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Analyzing a circuit with multiple batteries

  • #1
Krushnaraj Pandya
Gold Member
697
71

Homework Statement


Find charge on 3 capacitors.
Join two squares with one common side. At the middle of the common edge, place C3=6uF (micro-farad).
On upper edge of left square, place C1=2uF. On upper edge of right square place C2=4uF. On lower edge of left square, a battery-10V and on lower edge of right square 20 V
Photo attached, but quality of camera is not really good.

Homework Equations


1)sum of capacitance=equivalent C when in parallel.
2)reciprocal addition when in series
3) all other related to capacitance

The Attempt at a Solution


This is the first time I've encountered multiple batteries in a circuit. Note-Current electricity is the next chapter in our textbook, so I'm not very familiar with Kirchhoff's laws etc. i.e. just the basics.
My intuition was that either two currents will flow from both or a single one from 20 V to to 10 V, how can I know for sure?
I'd greatly appreciate some help, thank you
 

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Last edited:

Answers and Replies

  • #2
cnh1995
Homework Helper
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Find charge on 3 capacitors.
Join two squares with one common side. At the middle of the common edge, place C3=6uF (micro-farad).
On upper edge of left square, place C1=2uF. On upper edge of right square place C2=4uF. On lower edge of left square, a battery-10V and on lower edge of right square 20 V
Instead of this description, a circuit diagram would be much more helpful, for you and for the helpers. It is going to be difficult for the helpers to explain and take you through the solution without a diagram. You can post the diagram using the UPLOAD button below the text-editor.

Homework Equations


All related
That's one of the important pre-requisites for posting in the HH forum. You need to post the relevant equations, with an attempt at a solution. Without any attempt from the OP, members are not allowed to help them.
 
  • #3
Krushnaraj Pandya
Gold Member
697
71
Instead of this description, a circuit diagram would be much more helpful, for you and for the helpers. It is going to be difficult for the helpers to explain and take you through the solution without a diagram. You can post the diagram using the UPLOAD button below the text-editor.

That's one of the important pre-requisites for posting in the HH forum. You need to post the relevant equations, with an attempt at a solution. Without any attempt from the OP, members are not allowed to help them.
Alright. I was avoiding a picture due to my poor webcam quality. I'll write out my intuitions in attempt at a solution and post a picture. Thank you.
 
  • #4
Krushnaraj Pandya
Gold Member
697
71
Anyone?
 
  • #5
ehild
Homework Helper
15,427
1,824

Homework Statement


Find charge on 3 capacitors.
Join two squares with one common side. At the middle of the common edge, place C3=6uF (micro-farad).
On upper edge of left square, place C1=2uF. On upper edge of right square place C2=4uF. On lower edge of left square, a battery-10V and on lower edge of right square 20 V
Photo attached, but quality of camera is not really good.

Homework Equations


1)sum of capacitance=equivalent C when in parallel.
2)reciprocal addition when in series
3) all other related to capacitance

The Attempt at a Solution


This is the first time I've encountered multiple batteries in a circuit. Note-Current electricity is the next chapter in our textbook, so I'm not very familiar with Kirchhoff's laws etc. i.e. just the basics.
My intuition was that either two currents will flow from both or a single one from 20 V to to 10 V, how can I know for sure?
I'd greatly appreciate some help, thank you
As relevant equations, you need the one how the voltage across a capacitor depends on the charge and capacitance. The other one is the addition of voltages, (potential differences) that is, if UAB is the voltage across points A and B and UBC is the voltage across B and C, then the voltage across A and C is UAC=UAB+UBC.
 
