To calculate current in a series circuit, add the resistances to get the total, then use I = V ÷ R. Free GCSE AQA Physics notes and 15 questions answered.
Updated 20 Mar 2026 · Revise Right Team
Series Circuits: Calculations, Design & Testing is part of Series and parallel circuits in GCSE AQA Higher Physics.
5 min
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24
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GCSE AQA Higher
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What this lesson covers
To calculate current in a series circuit, add the resistances to get the total, then use I = V ÷ R. Free GCSE AQA Physics notes and 15 questions answered.
Calculate current, voltage and resistance in series circuits using Ohm's law and circuit rules
Design series circuits for measurement and testing purposes using appropriate components
Construct and check series circuits using circuit diagrams and standard symbols
Apply the concept of equivalent resistance to solve problems with resistors in series
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Use this lesson to stay inside Series and parallel circuits while connecting the detail back to the wider Electricity topic.
Electricity
Series and parallel circuits
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Series Circuits: Calculations, Design & Testing before testing yourself from memory.
1. Understanding Series Circuits
Picture Christmas lights from twenty years ago - when one bulb failed, the whole string went dark. That's a series circuit in action. Components are connected in a single loop, like beads on a string, with current flowing through each one in turn.
This simple arrangement creates predictable patterns that make series circuits perfect for measurement and testing. You'll find them in multimeters, battery testers, and safety circuits where you need precise control over current flow.
The key insight is this: in a series circuit, current has only one path to follow. This creates three fundamental rules that govern everything from simple torch circuits to complex measuring instruments.
Many students think current 'gets used up' as it flows through components - it doesn't. Current stays constant throughout a series circuit because charge cannot be created or destroyed. What changes is the energy carried by that charge.
2. The Three Rules of Series Circuits
Series circuits follow three iron-clad rules that make calculations straightforward. Master these, and you can solve any series circuit problem in your GCSE exam.
Rule 1: Current is identical everywhere. Whether you measure at the start, middle, or end of the circuit, you get the same reading. This happens because charge cannot accumulate anywhere - what goes in must come out.
Rule 2: Voltages add up to the supply voltage. Each component takes its share of the total voltage, like runners sharing a relay race. The sum of all voltage drops equals the battery voltage.
Rule 3: Resistances add up directly. Total resistance equals the sum of individual resistances. This makes series circuits useful for creating precise resistance values by combining standard resistors.
3. Essential Calculations and Ohm's Law
Every series circuit calculation starts with Ohm's law: V = IR. This relationship, combined with the series rules, unlocks every problem you'll face in exams.
The strategy is always the same: find the total resistance first, then calculate the circuit current, then work out individual voltages. This systematic approach prevents the common mistake of trying to solve everything at once.
Remember that current is your friend in series circuits - once you know it, you can find the voltage across any component using V = IR. The current acts like a key that unlocks all other values.
Worked example: Series Circuit Calculation
A 12V battery is connected to three resistors in series: 2Ω, 3Ω, and 7Ω. Calculate the current and the voltage across the 3Ω resistor. [4 marks]
$I = \frac{V}{R} = \frac{12}{12} = 1.0\text{ A}$ — Using Ohm's law with total values [M1]
$V = I \times R = 1.0 \times 3 = 3.0\text{ V}$ — Current is same throughout series circuit [A1]
$V_1 = 1.0 \times 2 = 2.0\text{ V},\; V_2 = 1.0 \times 3 = 3.0\text{ V},\; V_3 = 1.0 \times 7 = 7.0\text{ V};\; 2.0 + 3.0 + 7.0 = 12.0\text{ V}$ ✓ — The individual voltage drops add to the supply voltage, so this checks the calculation using voltage values rather than resistance values.
Answer: Current = 1.0 A, Voltage across 3Ω resistor = 3.0 V
4. Designing Series Circuits for Measurement
Series circuits are useful when you want predictable current and controlled voltage drops across components. This makes them useful in current-limiting circuits, potential dividers and some sensor circuits where you need a known relationship between current, resistance and voltage.
The classic example is a potential divider - two resistors in series that split the supply voltage in a known ratio. Change one resistor (perhaps with a thermistor or light-dependent resistor), and you get a sensor that responds to temperature or light.
For current measurement, you place an ammeter in series with the circuit. The ammeter must have very low resistance so that it does not significantly change the current being measured. By contrast, a voltmeter is connected in parallel across a component. Many practical circuits include both series and parallel parts, so it is important to recognise which arrangement is being used.
Series circuits also provide natural current limiting. Add a resistor in series with an LED, and you protect it from excessive current. The resistor 'drops' the excess voltage, leaving just enough for the LED to operate safely.
