Circuit Fundamentals

Sources shift in an electric circuit


Calculation of the electric circuit can be simplified by moving voltage and current sources. How to move them along the circuit is described here. According to the Law of Kirchhoff, currents in a circuit are determined by the summarised EMF of the loops on the circuit, whatever elements they consist of.

Then, changing of voltage source position in a circuit is irrelevant if summarised EMF is constant in all loops of a circuit – it does not change current in a circuit. Similarly, voltages in branches are determined by summarised currents in nodes, then changing of current source position in a circuit – when the summarised current in a node is invariable, it does not change circuit analysis.

For example, we have to exclude the voltage source in one of the branches of a circuit. We are adding compensatory voltage source onto the branch, and adding the same directed additional voltage sources (as compensatory ones) to all other branches connected to the shared node. So the voltage source is excluded from the considered branch, but all the other branches get additional voltage sources. Every loop in a circuit will have invariable summarised current and voltage. Thus, the voltage source can be moved from a particular branch of a circuit to another branch, with a shared node, with invariable summarised currents through this node. Converse state is true as well – let us say we have voltage sources in all the branches, which have converged, and are all directed to a node or opposite, they can be replaced by the voltage source in a branch where it was absent. This state is illustrated in Figure 28.

Circuit Fundamentals - Fig28
Figure 28

In case the current sources move, the latter connects to the circuit nodes to keep summarised currents in the nodes invariable. Current source can be replaced by a set of current sources, connected in parallel to all the branches that made a loop with described current source.

Thevenin's and Norton's theorems

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