Either there is a mistake in this lecture or I am confused.  Help? - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: October 12, 2023, 06:56:35 pm
content: I am using the Dilgent channel to learn ckt analysis.I think I am clear on passive signs, but I do not understand how the lecturer gets -6w for the current source in this screen capture. If the current is flowing out of the same positive node as designated for V+, why is P not equal +2X+3???Thanks,Dewey

author: AnalogTodd
author_rank: Regular Contributor
timestamp: October 12, 2023, 08:48:53 pm
content: Quote from: watchmaker on October 12, 2023, 06:56:35 pmThe way to think about it is this: circuit elements will either supply or dissipate power. Passive components, by definition, can only dissipate power. Active components can either supply or dissipate power. In a closed system like this, the supplied power must be equal to the dissipated power.Now, review KCL on the node at the top of the current source; I1+I2+Isrc = 0 by KCL. If I1 and I2 are +1A and +2A that means Isrc = -3A. Power dissipated in (2) is +1A * 2V = 2W, power in (1) and Vsrc together must be +2A * 2V = 4W, and power dissipated in the current source is -3A * 2V = -6W, meaning the current source isn't really dissipating power but instead supplying power to everything else. Once you have this, you can calculate that power dissipated in (1) is +2A * 1V = 2W, which means Vsrc must be dissipating 4W - 2W = 2W.Try changing the voltage across (1) to 3V and redo the calculations. You should now find that power dissipated in (1) is +2A * 3V = 6W, therefore power in Vsrc is 4W - 6W = -2W, which means the voltage source is supplying power instead of dissipating.

author: watchmaker
author_rank: Unknown
timestamp: October 12, 2023, 09:15:30 pm
content: Thanks. Once I drew in all the current arrows, I could see the opposition in the current source!DUH. I have to do this step by step.I have the same problem when I do not take my time with Zth and Eth. I have to label everything.Got lots of time, at least I am not under pressure for class time and such. Much less pressure when you are retired. No electives, no parties, no girlfriends, etc.

author: IanB
author_rank: Super Contributor
timestamp: October 12, 2023, 09:36:25 pm
content: A much easier way to think about this is that power balances apply to the system as a whole. So don't worry about nodes, just worry about arithmetic and keep track of signs. Resistors always consume power, so their power consumption is always positive. This gives you 2 W + 2 W = 4 W.Now the voltage source 3A could consume or provide power depending on which way the current is flowing. In this case the current is flowing in the same direction as the voltage, so it is acting as a power supply, and confusingly, it's power "consumption" is -6 W.Now the entire power in the system must add up to zero, so we have 4 W − 6 W + Pvs = 0, and that means that Pvs is 2 W. Since positive means power consumption in this example, the voltage source Vs is a sink and is consuming (absorbing) 2 W.

author: watchmaker
author_rank: Unknown
timestamp: October 13, 2023, 10:14:38 am
content: The main issue is I used to be able to "see" things easily. Now, when doing an analysis of a bridge ckt, I really have to take my time and pay attention to where the load was and redrawing the bridge. I cannot rely on my younger ability to "see", I have to draw everything, label and verify again.The thing that made me a decent watchmaker is I could "see" the forces at work. Now of, they are imprinted my brain but that does not help with maintenance of cognition. It is simple recall.Not a complaint at all, it is a puzzle to be solved. And the cognitive exercise is the primary reason I am doing this. Like doing trail work for my body. But probably more fun except less physically social.