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In a series circuit, the same current flows through each component, the supply pd is shared, and total resistance is the sum of the resistances. Free notes.
Series Circuits: Current, PD & Resistance is part of Series and parallel circuits in GCSE AQA Higher Physics.
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In a series circuit, the same current flows through each component, the supply pd is shared, and total resistance is the sum of the resistances. Free notes.
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Use this lesson to stay inside Series and parallel circuits while connecting the detail back to the wider Electricity topic.
Read the core explanations from Series Circuits: Current, PD & Resistance before testing yourself from memory.
Picture Christmas lights that all go out when one bulb breaks. That's a series circuit - one continuous loop where everything depends on everything else. Understanding how current, voltage, and resistance behave in series circuits is crucial for GCSE Physics because these principles appear in every electricity exam question.
Series circuits follow three fundamental rules that seem simple but catch many students out. Get these right, and you'll unlock marks in calculations, explanations, and practical work.
If you have a 12V battery and two resistors, the voltages across each resistor will add up to 12V. The bigger the resistance, the bigger its share of the voltage.
Adding more resistors in series always increases the total resistance because every resistor adds extra opposition to the current. In parallel circuits the opposite happens: adding another branch gives charge more than one path to follow, so the total resistance decreases and is always less than the smallest branch resistance. This qualitative contrast is a standard GCSE explanation and helps you decide which rule to apply before you start calculating.
These two equations are your toolkit for every series circuit calculation. The key insight is that current stays the same, so you can use Ohm's law on individual components or the whole circuit.
Worked example: Series Circuit Calculation
A 9V battery is connected to two resistors in series: 20Ω and 40Ω. Calculate the current and voltage across each resistor. [6 marks]
Answer: Current = 0.15A, Voltage across 20Ω = 3.0V, Voltage across 40Ω = 6.0V
This practical proves the theory works in real life. Set up a series circuit with a power supply, resistor, ammeter and voltmeter. Change the resistor combination, record current and potential difference, then compare the measured values with your calculated totals. Keep the supply voltage controlled, read meters at eye level, and switch off the circuit between changes to reduce heating errors. The aim is to show that current stays the same in series and that the voltage drops add to the supply voltage.
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same through all components | Splits between branches |
| Voltage | Shared between components | Same across all components |
| Total Resistance | Sum of all resistances | Less than smallest resistance |
| Effect of adding resistors | Increases total resistance | Decreases total resistance |
| If one component breaks | Whole circuit stops | Other components keep working |
The key difference is the path for current. Series has one path (like a single-lane road), while parallel has multiple paths (like a motorway with several lanes).
Series circuit questions follow predictable patterns. First identify whether the circuit is truly series or whether there are parallel branches hidden in the diagram. Then write the rule you need: current is the same, voltages add to the supply, and resistances add directly. Once you have stated the correct rule, substitute values with units and show each step so method marks are protected even if the arithmetic slips.
Common exam traps include mixing up series and parallel rules, forgetting to convert units (mA to A), and not showing working clearly. Always state your formula, substitute values with units, and give your final answer with correct units.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
In a series circuit, the total potential difference of the power supply is shared between the components in proportion to their resistance. The bigger the resistance, the bigger its share, and the voltages across each component add up to the supply voltage, such as 12V.
Series circuits follow three rules: the same current flows through each component, the potential difference of the supply is shared between the components, and the total resistance is the sum of all the individual resistances.
Adding more resistors in series always increases the total resistance because every resistor adds extra opposition to the current. In parallel circuits the opposite happens: adding another branch decreases the total resistance.
Find the total resistance, calculate the current with I = V ÷ R total, then use V = IR for each resistor. A 9V battery with 20Ω and 40Ω in series gives 60Ω and 0.15A, so the voltages are 3.0V and 6.0V, which add up to 9.0V.
If one component breaks in a series circuit, the whole circuit stops because there is only one path for current, like Christmas lights that all go out when one bulb breaks. In a parallel circuit, the other components keep working.
The current is the same through each component in a series circuit. Current is not used up by components; the potential difference of the supply is shared between them instead.
The series circuits practical uses a power supply, resistor, ammeter and voltmeter. The resistor combination is changed and current and potential difference are recorded, then compared with calculated totals to show the current stays the same and the voltage drops add to the supply voltage.
Errors in the series circuit practical are reduced by keeping the supply voltage controlled, reading meters at eye level, and switching off the circuit between changes to reduce heating errors.
A 9 V battery with 20 Ω and 40 Ω resistors in series gives a current of 0.15 A. The total resistance is 20 + 40 = 60 Ω, so I = 9 ÷ 60 = 0.15 A, with 3.0 V across the 20 Ω resistor and 6.0 V across the 40 Ω resistor.
The voltages across components in a series circuit add up to the supply voltage. With a 12 V battery and two resistors, the voltages across each resistor add up to 12 V, and the bigger resistance gets the bigger share.
A series circuit calculation is checked by adding the voltages across each component, which should equal the supply voltage. For 3.0 V across 20 Ω and 6.0 V across 40 Ω with a 9 V battery, 3.0 + 6.0 = 9.0 V shows the answer is correct.
In series, current is the same through all components, voltage is shared, total resistance is the sum, and one broken component stops the whole circuit. In parallel, current splits between branches, voltage is the same across all components, and total resistance is less than the smallest resistance.
A series circuit question starts by checking the circuit is truly series with no hidden parallel branches, then writing the rule needed: current is the same, voltages add to the supply, and resistances add directly. Values are then substituted with units, showing each step.
Adding more resistors in series always increases the total resistance, because every resistor adds extra opposition to the current. For the same supply voltage, I = V ÷ R, so the current in the circuit falls.
The potential difference of the power supply is shared between components in series in proportion to their resistance. The bigger the resistance, the bigger its share of the voltage, so in a 9 V circuit a 40 Ω resistor gets 6.0 V and a 20 Ω resistor gets 3.0 V.