What you'll learn
This revision guide covers everything you need to know about series and parallel circuits for AQA GCSE Physics. You'll learn how to identify different circuit configurations, apply the rules for current, voltage and resistance in each type, and solve calculation problems involving multiple components. These concepts appear regularly in Paper 1 (Foundation and Higher tier) and are essential for securing marks in both multiple-choice and calculation questions.
Key terms and definitions
Series circuit — a circuit where components are connected in a single loop, so current flows through each component one after another
Parallel circuit — a circuit where components are connected across separate branches, providing multiple paths for current to flow
Potential difference (voltage) — the energy transferred per unit charge between two points in a circuit, measured in volts (V)
Current — the rate of flow of electrical charge around a circuit, measured in amperes (A)
Resistance — the opposition to current flow in a component or circuit, measured in ohms (Ω)
Ammeter — a device connected in series to measure current flowing through a component
Voltmeter — a device connected in parallel across a component to measure potential difference
Total resistance — the overall opposition to current in a circuit, calculated differently for series and parallel arrangements
Core concepts
Identifying series and parallel circuits
A series circuit has all components connected end-to-end in one continuous loop. If you trace the path from the positive terminal of the cell through the circuit back to the negative terminal, you pass through each component exactly once. There is only one route for current to take.
A parallel circuit has components connected across separate branches. The current splits at junctions where the circuit divides into multiple paths, then recombines later. Components on different branches are independently connected across the same two points.
Real circuits often combine both configurations. For example, two bulbs might be connected in parallel with each other, while this parallel combination is in series with a switch and cell.
Current rules in series and parallel circuits
In series circuits:
- Current is the same at all points in the circuit
- The same current flows through each component
- I₁ = I₂ = I₃ (where I represents current)
- If one component breaks or is removed, the circuit is broken and current stops flowing everywhere
This occurs because there is only one path for charge to flow. The rate at which charge passes any point must be identical throughout the loop.
In parallel circuits:
- Current is shared between the branches
- The total current from the cell equals the sum of currents in each branch
- I_total = I₁ + I₂ + I₃
- If one branch breaks, current continues flowing in the other branches
At junctions, charge is conserved. The number of electrons entering a junction per second equals the number leaving per second, but they split between available paths.
Potential difference rules in series and parallel circuits
In series circuits:
- Potential difference is shared between components
- The total p.d. from the cell equals the sum of p.d.s across each component
- V_total = V₁ + V₂ + V₃
- Components with greater resistance have a larger share of the total p.d.
This happens because energy is transferred from the charge as it passes through each component. The cell provides the total energy per coulomb, which gets used up progressively around the circuit.
In parallel circuits:
- Potential difference is the same across each branch
- Each component (or branch) has the full cell p.d. across it
- V₁ = V₂ = V₃ = V_total
- Each parallel path connects directly to both terminals of the supply
Because each branch connects to the same two points (positive and negative terminals), the energy transferred per coulomb must be identical regardless of which path the charge takes.
Resistance rules in series and parallel circuits
In series circuits:
- Total resistance equals the sum of individual resistances
- R_total = R₁ + R₂ + R₃
- Adding more components increases total resistance
- This reduces the current flowing through the circuit
Each component opposes current flow, and these oppositions add up when components are arranged in series. The total resistance is always greater than any individual resistance.
In parallel circuits:
- Total resistance is less than the smallest individual resistance
- For two resistors: 1/R_total = 1/R₁ + 1/R₂
- Adding more parallel branches decreases total resistance
- This increases the total current drawn from the cell
Parallel branches provide additional paths for current. More paths mean easier overall flow, reducing the circuit's total resistance. For identical resistors in parallel, the total resistance equals the resistance of one divided by the number of resistors.
Practical applications and real-world examples
Series circuits are used when:
- Components need to be controlled by a single switch (like a string of old Christmas lights)
- You want components to share voltage (like resistors in a voltage divider)
- Failure of one component should stop the entire system (safety circuits)
Parallel circuits are used when:
- Components need to operate independently (household lighting circuits)
- Each device requires the full supply voltage (appliances in your home)
- System reliability matters—other components continue working if one fails (car lights, computer circuits)
Household electrical circuits connect appliances in parallel across the 230V mains supply. This ensures each device receives 230V regardless of what else is switched on. Lights, televisions, and kettles can be used independently.
Using ammeters and voltmeters
Ammeters must be connected in series with the component you're measuring. This allows all the current flowing through the component to pass through the ammeter. The ammeter has very low resistance to avoid affecting the current it's measuring.
Voltmeters must be connected in parallel across the component. This places the voltmeter between the same two points as the component, measuring the potential difference. Voltmeters have very high resistance so they draw negligible current and don't affect the circuit.
A common exam error is drawing these instruments connected incorrectly in circuit diagrams.
