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#130283 - 03/02/06 04:27 PM superposition theorem  
james S  Offline
Member
Joined: May 2003
Posts: 107
West England
Am i right in saying that you can't use superposition theorem on ac networks unless j notation is used?
I have a parallel network with the components and supply sources labled in j notation,so to avoid messy equations i have tried to convert these values into impedances and supply voltages (with no phase angles) , and continue like you would in a dc network.
The reason why i think you can't do this is because there are inductive and capacitive currents meeting each other changing the characteristics of the circuit currents.Is this correct?


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#130284 - 03/02/06 05:23 PM Re: superposition theorem  
Bob  Offline
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Joined: Feb 2002
Posts: 182
Mobile, AL, USA
Not sure what you are doing but you can not work with caps and inductors in a DC circuit.


#130285 - 03/02/06 07:55 PM Re: superposition theorem  
winnie  Offline
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Joined: Sep 2003
Posts: 649
boston, ma
"j" notation is not strictly necessary, but you must do _something_ to account for the energy stored and released from the reactive components. using the concept of 'phase angle' and "j" notation is a method for doing this accounting that works quite well when you are working at a single frequency with fixed value components.

Another technique that you could use to figure out these sort of circuits is to calculate how the current changes with time, using equations that have time components (eg V*sine(t) for voltage, C*dV/dt for the current through the capacitor, etc) . The "V/I characteristic" of components ("resistance" generalized) changes with time, so V = I * V/I (Ohm's law) quickly becomes a pretty serious differential equation. This sort of approach is generally not necessary for 'human' calculations, but is quite often used in computer simulations. Also the basic concepts of how a capacitor works are more easily understood in terms of thinking of change in charge over time, so it is a nifty exercise to go from I=C*dV/dt (and the other associated equations) to phase angle and reactance notation.

-Jon


#130286 - 03/02/06 08:28 PM Re: superposition theorem  
james S  Offline
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Joined: May 2003
Posts: 107
West England
Thanks jon for your quick reply!
you have just confirmed my thinking of such circuits.Since posting the problem i have worked out the different branch currents using j notation (it took a while),all the text books i have dont venture passed the DC sinario,and say things like this method can be employed for AC circuits changing the resistance for reactance,hence my initial assumption.
thanks again

bob
"and continue like you would in a dc network."
i know you can't work with caps/inductors in dc ,i meant the working out to my problem is treated like a DC circuit.



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