Capacitor Key: Potential Difference between K-L

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The discussion revolves around calculating the potential difference between points K and L in a circuit with capacitors, specifically when a key is open versus closed. When the key is closed, two capacitors are in parallel, resulting in a combined capacitance of 2C, and the potential difference is calculated to be 12V. When the key is open, only one capacitor is present, leading to a different combined capacitance and a potential difference of 3/5V. Participants discuss the rules for combining capacitors in series and parallel, as well as the implications of DC versus AC current, ultimately arriving at the conclusion that the potential difference when the key is open is 3/5V. The conversation highlights the complexity of the calculations and the importance of understanding capacitor behavior in electrical circuits.
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Homework Statement


It says,
The potantial difference between K-L = V , when the key is closed...
And asks what's the potantial difference between K-L, when the key is open ?

-----
You see 2 drawings,
1 above the question,
1 below the question,

I have drawed these,
these shows us the different waypaths, electric would follow as being related to the key.
But i couldn't bring this question to the solution... Can you show me ?
Exactly i don't know what i have to do too...
I did, what i could do.
 

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Hi FiskiranZeka! :smile:

(are all the capacitances the same?)

When the key is open, from K to N has only one capacitor

but when the key is closed, it has two capacitors in parallel …

what is the combined capacitance? :wink:
 
tiny-tim said:
Hi FiskiranZeka! :smile:

(are all the capacitances the same?)

When the key is open, from K to N has only one capacitor

but when the key is closed, it has two capacitors in parallel …

what is the combined capacitance? :wink:


( yeah all Capacitors are Same ( C ) )

Key Closed -->
v(KL) = V, so -> v(KN) = 2V, so -> v(MK)=4V, so -> v(MN)=12Volt...
C Combined = C+C --> 2C ; 2C + C --> 2/3 C ; 2/3 C + C --> 5/3 C

Key Open -->
The batteries are same :) So still must be 12 Volt,
C Combined = C + C --> 1/2 C ; 1/2 C + C --> 3/2 C ; 3/2 C + C --> 3/5 C
q = V.C and ? How to continue ?
Must find V (KL)
 
<< Not solved yet.
 
Hi FiskiranZeka! :smile:

I'm honestly not following what you're doing …

remember the capacitor rule: in parallel, you add the capacitances, and in series, you add the inverses of the capacitances.
 
tiny-tim said:
Hi FiskiranZeka! :smile:

I'm honestly not following what you're doing …

remember the capacitor rule: in parallel, you add the capacitances, and in series, you add the inverses of the capacitances.

Yes, i know that...
I found the Combined Capacitence for both positions ( Key open / Key closed )
Look at above...
And then ? What should i do now ?

OFF Topic;
In the country i live, education is different then you are used to see...
Those things aren't my homework :) Here we have to do these to learn the unit...
Pratically or teorically, education system doesn't prove/teach us many things in physic.
And they also don't want from us to make an electric motor or something imporant/usable in real life... So, they ask us simple questions like above and we have to learn how to solve these questions and this is enough for us to do.
Now got to learn how to solve this...
Don't expect from me to do it, 'cos in school we don't even do it in the lesson.
But we are expected to do it in exam.
 
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hey, I 'm trying to solve it but still, this is an DC or AC ?
If DC, it remains V.
If AC, a lots trouble now ( u must use triangle or Y formation to find the total C). I'll post the formula ASAP.
 
Thanks for your interrest and,
I don't know what is DC and what is AC...
Also there is no info in the question if its AC or DC...

There is a solution in book but honestly i don't understand it;
'Cos book shows me different electric waypaths ( Cycles / Contrivances ) as being compared to the 2 electric waypaths i added to the question image...( You see 1 electric waypath for closed key ( at the top of paper ), 1 electric waypath for open key ( at the bottom of paper ) Exactly those waypaths are what I'm thinking and what seems true for me )... Anyway, i couldn't understand books solution but i'll took important points from there, so you may help me on my own solution...

Book says;
In both positions ( Key On / Off ) Voltage of Cycle / Contrivance is 3 Volt...
And Correct answer is V2 = 3/5 V

By the way, what is AC / DC ?
I have seen AC / DC button on an Electric Lamp...
Was switching from Home electric to Battery Electric.

hey, I 'm trying to solve it but still, this is an DC or AC ?

* If AC/ DC is related with stable Voltage and Changable Voltage,
Then i think in this question the battery is same ( Steady / Stable ),
Voltage shouldn't change when we open the key or we close.

*Sorry for my bad english skills.
 
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HI FiskiranZeka! :smile:
FiskiranZeka said:
Key Closed -->

C Combined = C+C --> 2C ; 2C + C --> 2/3 C ; 2/3 C + C --> 5/3 C

Key Open -->

C Combined = C + C --> 1/2 C ; 1/2 C + C --> 3/2 C ; 3/2 C + C --> 3/5 C

Yes, that's right. :smile:
v(KL) = V, so -> v(KN) = 2V, so -> v(MK)=4V, so -> v(MN)=12Volt...

