# 4.3.3 Resistors in Series & Parallel

### Resistors in Series

• When two or more components are connected in series:
• The combined resistance of the components is equal to the sum of individual resistances When several components are connected in series, their combined resistance is equal to the sum of their individual resistances

### Resistors in Parallel

• When resistors are connected in parallel, the combined resistance decreases and is less than the resistance of any of the individual components
• If two resistors of equal resistance are connected in parallel, then the combined resistance will halve The above resistors will have a combined resistance of 2 Ω − half the value of each resistor

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### Determining Resistance in Parallel

• More generally, to determine the combined resistance of any combination of two resistors, you must use the equation:  The above equation is not the same as R = R1 + R2  – a common, incorrect simplification that is made

• To calculate the resistance:
• First find the value of 1/R (by adding 1/R1 + 1/R2)
• Next find the value of R by using the reciprocal button on your calculator (labelled either x-1 or 1/x, depending on your calculator) ### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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