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RC circuits and potential difference 
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#1
May712, 03:13 PM

P: 16

1. The problem statement, all variables and given/known data
Circuit is at this link: http://www.colorado.edu/physics/phys...s/gian1946.gif 1. What is the potential at point a with S open (let V=0 at the negative terminal of the source, and assume it's a long time after the circuit was connected to the potential difference source). 2. What is the potential at point b with the switch open? Again assume it's a long time after the circuit was connected. 3. After the switch is closed for a long time, what is the final potential of point b? I know what the answers are (some I solved through a happy coincidence, and some I was told the answer to this problem has been discussed on PF and yahoo answers already) but I don't understand why the answers are what they are. I think I'm confused about the foundation of this problem. See questions below. 2. Relevant equations V = IR C = Q/C Capacitors in series have same charge RC circuits are timedependent Resistors in series are additive 3. The attempt at a solution 1. I got this one right myself, but when I looked back at it I confused myself. The answer is, as you can find elsewhere on the internet: Va = ItotR2 = VtotR2/(R1+R2) from V = IR, Itot = Vtot/Rtot, and the knowledge that resistors in series are additive and that the same current goes through both of them. However, why is the voltage across a the voltage across R2, and not R1? Also, am I right in thinking that all current goes through the resistors, since it's been a long time since the circuit was connected and the capacitors are fully charged? 2. Using Vtot = V1 + V2 and V = Q/C and Q1 =Q2 when in series, Vb = V2 = Q/C2 and so forth... you can get Q in terms of the total voltage and total capacitance. My confusion is related to above: why do we use C2, and not C1 in the equation Vb = Q/C2? 3. Same as #1, because the capacitors become fully charged and all I again goes through the resistors. This one I understand. So, I think I'm missing something really elementary but it would be helpful to me if someone lets me know my dumb mistake. 


#2
May712, 04:00 PM

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#3
May712, 09:02 PM

P: 16

Thanks again! 


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