Electric circuits are an important element of physics classes in Austria, Germany, and most countries around the world. However, many students leave secondary school without having an appropriate conceptual understanding of simple circuits. Voltage in particular has proven to be a difficult concept as students think of it as a property or component of the electric current. Furthermore, research has shown that girls tend to have a lower interest in physics than boys and that context-based physics instruction is a promising approach to increase girls' interest. However, it is unclear whether decades of research on students' conceptual difficulties e.g. with voltage as well as research into ways to promote girls' interest have had a significant impact on physics classrooms. For this reason, the conceptual understanding of electric circuits as well as the interest in physics of N = 1207 traditionally taught students in Germany and Austria was assessed using a valid and reliable multiple-choice test. The empirical evaluation of the data shows that female students are still not as interested in physics as their male peers, despite achieving the same total learning gain in the multiple-choice test. An analysis of two items of the test instrument focusing on potential differences furthermore suggests that traditional instruction still fails to provide students with an adequate conceptual understanding of voltage as an independent physical quantity that refers to a difference in electric potential between two points in a circuit. These results underline the need to develop research-based curricula on electric circuits that take into account the findings on students' interests and alternative conceptions.