# isaac physics

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#1
Does anyone know how to do this question? Sorry i don't really understand capacitors with AC voltage supply/

https://isaacphysics.org/questions/c...d-5d7cabf77076
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1 month ago
#2
(Original post by amell1234)
Does anyone know how to do this question? Sorry i don't really understand capacitors with AC voltage supply/

https://isaacphysics.org/questions/c...d-5d7cabf77076
Half looked at it. As the question is (partially about) capacitors and ac voltage, what do you understand and what is causing problems in the question?
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#3
(Original post by mqb2766)
Half looked at it. As the question is (partially about) capacitors and ac voltage, what do you understand and what is causing problems in the question?
Hi sorry I'm kinda confused by all of it. I tried using Q=CV which gives Q=CVmaxsin(wt) but i was then unsure of what to do next as I tried subbing in wt =2pi but that just gives the maximum charge in the capacitor. I am also unsure of how the resistor affects it.
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1 month ago
#4
(Original post by amell1234)
Does anyone know how to do this question? Sorry i don't really understand capacitors with AC voltage supply/

https://isaacphysics.org/questions/c...d-5d7cabf77076
Try a search - I've seen this question before.

Basically, the capacitor charges over the first quarter cycle, then barely discharges over the last 3/4, keeping an essentially constant voltage across R. You need to calculate the average current over that first, quarter cycle, to get the charge that passes, then add that from the constant voltage over the other 3/4 cycle.
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#5
(Original post by RogerOxon)
Try a search - I've seen this question before.

Basically, the capacitor charges over the first quarter cycle, then barely discharges over the last 3/4, keeping an essentially constant voltage across R. You need to calculate the average current over that first, quarter cycle, to get the charge that passes, then add that from the constant voltage over the other 3/4 cycle.
Ahh yes I've found the other thread on this question. Thank you so much that is so helpful. I have tried it but i'm still getting the wrong answer. Here is my working:
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1 month ago
#6
(Original post by amell1234)
Ahh yes I've found the other thread on this question. Thank you so much that is so helpful. I have tried it but i'm still getting the wrong answer. Here is my working:
Check your precision and rounding. I don't know what you're taught to use, but it looks slightly off to me.
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#7
(Original post by RogerOxon)
Check your precision and rounding. I don't know what you're taught to use, but it looks slightly off to me.
ahh yes you're right my calculations actually give 75.56 microC which to 2sf would be 76, but that still seems to be wrong?
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1 month ago
#8
(Original post by amell1234)
ahh yes you're right my calculations actually give 75.56 microC which to 2sf would be 76, but that still seems to be wrong?
Sorry - I don't see an error. I got slightly different values using very high precision for pi, but I don't know what precision they expect.
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1 month ago
#9
(Original post by amell1234)
Ahh yes I've found the other thread on this question. Thank you so much that is so helpful. I have tried it but i'm still getting the wrong answer. Here is my working:

For the first T/4, the charges through the resistor is 13 μC.
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1 month ago
#10
(Original post by RogerOxon)
Sorry - I don't see an error. I got slightly different values using very high precision for pi, but I don't know what precision they expect.
In terms of μC, 2 sig fig.
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