Resistances in Parallel
Figure 28.-8a shows three resistances connected in parallel to an ideal battery of emf . The term “in parallel” means that the resistances are directly wired together on one side and directly wired together on the other side, and that a potential difference V is applied across the pair of connected sides. Thus, all three resistances have the same potential ifference V across them, producing a current through each. In general,
When a potential difference V is applied across resistances connected in parallel, the resistances all have that same potential difference V.
You might remember that Req and all the actual parallel resistances have the same potential difference V with the nonsense word “par-V.” To derive an expression for Req in Fig. 28-8b, we first write the current in each actual resistance.