The problem is to determine the response I in the in the resistor R2. One may consider the resistances R1 and R3 are the internal resistances of the voltage sources whereas the resistance R4 is considered as internal resistance of the current source. Superposition theorem can be explained through a simple resistive network as shown in figure and it has two independent practical voltage sources and one practical current source. In any linear bilateral network containing two or more independent sources (voltage or current sources or combination of voltage and current sources ), the resultant current voltage in any branch is the algebraic sum of currents / voltages caused by each independent sources acting along, with all other independent sources being replaced meanwhile by their respective internal resistances. The electric circuit superposition theorem is analogous to Dalton's law of partial pressure which can be stated as the total pressure exerted by an ideal gas mixture in a given volume is the algebraic sum of all the pressures exerted by each gas if it were alone in that volume.The superposition theorem states that in any linear network containing two or more sources, the response (current) in any element is equal to the algebraic sum of the response (current) caused by individual sources acting alone, while the other sources are inoperative. If at least two independent sources have the same frequency (for example in power systems, where many generators operate at 50 Hz or 60 Hz), then superposition can't be used to determine average power. However, if the linear network is operating in steady-state and each external independent source has a different frequency, then superposition can be applied to compute the average power or active power. To calculate power we first use superposition to find both current and voltage of each linear element and then calculate the sum of the multiplied voltages and currents. In other words, the sum of the powers of each source with the other sources turned off is not the real consumed power. Superposition works for voltage and current but not power. The theorem is applicable to linear networks (time varying or time invariant) consisting of independent sources, linear dependent sources, linear passive elements ( resistors, inductors, capacitors) and linear transformers. It is used in converting any circuit into its Norton equivalent or Thevenin equivalent. The superposition theorem is very important in circuit analysis. The resultant circuit operation is the superposition of the various voltage and current sources. This procedure is followed for each source in turn, then the resultant responses are added to determine the true operation of the circuit. I=0 internal impedance of ideal current source is infinite (open circuit)). Replacing all other independent current sources with an open circuit (thereby eliminating current i.e.V=0 internal impedance of ideal voltage source is zero ( short circuit)).
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