The superposition theorem states that for a circuit having multiple independent sources, the current through or voltage across an element is equal to the algebraic sum of the current through or voltage across that element due to each independent source acting alone.

For solving circuit using superposition theorem having multiple independent sources, consider one independent source (voltage or current) at a time while all other independent sources are turned off. Turned off implies that we replace every current source by 0A or an open circuit and every voltage source by 0V or a short circuit. Dependent sources are left intact because they are controlled by circuit variables.

Superposition theorem is applicable to linear circuit only. A linear circuit is one whose output is linearly related or directly proportional to its input.

 Hence, Superposition theorem is not applicable to power due to each source. If the power value is needed, the voltage across (or current through) the element be calculated first by superposition theorem.  Hence, calculate the value of power.

One disadvantage of analyzing a circuit by superposition theorem is that it is a lengthy process. If the circuit has say four independent sources, we may have to analyse four simple circuits each providing the contribution due to the respective individual source. But superposition theorem helps to reduce a complex circuit to simple circuits through replacement of current sources by open circuits and voltage sources by short circuits.

Solution: The given circuit consists of two sources [40V voltage source and 4A current source]. We shall determine voltage V₁ due to each source acting alone.