Chapter10
Network Theorems
This chapter will introduce the important fundamental theorems of network analysis.
Included are the superposition, Thévenin’s, Norton’s, maximum power transfer,
substitution, Millman’s, and reciprocity theorems.
1-SUPERPOSITION THEOREM :
The current through, or voltage across, an element in a linear bilateral network is
equal to the algebraic sum of the currents or voltages produced independently by
each source.
remove a voltage source when applying this theorem, the difference in potential
between the terminals of the voltage source must be set to zero (short circuit);
removing a current source requires that its terminals be opened (open circuit). Any
internal resistance or conductance associated with the displaced sources is not
eliminated but must still be considered.
Figure (A) reviews the various substitutions required when removing an ideal
source, and Figure (B) reviews the substitutions with practical sources that have an
internal resistance.
Fig. (A) Removing the effects of ideal sources.
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Network Theorems
Fig.(B) Removing the effects of practical sources.
Note : the total power delivered to a resistive element must be determined using the
total current through or the total voltage across the element and cannot be etermined
by a simple sum of the power levels established by each source.
EXAMPLE (1): Determine (I1) for the network of Fig. (1).
Fig.(1)
Solution:
1-Setting E = 0 V for the network of Fig. (1) results in the network of Fig. 1(a),
where a short-circuit equivalent has replaced the 30-V source.
Fig. 1(a)
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Chapter10
Network Theorems
As shown in Fig.1(a), the source current will choose the short circuit path, and I'1 ' 0 A. If
we applied the current divider rule,
2- Setting I to zero amperes will result in the network of Fig1(b), with the current source
replaced by an open circuit. Applying Ohm’s law,
Fig.1(b)
Since I'1 and I″1 have the same defined direction in Fig. 1(a) and (b), the current I1 is the
sum of the two, and:
EXAMPLE( 2). Using superposition, determine the current through the 4Ω resistor of
Fig.(2).
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Network Theorems
Fig.(2)
1-Considering the effects of a 54V source (Fig.2a)
Fig.2(a) The effect of E1on the current I3.
2-Considering the effects of the 48V source (Fig2b):
Fig(2b) The effect of E2on the current I3.
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Chapter10
Network Theorems
The total current through the 4Ω resistor (Fig. 2c) is:
Fig(2c) The resultant current for I3.
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