Substitution Theorem: The Substitution Theorem is used in circuit analysis to simplify complex networks by replacing a branch (like a resistor) with an equivalent voltage or current source, provided the voltage across or current through that branch remains identical, thus keeping the rest of the circuit’s behavior (voltages/currents) unchanged.
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Substitution Theorem
Read Also: Reciprocity Theorem
Defenition
In a linear network, any passive element can be replaced by a voltage source whose magnitude is the drop across that element, and the polarity of the source is the drop polarity across that element, provided the other branch currents and voltages are constant.

Problem 1
For the circuit shown in the following figure, the 1 Ω resistor can be replaced by

Solution: Using the substitution theorem, the equivalent circuit can be represented as follows.

So 1 Ω resistor can be replaced with voltage source of 1 V with the positive terminal upwards.
Read: Reciprocity Theorem
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Electrical Circuit Theorems
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