Help with RC circuits SPDT switch

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Discussion Overview

The discussion revolves around analyzing an RC circuit with a SPDT switch, focusing on the behavior of the circuit after the switch moves, particularly the voltage across capacitors and the current through a resistor. The scope includes homework-related problem-solving and technical reasoning regarding circuit behavior in response to switching actions.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states that the voltage across both capacitors is 10V at t=0- and t=0+, and proposes a general equation for voltage over time.
  • Another participant questions the assumption of V(infinity) being 0V, suggesting that if both capacitors have the same voltage, there would be no potential difference to drive current through the resistor.
  • A participant agrees that there would be no current, arguing that the disconnection of the voltage source leads to an open circuit for the capacitors in DC, implying that current would be 0A.
  • One participant reiterates that with no current flow, the charges on the capacitors cannot change, supporting the idea that the potentials remain constant.
  • Another participant suggests that the current is zero because the voltages across the two capacitors cancel each other out, leading to no potential difference across the resistor.

Areas of Agreement / Disagreement

Participants generally agree that there is no current flowing through the resistor due to the equal voltages across the capacitors, but there is disagreement regarding the value of V(infinity) and the implications of the circuit's behavior after disconnection from the voltage source.

Contextual Notes

There are unresolved assumptions regarding the behavior of the circuit after the switch is moved, particularly concerning the definition of V(infinity) and the impact of the capacitors' voltages on current flow.

asdf12312
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Homework Statement


Find v(t) after t=0, when the switch moves up
mj7pqg.png


need to know if i did this right.. V(C1)=V(C2)=10V (this is same for t=0- and 0+)
V(infinity)=0V (no voltage source)

T=RC=100k(ohm)*(4uF)=0.4 sec. (i got the 4uF by putting the 2 caps. in parallel with each other, because they are in series after t=0.)

v(t)=v(infinity)+[v(0)-v(infinity)]*e^(-t/T) <-- general equation
V(t)=10e^(-2.5t)V
where i got -2.5t by dividing -t/0.4

please help!

Homework Equations


The Attempt at a Solution

 
Last edited:
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Rethink your V(infinity). If both VC1(0+) and VC2(0+) are the same (10V at t = 0+), what potential difference exists to drive a current through the 100 kΩ resistor?
 
well there wouldn't be any current would there? and i thought V(infinity) would go to 0V once the voltage source gets disconected. the caps. in dc are replaced by an open circuit so i believe current would also be 0A.
 
asdf12312 said:
well there wouldn't be any current would there? and i thought V(infinity) would go to 0V once the voltage source gets disconected. the caps. in dc are replaced by an open circuit so i believe current would also be 0A.

With no current flow, the charges on the caps cannot change...
 
sorry for the long delay, was waiting for my teacher to post homework answers because i was unsure myself.
20g1sug.png


according to this, i(t) is 0. because i think the 2 capacitors voltage cancel out? 10V-10V=0V.
 
asdf12312 said:
sorry for the long delay, was waiting for my teacher to post homework answers because i was unsure myself.

:
:​

according to this, i(t) is 0. because i think the 2 capacitors voltage cancel out? 10V-10V=0V.
Yup. A more technically satisfying way to put it would be that because the potentials across the two capacitors are identical, the potential difference across the 100kΩ resistor is zero so that no current will flow through it. And since no current flows, the charges (and thus the potentials) on the capacitors will remain constant.
 

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