Worked example: Designing a Current-Limiting Circuit
An LED needs 2V and 20mA to operate safely. Design a series circuit using a 9V battery. What resistance is needed? [3 marks]
$I = 20\text{ mA} = 0.020\text{ A}$ — Always use base SI units in calculations
$R = \frac{V}{I} = \frac{7}{0.020} = 350\text{ Ω}$ — Using Ohm's law [A1]
Answer: 350Ω resistor needed to limit current to 20mA
5. Circuit Construction and Testing
Building series circuits requires systematic thinking. Start with the circuit diagram, identify the components you need, then connect them in the correct sequence. The key is following the current path - there should be only one route from positive to negative terminal.
When testing your circuit, measure current at different points to verify it's the same throughout. If readings differ, you have a wiring error or a faulty component. Current consistency is your best diagnostic tool.
Voltage measurements should add up to the supply voltage. If they don't, check for loose connections or short circuits. A short circuit bypasses components, reducing the total resistance and increasing current dangerously.
Common construction mistakes include parallel connections (creating multiple current paths) and reversed polarity on components like LEDs or electrolytic capacitors. Always double-check your wiring against the circuit diagram before switching on.
6. Exam Success and Common Pitfalls
Series circuit questions follow predictable patterns in GCSE exams. You'll typically need to calculate current using total resistance, then find individual voltages or powers. The key is working systematically through the three-step process.
Examiners love to test whether you understand that current stays constant. They might ask for current 'through the battery' or 'through the third resistor' - the answer is always the same value. Don't overthink it.
Watch out for unit conversion traps. If they give you milliamps (mA) or kiloohms (kΩ), convert to base units before calculating. A common mistake is forgetting that 500 mA equals 0.5 A, leading to answers that are 1000 times too large.
For design questions, show your working clearly. State what you're calculating at each step, substitute values with units, and give your final answer with appropriate units. This systematic approach maximises your method marks even if you make arithmetic errors.
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 short circuit bypasses components, reducing the total resistance and increasing the current dangerously. When testing a series circuit, voltage measurements that do not add up to the supply voltage can point to loose connections or short circuits.
How do you calculate current in a series circuit?
To calculate current in a series circuit, add the resistances to get the total, then use I = V ÷ R. A 12V battery with 2Ω, 3Ω and 7Ω in series gives 12Ω total, so the current is 12 ÷ 12 = 1.0 A everywhere in the circuit.
What is equivalent resistance?
Equivalent resistance is the single resistance value that could replace multiple resistors while maintaining the same current flow. In series circuits, equivalent resistance equals the sum of the individual resistances.
Why does an ammeter need a very low resistance?
An ammeter needs a very low resistance so that it does not significantly change the current being measured. It is placed in series with the circuit, whereas a voltmeter is connected in parallel across a component.
What resistor do you need in series with an LED?
The series resistor must drop the excess voltage at the LED's safe current. An LED needing 2V and 20mA on a 9V battery needs the resistor to drop 9 − 2 = 7 V at 0.020 A, so R = 7 ÷ 0.020 = 350 Ω.
What is a potential divider?
A potential divider is two resistors in series that split the supply voltage in a known ratio. Replacing one resistor with a thermistor or light-dependent resistor gives a sensor that responds to temperature or light.
What are the three rules of series circuits?
The three rules of series circuits are that current is identical everywhere, voltages add up to the supply voltage, and resistances add up directly. These rules follow from current having only one path, and charge cannot accumulate anywhere.
How do you find the voltage across one resistor in a series circuit?
The voltage across one resistor in series is found by calculating total resistance, then the current, then using V = IR for that resistor. A 12 V battery with 2 Ω, 3 Ω and 7 Ω in series gives 12 Ω, 1.0 A, and 1.0 × 3 = 3.0 V across the 3 Ω resistor.
How do you test a series circuit?
A series circuit is tested by measuring current at different points to check it is the same throughout, and checking that voltage measurements add up to the supply voltage. Differing current readings mean a wiring error or a faulty component.
What is the order for solving series circuit problems?
Series circuit problems are solved by finding the total resistance first, then calculating the circuit current with V = IR, then working out individual voltages. Once the current is known, the voltage across any component follows from V = IR.
What is the current through the battery in a series circuit?
The current through the battery in a series circuit is the same as the current through every other component, such as the third resistor. Current stays constant throughout a series circuit because charge cannot be created or destroyed.
Does current get used up in a series circuit?
Current does not get used up as it flows through components in a series circuit; it stays constant because charge cannot be created or destroyed. What changes is the energy carried by that charge.
Why are series circuits used for measurement?
Series circuits are used for measurement and testing because they give predictable current and controlled voltage drops across components. They are found in multimeters, battery testers, current-limiting circuits, potential dividers and some sensor circuits.
What are common mistakes when building series circuits?
Common mistakes when building series circuits include parallel connections that create multiple current paths and reversed polarity on components like LEDs. The wiring should be double-checked against the circuit diagram before switching on.
How can a potential divider be made into a sensor?
A potential divider becomes a sensor when one of its two series resistors is replaced with a thermistor or light-dependent resistor. The supply voltage is then split in a ratio that responds to temperature or light.