Two studies were conducted to describe how students perform direct current (D-C) circuit problems. It was hypothesized that problem solving in the electricity domain depends largely on good visual processing of the circuit diagram and that this processing depends on the ability to recognize when two or more electrical components are in series or in parallel, even if the drawings diverge from the canonical ones such as those which are largely taught. The first study examined how college students (N=10) recognized a given circuit among different circuit drawings. Results indicate that these students were able to recognize circuits in series or in parallel only if they have canonical shapes, such as those taught in class or drawn in textbooks. Tenth-grade students (N=22) participated in the second study which demonstrated how the current conservation law at a junction point is applied to a D-C circuit. These students solved the same problem by different procedures which depended directly on how their knowledge was memorized, indicating that current conservation is associated with a specific circuit similar to a canonical one, formulated as a mathematical relation without physical content, or formulated as a law applied to a junction point. Implications for science instruction are discussed. (JN)