yes i know that i must use kvl and kcl the question was which are independent of all kcl and kvl. i 've understood about kcl but have some problems sometimes to choose the correct kvl, but i think I've understood now
Well I hope so, I'm only trying to help.
Just remember that Kirchoff's theorems are only a means to an end.
They provide a formal and efficient way to analyse some circuit networks. There are other methods as well.
Analysing circuit networks is really about solving a system of simultaneous equations.
To do this you have to
1) Have the same number of equations as unknowns
2) Include at least one instance of each of the required unknowns in at least one of these equations.
3) Ensure that none of the equations must be a simple multiple of any other equation.
4) Ensure that you cannot form any of the equations by a simple combination of the others.
The answer to your original question and the others asked in this thread lie in knowing and applying this.
KCL cannot be used by itself because it contains no voltage terms so you cannot fully solve a network with any number of KCL equations.
You can obtain voltage drops from Ohm's law, but that does not tell you generator (battery) voltages.
The answer to the question which equations to choose is that you choose a set of those which contains all the unknowns you want to solve for (you may be lucky, you may have to solve for others as well along the way).
Some of the equations in your set may be KVL, some may be KCL, some may be Ohm's. That is fine.
It is sometimes possible (small correction Tim
) to pick linearly dependent (conditions 3 & 4) equations from both the possible KVL and KCL set. The maths will soon let you know if you have done this as the system is then insoluble.
Often KVL leads to fewer equations so many prefer it however judicious use of KCL can significantly reduce the calculation effort.
Spotting where this can be done comes with practice.
