In a previous Mathskit Sandy Loynd explained why reciprocals are used when combining resistors in parallel (‘Using reciprocals’, PHYSICS REVIEW Vol. 29, No. 4, pp. 20–21). Here we take a look at how circuit equations are manipulated when analysing circuits involving both series and parallel combinations of resistors, and how a complicated circuit can be simplified one step at a time.

From your GCSE (or equivalent) course you should remember that in a series circuit the current is the same everywhere around the single loop of the circuit. You might also recall that the total supply voltage, V T , is shared between all components, depending on their resistances.

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