Worked examples
Example 1: Calculating current in a series circuit (3 marks)
Question: A series circuit contains a 6V battery and three resistors of 2Ω, 3Ω and 7Ω. Calculate the current flowing through the circuit.
Solution:
Step 1: Calculate total resistance R_total = R₁ + R₂ + R₃ (1 mark) R_total = 2 + 3 + 7 = 12Ω
Step 2: Use V = IR rearranged to I = V/R I = 6/12 (1 mark) I = 0.5A (1 mark)
Mark scheme note: Award marks for correct formula, correct substitution, and correct answer with unit.
Example 2: Potential difference in parallel circuits (4 marks)
Question: A parallel circuit has a 12V battery connected to three branches. Branch 1 contains a 4Ω resistor, branch 2 contains a 6Ω resistor, and branch 3 contains a 12Ω resistor.
(a) State the p.d. across the 4Ω resistor. (1 mark) (b) Calculate the current through the 6Ω resistor. (2 marks) (c) Calculate the total current drawn from the battery. (1 mark)
Solution:
(a) 12V (1 mark) In parallel, p.d. across each branch equals the supply voltage
(b) I = V/R I = 12/6 (1 mark) I = 2A (1 mark)
(c) Current in branch 1: I = 12/4 = 3A Current in branch 2: I = 12/6 = 2A Current in branch 3: I = 12/12 = 1A Total current = 3 + 2 + 1 = 6A (1 mark)
Alternative for (c): Calculate total resistance using 1/R_total = 1/4 + 1/6 + 1/12 = 6/12, so R_total = 2Ω, then I = 12/2 = 6A
Example 3: Mixed series-parallel circuit (Higher tier, 5 marks)
Question: A circuit contains a 9V battery connected to two parallel branches. Each branch contains two resistors in series. Branch 1 has resistors of 2Ω and 4Ω. Branch 2 has resistors of 3Ω and 6Ω. Calculate the total current from the battery.
Solution:
Step 1: Calculate resistance of branch 1 R₁ = 2 + 4 = 6Ω (1 mark)
Step 2: Calculate resistance of branch 2 R₂ = 3 + 6 = 9Ω (1 mark)
Step 3: Calculate total circuit resistance (two resistors in parallel) 1/R_total = 1/6 + 1/9 = 3/18 + 2/18 = 5/18 (1 mark) R_total = 18/5 = 3.6Ω (1 mark)
Step 4: Calculate total current I = V/R_total = 9/3.6 = 2.5A (1 mark)
Common mistakes and how to avoid them
Confusing series and parallel rules — Make a quick reference table of the rules for current, voltage and resistance in both types. Learn which quantities are "same" and which are "shared/added" in each configuration.
Adding resistances incorrectly in parallel — Remember that parallel resistances use reciprocals: 1/R_total = 1/R₁ + 1/R₂. The total is always less than the smallest individual resistance. If your answer is bigger, you've used the series formula by mistake.
Drawing ammeters and voltmeters wrongly — Ammeter in series (current flows through it), voltmeter in parallel (measures across components). Use the mnemonic: "A in series, V across."
Forgetting units in calculations — Always include units (A for current, V for voltage, Ω for resistance) in your final answer. Marks are often lost for missing units even when the number is correct.
Assuming current is "used up" — Current is the same everywhere in a series circuit because charge is conserved. Components don't "use up" current; they transfer energy from it (measured as voltage drop).
Not showing working in calculations — Even if you get the wrong final answer, you can earn method marks by showing clear steps. Write out the formula, substitute values, then calculate the answer on separate lines.
Exam technique for "Series and parallel circuits"
Command word "Calculate" requires numerical working. Always show the formula first (e.g., V = IR), then substitute values with units, then give your answer. This earns method marks even if arithmetic errors occur. Typically worth 2-4 marks.
Command word "Explain" needs you to give reasons using physics principles. For example, "Explain why the total resistance decreases when a resistor is added in parallel" requires you to state that adding another branch provides an additional path for current, making it easier for current to flow overall. Worth 2-3 marks typically.
Circuit diagram questions — Use a ruler for straight lines, ensure components are drawn with correct symbols from the AQA specification, and check ammeters are in series while voltmeters are in parallel. One mark per correct element is standard.
Multi-step calculations — Break complex circuits down systematically: calculate resistance of series sections first, then treat these as single resistors in parallel combinations, then find total resistance, finally calculate current. Show each stage clearly for maximum marks.
Quick revision summary
Series circuits: current is identical throughout; voltage is shared (adds up); resistance adds directly. Parallel circuits: voltage is identical across branches; current splits (adds up at junctions); total resistance is less than the smallest branch resistance. Ammeters connect in series, voltmeters in parallel. Use V = IR and the specific series/parallel rules to solve problems. Real household circuits use parallel connections so appliances operate independently at full mains voltage.