Resistors in Series & Parallel (AQA GCSE Physics: Combined Science)

In my experience of teaching Resistors in Parallel to AQA GCSE students, it always seems difficult, at first, because it is counterintuitive. Students always asked me, 'How can adding resistors to a circuit actually reduce resistance?' and it's a great question!

First of all, we need to understand exactly what we mean by 'adding resistors'. Imagine a circuit with just a cell, some wires, and a 5 Ω resistor. We now want to add a 10 Ω resistor to that circuit. We have two options; we can add the 10 Ω resistor in series to the 5 Ω resistor or in parallel to it. That's what we mean by adding a resistor. 

If we add the 10 Ω resistor in series, then all of the electrons in the circuit have to flow through both resistors. That's why the resistances add together. The total resistance of the new circuit is 5 + 10 = 15 Ω.

If we add the 10 Ω resistor in parallel, then we are creating a new loop, which is a new path for the electrons to flow through. They either flow through the 5 Ω resistor or they flow through the 10 Ω resistor. Now, because of the extra loop, that extra path for the electrons, there is actually more charge flowing around the circuit than there was before that 10 Ω resistor was added. That means a higher current, which means an overall smaller resistance than the original 5 Ω circuit.