Sorry, I don't understand any of this. :confused:
FiskiranZeka said:
Book says;
In both positions ( Key On / Off ) Voltage of Cycle / Contrivance is 3 Volt...
And Correct answer is V2 = 3/5 V

Yes, in the key closed position you have to prove V(MN) = 3V.

Try that first, using the rules:

for capacitors in parallel, Q is the same
for capacitors in series, the Qs add to (total V)(total C) :smile:

(DC is direct current, and AC is alternating current. The voltages in these problems are always DC :wink:)
 
  • #10
I'm sorry about what i posted, it's totally wrong.
And here's why the Voltage of circle is 3:
These capacitors is the same, so all have the same Q and thus, same V then use the rule u've just said.
V(KL)=2V and V(KL)=V(KM)-> V(KM)=2V->V(MN)=2V+V=3V
 
  • #11
I will look at all these.. and check again, but I am away for a while ( not at home )
So can't check them right now... Thank you for your help...
Will check all answers and try to resolve when I am back at home.
 
  • #12
Didn't understand totally,
let me practice and see;
please check my calculations,

Look at new image;

New Image;
http://images.foraster.us/imgs/-647497902kondansator_anahtar.JPG

I'm trying to solve the main question, through Image 2 and Image 3...
First tell me, are both images drawed true ?

second;
Paralels has the same voltage;
The question says; Voltage of KL = V,
So, look at IMAGE 2;
v (KL) = V --> v ( KN ) = V too
( MN ) has 2 wires, underwire and upperwire;
v ( MN_Underwire ) = v ( KN ) + v ( MK ) { Because KN and MK are Serial }
v ( MN_Underwire ) = V + v ( MK )
q= V.C -->
v ( KN ) = V and C ( KN ) = 2C ! --> q ( KN ) = V. 2C
q ( KN ) = q ( MK ) --> V . 2C = v ( MK ) . C --> v ( MK ) = 2V !
v ( MN_Underwire ) = V + 2V = 3V

Voltage of Underwire = Voltage of Upperwire = Total Voltage ;
Voltage of System = 3V

-------------------

Now i go further to solve the problem;
IMAGE 3; (key open)
generator is same; as it is DC ...
So; System has still 3 Volt...
C_combined = 3/5 C
V2 = ?

q= V.C -->
q System = q (MK) { Because , Serial Q's are equal }
q (MK) = q (MK_Downwire) + q (MK_Upperwire)
q (MK_Upperwire) = Vupper . 0,5C
q (MK_Downwire) = Vdown . C
Vdown = Vupper ('cos parallel )
--> q (MK) = q (MK_Downwire) + q (MK_Upperwire)
--> q (MK) = Vdown.C + Vdown. 0,5C --> q (MK) = 1,5C . Vdown
q System = q (MK)
3V . 3/5 C = Vdown . 1,5C
Vdown = Vup = 6/5 V
q = V.C --> q (ML) = q (KL) = q Upper
v ( ML ) . C = v (KL) . C = 6/5 V . 1/2 C
v ( KL ) = 3/5 V
V2 = 3/5V = Solution

Huhh... Finally... Thanks... Check the maths all right ?:P
Any mental mistake ?
 
  • #13
Any mistake ?
 
  • #14
Hi FiskiranZeka! :smile:
FiskiranZeka said:
First tell me, are both images drawed true ?

second;
Paralels has the same voltage;
The question says; Voltage of KL = V,
So, look at IMAGE 2;
v (KL) = V --> v ( KN ) = V too
( MN ) has 2 wires, underwire and upperwire;
v ( MN_Underwire ) = v ( KN ) + v ( MK ) { Because KN and MK are Serial }
v ( MN_Underwire ) = V + v ( MK )
q= V.C -->
v ( KN ) = V and C ( KN ) = 2C ! --> q ( KN ) = V. 2C
q ( KN ) = q ( MK ) --> V . 2C = v ( MK ) . C --> v ( MK ) = 2V !
v ( MN_Underwire ) = V + 2V = 3V

Voltage of Underwire = Voltage of Upperwire = Total Voltage ;
Voltage of System = 3V

Yes, that's fine …

except that there's no need to say v ( MN_Underwire ) …

just say v(MN) (because the voltage difference between two points is independent of the path!)
Now i go further to solve the problem;
IMAGE 3; (key open)
generator is same; as it is DC ...
So; System has still 3 Volt...
C_combined = 3/5 C
V2 = ?

q= V.C -->
q System = q (MK) { Because , Serial Q's are equal }
q (MK) = q (MK_Downwire) + q (MK_Upperwire)
q (MK_Upperwire) = Vupper . 0,5C
q (MK_Downwire) = Vdown . C
Vdown = Vupper ('cos parallel )
--> q (MK) = q (MK_Downwire) + q (MK_Upperwire)
--> q (MK) = Vdown.C + Vdown. 0,5C --> q (MK) = 1,5C . Vdown
q System = q (MK)
3V . 3/5 C = Vdown . 1,5C
Vdown = Vup = 6/5 V
q = V.C --> q (ML) = q (KL) = q Upper
v ( ML ) . C = v (KL) . C = 6/5 V . 1/2 C
v ( KL ) = 3/5 V
V2 = 3/5V = Solution

