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# help needed.... fundmental question

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#### pbs681

##### Full Member level 3
Hi all,

How can we assume voltage sources are replaced by wires (shorts), and all current sources with open circuits (breaks) in circuit analysis? I mean, what is the justification that we use to short circuit the voltage source and open circuit the current source.... please clarify... thanks in advance

hey....
when u say that we replace voltage source with wires or current sources wid open ckt..means that we r talkin abt the ideal votlage or current sources.
so, let us first concern abt the voltage sourse...here we connect the voltage source in series wid the rest of the ckt. so, if we want this source to haev no effect on the rest of the ckt, wat is should be the condition? it should be havin zero resistance, so that the over all resistance of the ckt will be same n inc. is in current is only because of the addition of voltage n no change in resistance. hence, we consider voltage source to be replaced wid the short ckt.

on similar lines goes the argument for curreent source. as we connect it in parallel wid the rest of the ckt, for it to not to have any effect on the ckt is that it should have infinite resistance so that i doesn't draw any current from the ckt. hence, can be replaced by open ckt.

rakesh_vnit said:
hey....
when u say that we replace voltage source with wires or current sources wid open ckt..means that we r talkin abt the ideal votlage or current sources.
so, let us first concern abt the voltage sourse...here we connect the voltage source in series wid the rest of the ckt. so, if we want this source to haev no effect on the rest of the ckt, wat is should be the condition? it should be havin zero resistance, so that the over all resistance of the ckt will be same n inc. is in current is only because of the addition of voltage n no change in resistance. hence, we consider voltage source to be replaced wid the short ckt.

on similar lines goes the argument for curreent source. as we connect it in parallel wid the rest of the ckt, for it to not to have any effect on the ckt is that it should have infinite resistance so that i doesn't draw any current from the ckt. hence, can be replaced by open ckt.

Hey man, thanks a lot!!

Hi,
Voltage sources can be considered as an infinite capacitor and current sources as an infinite inductor. So at zero frequency, DC, a voltage source is a short circuit connection and a current source is an open circuit connection.

every source is modelled as a ideal source and a resistance to model the variations... if you take voltage source the voltage doesnt vary with current and when u make the source as zero how can you model something which doesnt vary with current it is a short circuit... similar argument for current source where the source is modelled with a parallel resistance...

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