C1xC2 / (C1+C2) = 0.09/0.6 = 0.15 uF

However, when the middle cap is shorted, you're left with just C3, 0.3uF

[tom66]

Re: Help with school problem

Infinite. A short circuit would appear like an infinite capacitive load. It would never store any charge (Q = CV) because the capacitance being so large would make V zero (ignoring series resistance etc.)

[mariushm]

Re: Help with school problem

Well, I put a 330uF and 3 100uF on a breadboard and tested it, then I removed the center 100uF and shorted with a wire.

With the original circuit, the capacity was 167 uF, which is the right value for 330uF and 300uF in series.

After shorting, the capacity between 1 and 2 resulted in the capacity of C1.

[tom66]

Re: Help with school problem

Ah, misread the question. The circuit can be treated as a short circuit (wire) connected to the single series capacitor C1, so the capacitance would be C1, 0.3uF.

[budm]

Re: Help with school problem

It will be 0.3 (the value of C1) since C2 C3, C4 will become just a piece of wire connecting node #2 (you can short C2, or C4 it does not matter) to the bottom leg of the C1.
This circuit can be resistors also with the same result. You shorted out the component, that component will no longer has any affect.
You can easily proof it with the real circuit.

[shorteddiode]

Re: Help with school problem

Thanks guys,, I think I understand

[shorteddiode]

Re: Help with school problem

so what you are saying is the short would talk the parellel circuit out of the equation?

[budm]

Re: Help with school problem

Yes.

[shorteddiode]

Re: Help with school problem

Thanks Guys, the little light came on and it makes sense now.

[budm]

Re: Help with school problem

Great!

[shorteddiode]

Re: Help with school problem

OK, let me ask just one more question. If it had been C2 or C4, would it have the same result?

[Tube_Dude]

Re: Help with school problem

OK, let me ask just one more question. If it had been C2 or C4, would it have the same result?

Absolutely!

[shorteddiode]

Re: Help with school problem

Thanks for all the help everyone