IGCSE Physics / Electrical resistance
Adding resistors: series against parallel revision notes
Edexcel International GCSE Physics (4PH1), chapter 8: Electrical resistance, specification point 2.10, 2.17, 2.18. Part of the full course.
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Adding a resistor in series increases total resistance, while adding one in parallel gives current an extra path and decreases total resistance. With the same supply voltage, lower resistance means more current from the battery.