AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Electricity
Practise Series and parallel circuits. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The rules for current, potential difference and resistance in series and parallel circuits, and how to use them in multi-step calculations. You need to explain why adding resistors in series increases the total resistance but adding them in parallel decreases it. You do not need to calculate the total resistance of resistors in parallel.
Key facts
Series: same current through each component
Series: Vtotal = V1 + V2 (pd is shared)
Series: Rtotal = R1 + R2
Parallel: same pd across each branch
Parallel: Itotal = I1 + I2
Parallel: total resistance is less than the smallest resistor
Add a resistor in series → total resistance up; in parallel → total resistance down
Notes
Series circuits
diagram
Components in series are connected one after another in a single loop.
The current is the same through each component.
The total pd of the supply is shared between the components: Vtotal = V1 + V2.
The total resistance is the sum of the resistances: Rtotal = R1 + R2.
The component with the bigger resistance gets the bigger share of the pd, because V = I R and the current is the same.
Series: same current; the pds add up to the supply pd (Rtotal = 6.0 Ω).
Parallel circuits
diagram
Components in parallel are each on their own branch, connected across the same two points.
The pd across each branch is the same. If each branch is connected straight across the supply, each branch has the full supply pd.
The total current is the sum of the currents through the branches: Itotal = I1 + I2.
Parallel: same pd across each branch; the branch currents add up.
The total resistance of two resistors in parallel is less than the smallest of the two resistances.
Each branch can have its own switch, so each component can be turned on and off separately.
Don’t lose marks
Adding the resistances of resistors in parallel as if they were in series.
Saying the current is shared in a series circuit. It is the pd that is shared; the current is the same.
Using the whole supply pd with just one resistor in a series circuit.
Thinking the total resistance in parallel is between the two values. It is less than the smallest.
Explaining total resistance with just 'there are more resistors' without mentioning pd and current.
More tips
Memory tricks
Series: Same current, Shared pd. Parallel: same Pd, current sPlits.
Check a series answer: the pds must add up to the supply pd.
Check a parallel answer: the total current must be bigger than any one branch current.
Parallel sense check: the total resistance is always smaller than the smallest resistor.
Exam technique
Label the circuit diagram with every value you know or work out (V, I and R).
Write the rule you are using, e.g. 'series, so same current', so the examiner can follow your method.
In 'explain' questions about total resistance, always mention both the pd and the current.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Tell series and parallel circuits apartSeries: one loop, one path. Parallel: two or more branches connected across the same two points.
Grade 4
State the series and parallel rulesSeries: same current, pd shared. Parallel: same pd across each branch, branch currents add up.
Grade 4
Add resistances in seriesUse Rtotal = R1 + R2.
Grade 5
Find a missing current or pde.g. the supply pd equals the sum of the pds in series; the total current equals the sum of the branch currents.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Components can be connected in series or in parallel.
(a) Which two statements about components connected in parallel are correct? Tick (✓) two boxes.[2]
The potential difference across each branch is the same.
The current in each branch is always the same.
The total current is the sum of the currents through the separate branches.
The total resistance is the sum of the resistances of the components.
(b) The lamps in a house are connected in parallel. Give two advantages of connecting the lamps in parallel.[2]
Show the answer and mark scheme
(a)Answer: The potential difference across each branch is the same.; The total current is the sum of the currents through the separate branches.
(b)Answer: Any two from: each lamp can be switched on and off separately; if one lamp breaks the others stay on; each lamp has the full supply potential difference.
each lamp can be switched on / off separately
if one lamp breaks / blows, the others still work
each lamp has the full (230 V) potential difference across it
Question 2Medium3 marks
The sockets and lights in a house are connected in parallel to the mains supply, not in series.
Give three reasons why household wiring uses parallel circuits rather than series circuits.[3]
Show the answer and mark scheme
Answer: Each appliance/lamp gets the full supply potential difference; each can be switched on/off independently; if one develops a fault or is disconnected, the others are unaffected.
each appliance / lamp receives the full (230 V) supply potential difference, so it works at its rated power regardless of how many others are connected
each appliance / lamp can be switched on or off independently of the others
if one appliance / lamp develops a fault or is disconnected, the others are unaffected and keep working
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