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# Very difficult capacitors question watch

1. A bomb timer consists of two capacitors in parallel, each with capacitance C=1.5μF, and a resistor of resistance R=10MΩ, connected in a circuit.
The initial charge on one capacitor is Q𝟢=30μC (the other has no charge initially).
The bomb will go off when the voltage across the pair of capacitors drops to a factor e^2.
After how long will the bomb go off?
2. (Original post by Laurasaur)
A bomb timer consists of two capacitors in parallel, each with capacitance C=1.5μF, and a resistor of resistance R=10MΩ, connected in a circuit.
The initial charge on one capacitor is Q𝟢=30μC (the other has no charge initially).
The bomb will go off when the voltage across the pair of capacitors drops by a factor e^2.
After how long will the bomb go off?
how far did you get?

the rule for capacitors in parallel is that the combined capacitance is the sum of the individual capacitances..
3. (Original post by Joinedup)
how far did you get?

the rule for capacitors in parallel is that the combined capacitance is the sum of the individual capacitances..
I got a couple of wrong answers...
4. I went

V/V0=1/e^2=e-2

V/V0=e-t/RC

-t/RC=-2
t/RC=2
t=2RC

RC=106 * (3*10-6)
RC=3.0s
2RC=6.0s

what's the answer supposed to be?
5. (Original post by Joinedup)
I went

V/V0=1/e^2=e-2

V/V0=e-t/RC

-t/RC=-2
t/RC=2
t=2RC

RC=106 * (3*10-6)
RC=3.0s
2RC=6.0s

what's the answer supposed to be?
Thanks for your answer, apparently it isn't correct though. It's a question from isaacphysics.org, so it only tells you if your answer is correct or not, but it won't give you the answer so I don't know

The thing is, I don't know if both capacitors need a voltage of V0/e2 or not. How could you get both of the capacitors to reach the same voltage at the same time and make sure it's that exact value? So I was thinking maybe you had to find the potential difference between the two capacitors and have that be V0/e2
6. (Original post by Joinedup)
I went

V/V0=1/e^2=e-2

V/V0=e-t/RC

-t/RC=-2
t/RC=2
t=2RC

RC=106 * (3*10-6)
RC=3.0s
2RC=6.0s

what's the answer supposed to be?
(Original post by Laurasaur)
Thanks for your answer, apparently it isn't correct though. It's a question from isaacphysics.org, so it only tells you if your answer is correct or not, but it won't give you the answer so I don't know
Resistance is 107 ohms, the answer should be 60s.
7. (Original post by alow)
Resistance is 107 ohms, the answer should be 60s.
Thanks for your help though, this is what some of my teachers thought too. The sf are correct and I did put the unit, s, in but it doesn't show up in the box (just a glitch)
8. (Original post by Laurasaur)
Thanks for your help though, this is what some of my teachers thought too. The sf are correct and I did put the unit, s, in but it doesn't show up in the box (just a glitch)
Could you try 39.21s.
9. (Original post by alow)
Could you try 39.21s.
CORRECT!! Thank you so much, oh my God I've been stuck on this question for months! How did you do it?
10. (Original post by Laurasaur)
CORRECT!! Thank you so much, oh my God i've been stuck on this question for months! How did you do it?
So as soon as the uncharged capacitor is connected, half of the charge is transferred to it. This means the voltage (effectively instantaneously) drops by a factor of a half. Then you need to calculate the time for the voltage to change from . i.e.

11. (Original post by alow)
So as soon as the uncharged capacitor is connected, half of the charge is transferred to it. This means the voltage (effectively instantaneously) drops by a factor of a half. Then you need to calculate the time for the voltage to change from . i.e.

I'm at the borderline of tears of joy right now, thank you!
12. (Original post by Laurasaur)
I'm at the borderline of tears of joy right now, thank you!
No worries. I haven't thought about circuits in a while so it was pretty fun.
13. FWIW I think it'd be clearer whether they started timing before or after the 2 capacitors were connected if you ran into that on an A level paper
14. (Original post by Joinedup)
FWIW I think it'd be clearer whether they started timing before or after the 2 capacitors were connected if you ran into that on an A level paper
Agreed. It's not a well worded question.
15. (Original post by Joinedup)
FWIW I think it'd be clearer whether they started timing before or after the 2 capacitors were connected if you ran into that on an A level paper
(Original post by alow)
Agreed. It's not a well worded question.
As the OP has stated the question is not an A-level question. If we look at the original question, I think the wording is clear. Hopefully, the image would display properly after submission.
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