If you're confused by this kind of thing, you could probably give your intuitions a good training by studying the basics of electrical circuits (i.e. Ohm's law, Kirchoff's laws, the relations between current, voltage, resistance, and power, the Thevenin-Norton transformations, the series and parallel combinations of resistors, etc.). This will stretch your brain very nicely with a whole bunch of problems where you must be careful about what's being held constant or otherwise your intuition leads you into awful contradictions. (For example, power is proportional to resistance if you hold the current constant, but inversely proportional to it if you hold the voltage constant.) The concepts are very simple mathematically and don't require any background beyond very basic physics.