  • #6
ehild
Homework Helper
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1,824
Alright. I was avoiding a picture due to my poor webcam quality. I'll write out my intuitions in attempt at a solution and post a picture. Thank you.
You certainly have some drawing program on your computer, Paint, for example. Make a drawing, copy, and paste it into your post, like I did :

upload_2018-8-8_6-48-59.png
 

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  • #7
Krushnaraj Pandya
Gold Member
697
71
You certainly have some drawing program on your computer, Paint, for example. Make a drawing, copy, and paste it into your post, like I did :

View attachment 229030
That's a really good idea! I'll do that from next time. Thanks a lot for drawing this :D
 
  • #8
Krushnaraj Pandya
Gold Member
697
71
As relevant equations, you need the one how the voltage across a capacitor depends on the charge and capacitance. The other one is the addition of voltages, (potential differences) that is, if UAB is the voltage across points A and B and UBC is the voltage across B and C, then the voltage across A and C is UAC=UAB+UBC.
The edit button is gone, I can't add the equation or your drawing...
 
  • #9
ehild
Homework Helper
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1,824
The edit button is gone, I can't add the equation or your drawing...
Write an other post with the new equations and drawing. You can copy my picture and paste into your new post.
 
  • #10
Krushnaraj Pandya
Gold Member
697
71
Write an other post with the new equations and drawing. You can copy my picture and paste into your new post.
how do I delete this one though?
 
  • #11
Delta2
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You said Kirchoff's laws are in the next chapter of your textbook, I have big trouble understanding how we can solve this without using Kirchoff's laws.

Using kirchoff's laws ,specifically KVL, you can make one equation for the closed loop that contains C1, C0 and 10V, another equation from the closed loop that contains C2,C0 and 20V, and the third equation will come from KCL and will be an equation between the charges on the capacitors Q0,Q1 and Q2.
 
  • #12
ehild
Homework Helper
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how do I delete this one though?
No need to delete. Just post an answer with picture, equation and attempts.
 
  • #13
ehild
Homework Helper
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You said Kirchoff's laws are in the next chapter of your textbook, I have big trouble understanding how we can solve this without using Kirchoff's laws.

Using kirchoff's laws ,specifically KVL, you can make one equation for the closed loop that contains C1, C0 and 10V, another equation from the closed loop that contains C2,C0 and 20V, and the third equation will come from KCL and will be an equation between the charges on the capacitors Q0,Q1 and Q2.
Kirchhoff's Laws refer to voltages and currents, but you can apply more basic rules here: The definition of voltage between two points as the work done by the electric field between those points, and as work, it is additive.
The other basic law is that charge can not be created or destroyed. If three capacitor plates are connected, and no charge was given to the junction, the net charge on the plates remains zero. Kirchhoff's Laws follow from these more basic Laws, + Ohm's Law.
 
  • #14
Delta2
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Kirchhoff's Laws refer to voltages and currents, but you can apply more basic rules here: The definition of voltage between two points as the work done by the electric field between those points, and as work, it is additive.
The other basic law is that charge can not be created or destroyed. If three capacitor plates are connected, and no charge was given to the junction, the net charge on the plates remains zero. Kirchhoff's Laws follow from these more basic Laws, + Ohm's Law.
Thanks ehild, I had almost forgot that KCL is a consequence of the principle of conservation of charge, and KVL is a consequence of principle of conservation of energy.
 
  • #15
Krushnaraj Pandya
Gold Member
697
71
You said Kirchoff's laws are in the next chapter of your textbook, I have big trouble understanding how we can solve this without using Kirchoff's laws.

Using kirchoff's laws ,specifically KVL, you can make one equation for the closed loop that contains C1, C0 and 10V, another equation from the closed loop that contains C2,C0 and 20V, and the third equation will come from KCL and will be an equation between the charges on the capacitors Q0,Q1 and Q2.
Alright, so I've come back after learning Kirchhoff's Laws. Simple question- Why do we consider only the current coming from 10 V while considering the loop containing C1, C0 and 10V and similarly for the other loop.
Isn't a part of the current from the other battery also entering the second loop?
 
  • #16
Delta2
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first of all in the equilibrium state all the currents will be zero. But in the transient state, yes there will be part of current from one battery entering the other. To see this clearly, what equation can you make for the currents, using KCL at point A (or at point B)?
 

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