AQA GCSE Physics

Revision Notes

2.2.2 Resistors in Series & Parallel

Resistors in Series

  • When two or more resistors are connected in series, the total resistance is equal to the sum of their individual resistances
  • For two resistors of resistance R1 and R2, the total resistance can be calculated using:

Resistors in series equation, downloadable AS & A Level Physics revision notes

  • Where R is the total resistance, in Ohms (Ω)
  • Increasing the number of resistors increases the overall resistance, as the charge now has more resistors to pass through

combined-resistance, IGCSE & GCSE Physics revision notes

Resistors connected in series

Worked Example

The combined resistance R in the following series circuit is 60 Ω.

What is the resistance value of R2?

A.     100 Ω               B.     30 Ω               C.     20 Ω               D.     40 Ω

ANSWER:  C

Step 1: Write down the equation for the combined resistance in series

R = R1 + R2 + R3

Step 2: Substitute the values for total resistance R and the other resistors

60 Ω = 30 Ω + R2 + 10 Ω

Step 3: Rearrange for R2

R2 = 60 Ω – 30 Ω – 10 Ω = 20 Ω

Resistors in Parallel

  • When two or more resistors are connected in parallel, the combined resistance decreases
  • In the below circuit, the combined resistance of the resistors R1 and R2 is less than if they were connected in series

resistors-parallel, IGCSE & GCSE Physics revision notes

Resistors connected in parallel

  • This happens because each resistor creates an extra path along which the charge can flow
    • This allows more charge to flow overall
    • This leads to a smaller overall resistance
  • The advantages of this kind of circuit are:
    • The components can be individually controlled, using their own switches
    • If one component stops working the others will continue to function

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