Resistors in Series and Parallel Revision Notes | GCSE AQA Higher Physics
Adding more resistors in parallel decreases the total resistance, because each extra branch gives current more routes to flow. Free GCSE Physics notes.
Updated 20 Mar 2026 · Revise Right Team
Resistors in Series and Parallel is part of Current, potential difference and resistance in GCSE AQA Higher Physics.
3 min
Reading time
24
Lessons in this topic
GCSE AQA Higher
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Reviewed by Revise Right
Content review
What this lesson covers
Adding more resistors in parallel decreases the total resistance, because each extra branch gives current more routes to flow. Free GCSE Physics notes.
State the conclusion for resistors connected in series circuits and compare parallel circuits qualitatively
Explain how current and voltage behave differently in series versus parallel circuits
Apply circuit rules to solve problems involving combinations of resistors
Compare the advantages and disadvantages of series and parallel circuits in real applications
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Use this lesson to stay inside Current, potential difference and resistance while connecting the detail back to the wider Electricity topic.
Electricity
Current, potential difference and resistance
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Resistors in Series and Parallel before testing yourself from memory.
1. Why Circuit Design Matters
Your phone charger, Christmas lights, and car headlights all use different ways of connecting components. Get it wrong and your Christmas lights go out when one bulb fails, or your phone takes forever to charge.
The secret lies in how resistors combine. Connect them in series and you add their resistances together. Connect them in parallel and the total resistance actually gets smaller. This changes everything about how current flows and voltage divides.
2. Series Circuits - The Single Path
Picture water flowing through a single pipe with three narrow sections. The water has no choice but to flow through each section in turn. That's exactly how current behaves in a series circuit.
In series circuits, current stays constant throughout the loop. If 2A flows through the first resistor, exactly 2A flows through the second and third resistors too. Current doesn't get 'used up' - it's the same everywhere.
**Trap**: Many students think current gets smaller as it passes through each resistor. Actually, current stays the same - it's the voltage that gets shared out between the resistors.
3. Parallel Circuits - Multiple Pathways
Now imagine water flowing through three pipes side by side. The water can choose any pipe, and more water flows through the wider pipes. That's parallel circuits - current splits up and takes multiple routes.
In parallel circuits, voltage stays the same across each branch. If the battery provides 12V, each resistor gets the full 12V. But current divides up - more current flows through lower resistance branches.
The key insight: adding parallel branches gives current more routes to flow, so total resistance decreases. It's always smaller than the smallest individual resistor.
4. Calculating Circuit Values
Theory is one thing, but exam questions demand precise calculations. Let's work through the standard methods that win marks.
Worked example: Series Circuit Calculation
A 12V battery connects to three resistors in series: 4Ω, 6Ω, and 8Ω. Calculate the total resistance and current. [4 marks]
$I = \frac{V}{R} = \frac{12}{18}$ — Use V = IR rearranged [M1]
$I = 0.67 A$ — Current is the same throughout series circuit [A1]
Total resistance = 18Ω, Current = 0.67A — Always include units for full marks [A1]
Answer: Total resistance = 18Ω, Current = 0.67A
Worked example: Comparing Resistance in Parallel Circuits
Two resistors are connected in parallel. Explain qualitatively how the total resistance compares with the resistance of each branch. [3 marks]
Adding branches gives current more than one path to flow. — More paths means lower total resistance [M1]
The total resistance is less than the resistance of the smallest branch. — This is the key comparison required at GCSE [M1]
The total resistance in parallel is always less than the smallest resistor. — State the qualitative rule clearly [A1]
This is because adding branches gives current more than one route through the circuit. — Link the idea to extra current paths [M1]
The total resistance is smaller than the resistance of the smallest branch. ✓ — This is the correct GCSE conclusion for parallel resistance [A1]
Answer: In parallel circuits, the total resistance is less than the smallest branch resistance.
5. Real-World Applications
Understanding series and parallel circuits explains why your house wiring works the way it does, and why some designs are better than others.
