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# 2Cs, 3Rs and 4 hours of "I have no clue about capacitors" watch

1. https://isaacphysics.org/s/E4o9f7

Okay the 4 hours is a small exaggeration but I don't know to do this and I keep trying new methods and nothing seems to work because frankly I don't think I understand what is going on.

I got the first question right without an issue but the second one I can't seem to get. So I know that the capacitance adds up like how resistors add up in parallel. The thing is I don't think I quite understand how the voltage works across them? I think there is a difference in potential between the two capacitors but I don't really want to say. And then there is fact how current flowing through one half of the circuit is opposite through R3 to the current in the other half of the circuit when you use the other capacitor as the source. So does the this reverse current seem to cancel out or?

Yeah some better understand would be appreciated thanks.
2. Haven't tried to solve it yet tbh... I'm just going out

But the rule for 2 capacitors in series is that they both hold the same amount of charge - cos the 'middle' plates are isolated from the rest of the circuit.
So the any charge coming off the right hand plate of the left hand capacitor has to go into the left hand plate of the right hand capacitor (and the same for charge going the other way of course)

The important consequence of this is that series capacitors of different values will charge up to different voltages because C=Q/V - but those voltages have to sum to give the supply voltage of 2.4V

more on capacitors in series and parallel here
http://farside.ph.utexas.edu/teachin...es/node46.html
3. (Original post by DrSebWilkes)

https://isaacphysics.org/s/E4o9f7

Okay the 4 hours is a small exaggeration but I don't know to do this and I keep trying new methods and nothing seems to work because frankly I don't think I understand what is going on.

I got the first question right without an issue but the second one I can't seem to get. So I know that the capacitance adds up like how resistors add up in parallel. The thing is I don't think I quite understand how the voltage works across them? I think there is a difference in potential between the two capacitors but I don't really want to say. And then there is fact how current flowing through one half of the circuit is opposite through R3 to the current in the other half of the circuit when you use the other capacitor as the source. So does the this reverse current seem to cancel out or?

Yeah some better understand would be appreciated thanks.

I would outline how can you go about attempting part b.

1. First, find the p.d. across each capacitor.

2. Then treat each capacitor as a cell with the voltage you find in (1).

3. Apply Kirchhoff’s current and voltage law to the circuit to find the current through R3.
4. (Original post by Joinedup)
But the rule for 2 capacitors in series is that they both hold the same amount of charge - cos the 'middle' plates are isolated from the rest of the circuit.
So the any charge coming off the right hand plate of the left hand capacitor has to go into the left hand plate of the right hand capacitor (and the same for charge going the other way of course)
Yeah I did this and now it makes sense actually - big thanks for that!

(Original post by Eimmanuel)
I would outline how can you go about attempting part b.

1. First, find the p.d. across each capacitor.

2. Then treat each capacitor as a cell with the voltage you find in (1).

3. Apply Kirchhoff’s current and voltage law to the circuit to find the current through R3.
I wasn't sure how to solve it so I gave it a stab taking one current away from the other by treating each "side" as its own circuit. I got the right answer according to Issac but I'm not really convinced by my method.

Is that how to solve these questions where there is a load in between two batteries and each in between other loads?
5. (Original post by DrSebWilkes)
Yeah I did this and now it makes sense actually - big thanks for that!

I wasn't sure how to solve it so I gave it a stab taking one current away from the other by treating each "side" as its own circuit. I got the right answer according to Issac but I'm not really convinced by my method.

Is that how to solve these questions where there is a load in between two batteries and each in between other loads?
Could you post your working and say what is the stuff that you are uncertain according to your working? I am not sure what are you not convinced of.

Kirchhoff voltage and current rules are valid for DC circuits.

If you want to find the current through R3 at any time, it is a bit tedious. You would need to solve differential equations. I think you are seldom asked to solve this kind of problem at A level, but you can be given such problems in the first year of university study in physics or engineering.
6. (Original post by Eimmanuel)
Could you post your working and say what is the stuff that you are uncertain according to your working? I am not sure what are you not convinced of.

Kirchhoff voltage and current rules are valid for DC circuits.

If you want to find the current through R3 at any time, it is a bit tedious. You would need to solve differential equations. I think you are seldom asked to solve this kind of problem at A level, but you can be given such problems in the first year of university study in physics or engineering.

Basically this but it was pure luck
7. (Original post by DrSebWilkes)
Yeah I did this and now it makes sense actually - big thanks for that!

I wasn't sure how to solve it so I gave it a stab taking one current away from the other by treating each "side" as its own circuit. I got the right answer according to Issac but I'm not really convinced by my method.

Is that how to solve these questions where there is a load in between two batteries and each in between other loads?
Well it's got a fancy name... superposition theorem and it's a valid thing to do when you haven't got any non linear components e.g. diodes in circuit.
8. (Original post by DrSebWilkes)

Basically this but it was pure luck
It may be "pure luck" that you had guessed the superposition theorem. But if you have learnt the Kirchhoff rules, it should be a systematic ways for you to use to solve DC circuit problems.

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