# Kirchhoff's Voltage Law - Closed Loop?

288
1. The problem statement, all variables and given/known data

Determine VC for the network in Fig. 7.24 (left-hand 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:

$$V_{C}+V_{R_C}-V_{CC}=0$$

Why are we subtracting VCC?

Can somebody post a Microsoft Paint drawing to show me how Fig. 7.25 is a closed loop?

2. ### ehild

12,245
Look at the closed loops. They did not draw the battery, but it is there!

ehild

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• ###### transistorcirc.JPG
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288
People like you are what make this site among the best on the 'net. You responded within minutes of my OP. Thank you.

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. ### ehild

12,245
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

Last edited: Jun 30, 2012

288
OK, now I see it...

The branch you added to the left was there to begin with and it is the long-hand way of drawing in VCC. Correct?

6. ### ehild

12,245
I do not know what long-hand 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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