Hi
Please, I need help to understand this issue. I made some exercises on Thévenin equivalent but I'm stuck in this one.
Here I attached two files taken off from O'Malley's book ("Basic circuit analysis" edited by schaum...) (pages 103104)
I have two doubts:
First  why we consider the 30 V from 30 voltage if we DON'T consider the 8resistor? "The 822 resistor has no effect on this voltage because there is zero current flow through it as a result of the open circuit." It is ok, but if there is no current why we must consider the 30 V to calculate V th?
I= V / R if there is no current therefore 0 A = 30V / R internal (of the voltage source). Confused. :
Second  Why you cannot consider the "5R and 4R resistors" to calculate the Vth? O'Malley said that "Notice that the 5R and 4R resistors have no effect on VTh." but no explanation given.
I thought that the correct would be:
(V  100)/10 + V / 40 + 20 + V / 5 = 0 (the 4resistor was not used, because we have the current known... I think.)
Why not put Vth = 20 * 4 = 80 V? Directly... I confused about the adding of 30 V if there is no current in that voltage source...
Concerning Rth, no problem.
Thanks for your help,
Jorge Almeida
Please, I need help to understand this issue. I made some exercises on Thévenin equivalent but I'm stuck in this one.
Here I attached two files taken off from O'Malley's book ("Basic circuit analysis" edited by schaum...) (pages 103104)
I have two doubts:
First  why we consider the 30 V from 30 voltage if we DON'T consider the 8resistor? "The 822 resistor has no effect on this voltage because there is zero current flow through it as a result of the open circuit." It is ok, but if there is no current why we must consider the 30 V to calculate V th?
I= V / R if there is no current therefore 0 A = 30V / R internal (of the voltage source). Confused. :
Second  Why you cannot consider the "5R and 4R resistors" to calculate the Vth? O'Malley said that "Notice that the 5R and 4R resistors have no effect on VTh." but no explanation given.
I thought that the correct would be:
(V  100)/10 + V / 40 + 20 + V / 5 = 0 (the 4resistor was not used, because we have the current known... I think.)
Why not put Vth = 20 * 4 = 80 V? Directly... I confused about the adding of 30 V if there is no current in that voltage source...
Concerning Rth, no problem.
Thanks for your help,
Jorge Almeida
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