The power absorbed or supplied by each element can be computed directly from its current and voltage, regardless of the nature of the element, as discussed earlier. Therefore, we will consider only the currents and voltages to be our unknowns. For a circuit with b branches, we therefore have 2b unknowns (e.g., 14 unknowns for the above example with 7 elements). (Arguably, some of these variables are not really unknown, such as the voltage of a voltage source or the current of a current source. For consistency, however, we treat those as unknowns and apply "constraint equations" based on the nature of the source to determine them.) There are only two laws that can be used to solve circuits, namely Kirchhoff's voltage law (KVL) and Kirchhoff's current law (KCL). (Note: If you cannot state those now from memory, please review your prior notes before proceeding, as it is impossible to learn to solve circuits without having both of those laws committed to memory.) In addition to these laws, we need the constitutive relationship that defines each circuit element. For resistors, that is of course just Ohm's law (with the correct sign). For voltage sources, the property is that the voltage