Kirchhoff's Voltage Law  Closed Loop?by JJBladester Tags: circuit, closed loop, kirchhoff, kvl, voltage 

#1
Jun3012, 07:47 AM

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1. The problem statement, all variables and given/known data
Determine V_{C} for the network in Fig. 7.24 (lefthand image). 2. Relevant equations Kirchhoff's Voltage Law: The algebraic sum of the potential rises and drops around a closed path (or closed loop) is zero. 3. The attempt at a solution This is an example problem in my book "Introductory Circuit Analysis" by Boylestad, 12th Ed., page 256. I understood KVL in the chapter on series dc circuits. There, closed loops were obvious. My hangup with this question is I don't see how Fig. 7.25 represents a closed loop. The book states: [tex]V_{C}+V_{R_C}V_{CC}=0[/tex] Why are we subtracting V_{CC}? Can somebody post a Microsoft Paint drawing to show me how Fig. 7.25 is a closed loop? 



#2
Jun3012, 08:01 AM

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Look at the closed loops. They did not draw the battery, but it is there!
ehild 



#3
Jun3012, 08:11 AM

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So is there a rule of thumb that I can use when I run into this dilemma? What about the circuit told you that the battery was definitely hooked up as in your image? I mean, it would be feasible to draw the battery into the circuit a different way (as in below) and get a different result, right? 



#4
Jun3012, 08:21 AM

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Kirchhoff's Voltage Law  Closed Loop?
Your circuit is different from the original one. The emf of your battery is 2 V, Vcc =22 V can not be supplied with such an arrangement, without an additional battery.
The potential must be 22 V at the point shown with respect to the ground, which is at zero potential. That is the maximum potential shown: there must be a battery with that emf. The real transistor circuits have some supply voltage, a battery. That is what I drew. The Base voltage is obtained by the voltage divider R1 and R2 in series. I know it because I have made a lot of such circuits... ehild 



#5
Jun3012, 08:22 AM

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OK, now I see it...
The branch you added to the left was there to begin with and it is the longhand way of drawing in V_{CC}. Correct? 



#6
Jun3012, 08:32 AM

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I do not know what longhand way is, but the battery was there. Maybe, not really a battery, but some other appliance for power supply: transformer connected to 230 V ac, with rectifier and stabiliser and so on, with output 22 V dc.
ehild 


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