Huhh... Finally... Thanks... Check the maths all right ?:P
Any mental mistake ?

hmm … a bit long …

you could say that C(MK) = 3/2 times C(KN),

so V(MK) = 2/3 times C(KN), so V(MK) = 2/5 V(KN)

so V(LK) = 1/2 times V(MK) = 1/5 V(KN) = 3/5 V …

oh … but isn't V(KL) = -V(LK)? :smile:
 
  • #15
hmm … a bit long …

you could say that C(MK) = 3/2 times C(KN),
understood this...
so V(MK) = 2/3 times C(KN), so V(MK) = 2/5 V(KN)
Didn't understand above;
Below must be true ? why not ?
V(MK)= 2/3 times V(KN) ... so, V(MK) = 6/5V and V(KN)=9/5 V total = 3V
so V(LK) = 1/2 times V(MK) = 1/5 V(KN) = 3/5 V …
V(LK) = 1/2 times V(MK) , V(LK)=3/5V

and yes, V(LK) = -V(KL) right. But as it seems Turks don't care about saying V(KL) or saying V(LK) :) 'cos the book says 3/5 V is true... That means they don't care for the direction of electric... But what you said is right, correct answer must be -3/5 V

* I think you made a mistake at calculating...
I solved it from the way you showed me and found different numbers then you have finden for voltage, but even i found different numbers; those numbers bringed me to the true answer.
 
  • #16
up!

up!
 
  • #17
merry fishmas!

Hi FiskiranZeka! :smile:

(i didn't reply yesterday. 'cos it was xmas, so i took the day off to do important things like eat and drink and watch doctor who, and wallace and gromit :wink:)
FiskiranZeka said:
so V(MK) = 2/3 times C(KN), so V(MK) = 2/5 V(KN)

Didn't understand above;

oops! :redface:

should have been V(MK) = 2/3 times V(KN), so V(MK) = 2/5 V(KN) :redface:
I solved it from the way you showed me and found different numbers then you have finden for voltage, but even i found different numbers; those numbers bringed me to the true answer.

Yes, the way you did it is correct also :smile:

but my reason for preferring my way is that I used the rule that in capacitors in series, V is inversely proportional to C (so you can "balance" the voltages, a bit like balancing weights in mechanics) … so I didn't need to spend time finding Q …

you did find Q, which is ok, but it takes longer! :wink:
 
  • #18
tiny-tim said:
Yes, the way you did it is correct also :smile:

but my reason for preferring my way is that I used the rule that in capacitors in series, V is inversely proportional to C (so you can "balance" the voltages, a bit like balancing weights in mechanics) … so I didn't need to spend time finding Q …

you did find Q, which is ok, but it takes longer! :wink:

No, i made a new solution (2nd solution); i didnt find Q,
C and V are inversely proportional... I used this rule to solve the problem as you teached me.

My 2nd Solution is made by your way; Using the C and V inversation :))
Look at it carefully ;

C(MK) = 3/2 times C(KN)
V(MK) = 2/3 times V(KN),
so, V(MK) = 6/5V and V(KN)=9/5 V total = 3V
V(LK) = 1/2 times V(MK) , V(LK)=3/5V

I didn't write the maths ( calculated at mind ), if you don't understand i can write the maths too so you see how i found those numbers...
 
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  • #19
FiskiranZeka said:
No, i made a new solution (2nd solution); i didnt find Q,
C and V are inversely proportional... I used this rule to solve the problem as you teached me.

My 2nd Solution is made by your way; Using the C and V inversation :))
Look at it carefully ;

C(MK) = 3/2 times C(KN)
V(MK) = 2/3 times V(KN),
so, V(MK) = 6/5V and V(KN)=9/5 V total = 3V
V(LK) = 1/2 times V(MK) , V(LK)=3/5V

I didn't write the maths ( calculated at mind ), if you don't understand i can write the maths too so you see how i found those numbers...

ah, i didn't realize you calculated it in your head …

there were so many q's i assumed you used them!

is everything ok now? :smile:
 
  • #20
Yes, everything well,
i learn both ways;

In my first solution surely there were too many Q's because i solved the problem through Q's...( Not at mind )

But the point you missed is, that after you showed me an alternative and a shorter way that, " C's and V's are opposite related " ; i made a new solution that you may missed ( you may not saw ), that new solution is the one that i calculated at mind...
its like k + 2/3 k = 5/3k , 5/3k =150, k = 90 :P

Happy x mas happy holiday thank you...
Here in Turkey there is no holiday for X mas :)
We don't celebrate it.

// Problem Solved...
 
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