Download Kirchoff Law Problem Solving

Junction Rule: sum of current entering junction equals sum of
current leaving junction.
Loop Rule: the sum of potential differences around a close loop equals
zero.
Problem-Solving Strategy: Using Kirchhoff ’s Rules to Analyze a Circuit
1.
Replace any series or parallel combinations with their equivalents.
2.
Assign variables to the currents in each branch of the circuit(I1,I2,...)and
choose directions for each current. Draw the circ uit with the current
directions indicated by arrows. It does not matter whether or not you choose
the correct direction.
3.
Apply Kirchhoff’s junction rule to all but one of the junctions in the circuit.
(Applying it to every junction produces one redundant equation.) Remember
that current into a junction is positive; current out of a junction is negative.
4.
Apply Kirchhoff’s loop rule to enough loops so that, together with the
junction equations, you have the same number of equations as unknown
quantities. For each loop, choose a starting point and a direction to go around
the loop. Be care- ful with signs. For a resistor, if your path through a resistor
goes with the current (“downstream”), there is a potential drop; if your path
goes against the current (“upstream”), the potential rises. For an emf, the
potential drops or rises depend- ing on whether you move from the positive
terminal to the negative or vice versa; the direction of the current is irrelevant.
A helpful method is to write “+” and “–” signs on the ends of each resistor
and emf to indicate which end is at the higher potential and which is at the
lower potential.
5.
Solve the loop and junction equations simultaneously. If a current comes out
negative, the direction of the current is opposite to the direction you chose.
6.
Checkyourresultusingoneormoreloopsorjunctions.Agoodchoiceisaloop that
you did not use in the solution.