**Christmas lights**: Old-style lights used series circuits. When one bulb failed, the whole string went dark because current had no alternative path. Modern LED strings use parallel circuits - each bulb gets full voltage and the others keep working if one fails.
**House wiring**: Every socket and light in your house connects in parallel to the mains supply. This means each device gets the full 230V, and switching off your bedroom light doesn't affect the kitchen lights.
**Car headlights**: Connected in parallel so if one bulb blows, you still have one working headlight. In series, both would go dark - dangerous for night driving.
6. Exam Success Strategy
Circuit questions follow predictable patterns. Master these techniques and you'll spot the traps before they catch you.
Common exam trap: questions often show a circuit diagram and ask for the total resistance. First decide whether the components are in series or in parallel. In series, add the resistances directly. In parallel, the total resistance is always smaller than the smallest branch resistance because adding branches gives current more than one path through the circuit.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
A parallel circuit is a circuit where components are connected on separate branches, providing multiple paths for current to flow. The voltage is the same across each branch, and the current divides up between the branches.
What is the difference between series and parallel circuits?
In a series circuit, components are connected end-to-end in a single loop, so the same current flows through each and the voltage is shared. In a parallel circuit, components are on separate branches, so each branch gets the same voltage and the current divides up between the branches.
What is a series circuit?
A series circuit is a circuit where components are connected end-to-end in a single loop, so the same current flows through each component. The current stays constant throughout the loop, while the voltage is shared out between the resistors.
What happens to total resistance when more resistors are added in parallel?
Adding more resistors in parallel decreases the total resistance, because each extra branch gives current more routes to flow. The total resistance in parallel is always smaller than the smallest individual resistor.
How do you calculate total resistance in series?
Total resistance in series is the sum of the resistors, found by adding all the resistances. Three resistors of 4 Ω, 6 Ω and 8 Ω in series give 4 + 6 + 8 = 18 Ω, so with a 12 V battery the current is 12 ÷ 18 = 0.67 A throughout the circuit.
How does current split in a parallel circuit?
Current in a parallel circuit splits up and takes multiple routes, with more current flowing through the lower resistance branches. Adding more parallel branches gives current more routes, so the total resistance decreases.
Why is total resistance lower in parallel?
Total resistance in parallel is lower because adding branches gives current more than one route through the circuit. The total resistance of resistors in parallel is always less than the smallest individual resistor.
Why are houses wired in parallel?
Houses are wired in parallel so every socket and light gets the full 230V from the mains supply. It also means switching off one light, such as a bedroom light, does not affect the others, because each device has its own path for current.
Why do Christmas lights go out when one bulb fails?
Old-style Christmas lights used series circuits, so when one bulb failed the whole string went dark because the current had no alternative path. Modern LED strings use parallel circuits, so each bulb gets full voltage and the others keep working if one fails.
Is the voltage the same in each branch of a parallel circuit?
The voltage is the same across each branch of a parallel circuit. If the battery provides 12 V, each resistor in parallel gets the full 12 V, but the current divides up between the branches.
Why are car headlights wired in parallel?
Car headlights are wired in parallel so that if one bulb blows, the other headlight still works. In series, both would go dark, which would be dangerous for night driving.
How is voltage shared in a series circuit?
In a series circuit, the voltage is shared out between the resistors, while the current stays the same through every component. Many students wrongly think current gets smaller as it passes through each resistor.
How do you work out total resistance from a circuit diagram?
To find total resistance from a circuit diagram, first decide whether the components are in series or in parallel. In series, add the resistances directly. In parallel, the total resistance is always smaller than the smallest branch resistance.
Why are modern LED light strings wired in parallel?
Modern LED light strings are wired in parallel so that each bulb gets the full voltage and the others keep working if one fails. Old-style series strings went dark when one bulb failed, because current had no alternative path.
What is the disadvantage of a series circuit?
The main disadvantage of a series circuit is that when one component fails, the whole circuit stops working because current has no alternative path. Old-style Christmas lights used series circuits, so one failed bulb made the whole string go dark.