The Sum of Capacitance Question Help Pls

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Obolinda
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#1
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#1
I have to sum the capacitance (The answer is 400 and I keep getting 300). no idea how else to approach lol
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sorry! no idea y it's upside down...
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BlueChicken
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Do you not have 3 in series which are in parallel with the other one? Second picture.
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Obolinda
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(Original post by BlueChicken)
Do you not have 3 in series which are in parallel with the other one? Second picture.
the second picture was me trying to draw what I thought was an equivalent circuit to make the question easier to do. this is the original q:
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BlueChicken
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(Original post by Obolinda)
the second picture was me trying to draw what I thought was an equivalent circuit to make the question easier to do. this is the original q:
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oh, I meant second, if we turn both pictures as are the correct way up! That's the picture I was referring to.

So, imagine 2 capacitors top and bottom of the picture are drawn next to the one on the left - there are 3 capacitors in series, which are in parallel with the one on the right. Just looking at the answer, the maths appears to be [1/(3/300)] + 300, which would tie with the above. Does that make sense?
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Obolinda
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(Original post by BlueChicken)
oh, I meant second, if we turn both pictures as are the correct way up! That's the picture I was referring to.

So, imagine 2 capacitors top and bottom of the picture are drawn next to the one on the left - there are 3 capacitors in series, which are in parallel with the one on the right. Just looking at the answer, the maths appears to be [1/(3/300)] + 300, which would tie with the above. Does that make sense?
that makes sense thank you! i just don't know how id answer a question like this in the future, how did you know to think of the circuit as 3 capacitors in series parallel to 1 capacitor ?
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Joinedup
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I think you've probably not paid enough attention to where the test points are in relation to the components...


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Joinedup
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... so you need to combine the caps in series that I've called B,C &D

and then combine that equivalent value with the capacitor A that it's in parallel with.
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BlueChicken
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(Original post by Obolinda)
that makes sense thank you! i just don't know how id answer a question like this in the future, how did you know to think of the circuit as 3 capacitors in series parallel to 1 capacitor ?
So, first way, practice! The logical way, is to consider the current/charge going through them (these are how the laws are derived). In the diagram, assume current is flowing from R to S (assume they are just points and R and S don't refer to anything - I feel like they threw those in as a trick). The top junction, i.e. the black dot at top, the current will split, i.e. some current will go through 3 of the resistors in a line and the rest will go through the resistor on the right (here, we don't care about sizes of current/charge, just where it is going). Therefore, 3 are in series and are in parallel with the other one.

This link has pictures for what I mean about the charge (therefore, current) splitting: https://courses.lumenlearning.com/ph...-and-parallel/
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Obolinda
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(Original post by Joinedup)
I think you've probably not paid enough attention to where the test points are in relation to the components...


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(Original post by BlueChicken)
So, first way, practice! The logical way, is to consider the current/charge going through them (these are how the laws are derived). In the diagram, assume current is flowing from R to S (assume they are just points and R and S don't refer to anything - I feel like they threw those in as a trick). The top junction, i.e. the black dot at top, the current will split, i.e. some current will go through 3 of the resistors in a line and the rest will go through the resistor on the right (here, we don't care about sizes of current/charge, just where it is going). Therefore, 3 are in series and are in parallel with the other one.
ahh... i see. i think i get this now. thank you very much guys, you are life savers !!! :heart:
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Joinedup
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#10
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Maybe think of circuit diagrams as being like tube maps with the components for the stations *

if the train comes in from the left and goes right at the first junction it has to go through B and then C and then D... cos those 3 are in series.


* The guy who designed the 'iconic' modern london tube map was IIRC an electrical engineer
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