Find an Expression For the Voltage

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Homework Help Overview

The discussion revolves around finding an expression for voltage in a circuit involving capacitors, specifically using the concept of equivalent capacitance and charge relationships.

Discussion Character

  • Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants explore the calculation of equivalent capacitance and the relationship between charge and voltage across capacitors. Questions arise regarding the correctness of the original poster's expressions and whether alternative methods, such as using complex impedance, could yield the same results.

Discussion Status

Several participants provide feedback on the original poster's calculations, with some suggesting alternative approaches and clarifying misunderstandings about the relationships between charge, capacitance, and voltage. There is an ongoing exploration of different interpretations and methods without reaching a consensus.

Contextual Notes

Participants are working within the constraints of an assignment, which may limit the information available and the approaches they can take. There is a focus on ensuring the accuracy of the mathematical relationships involved in capacitor circuits.

mmmboh
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Hey, so I have an assignment and this is one of the questions on it, I did it but I am not sure if what I did is right.
2chpe38.jpg


What I did was find Ceq=(C1C2)/(C1+C2))

So then Q=CV(t)=(C1C2)/(C1+C2))V(t)

Then I found the voltage across C2=Q/C2=(C1C2)/(C1+C2))V(t)/C2=(C1)/(C1+C2))V(t)

And then V0=V(t)-(C1)/(C1+C2))V(t)

Can anyone tell me if what I did is right or if I am completely off?
 
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mmmboh said:
Hey, so I have an assignment and this is one of the questions on it, I did it but I am not sure if what I did is right.
2chpe38.jpg


What I did was find Ceq=(C1C2)/(C1+C2))

So then Q=CV(t)=(C1C2)/(C1+C2))V(t)
For starters, shouldn't it be Q = CVT = (C1C2VT)/(C1+C2)?
 
mmmboh said:
Hey, so I have an assignment and this is one of the questions on it, I did it but I am not sure if what I did is right.
2chpe38.jpg


What I did was find Ceq=(C1C2)/(C1+C2))

So then Q=CV(t)=(C1C2)/(C1+C2))V(t)

Then I found the voltage across C2=Q/C2=(C1C2)/(C1+C2))V(t)/C2=(C1)/(C1+C2))V(t)

And then V0=V(t)-(C1)/(C1+C2))V(t)

Can anyone tell me if what I did is right or if I am completely off?
You could have just found the voltage across [tex]c_1[/tex] like you did for [tex]c_2[/tex] and stopped there.: [tex]\frac{q}{c_1}=v_o=v(t) \frac{c_2}{c_1+c_2}[/tex]
Your expression you found simplifies to that anyway:
[tex]v_o=v(t)[1 - \frac{c_1}{c_1+c_2}][/tex]
[tex]v(t)[\frac{c_1+c_2}{c_1+c_2} - \frac{c_1}{c_1+c_2}]= v(t) \frac{c_1+c_2 - c_1}{c_1+c_2}=v(t) \frac{c_2}{c_1+c_2}[/tex]
I don't know if you know about complex impedance yet but you can also use that to arrive to the same answer:
[tex]Z_T = Z_C_1 + Z_C_2[/tex]
[tex]Z_T = \frac{1}{\omega c_1}+\frac{1}{\omega c_2}[/tex]
These impedances work like resistance with v = IR. Therefore,voltage division works too.
[tex]v_o = v(t) \frac{ Z_C_1}{Z_T}[/tex]
[tex]v_o = v(t) \frac{ \frac{1}{\omega c_1}}{\frac{1}{\omega c_1}+\frac{1}{\omega c_2}}[/tex]
Omegas cancel and you come to the same answer.
 
Last edited:
Leptos said:
For starters, shouldn't it be Q = CVT = (C1C2VT)/(C1+C2)?
Just for the record: no, Q=CV for a capacitor, as the OP said.
 

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