Series and parallel circuits — AQA GCSE Physics
Test yourself on Series and parallel circuits with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Series and parallel circuits explained
Two components are in series when they sit in the same single loop, joined end to end, so every unit of charge that leaves the cell must pass through both.
Read the full explanation
They are in parallel when they sit on separate branches between the same two junctions, so charge divides and each branch is a complete path on its own. To read a circuit diagram, start at one terminal of the cell and trace the wire: if you reach a junction where the wire splits, whatever follows on the separate branches is in parallel. Many real circuits mix the two, for example a switch in series with the supply feeding two lamps in parallel. Measuring instruments follow the same rule for a reason: an ammeter goes in series because it must carry the current being measured, and a voltmeter goes in parallel because it compares the potential at two points.
For components connected in series: there is the same current through each component the total potential difference of the power supply is shared between the components the total resistance of two components is the sum of the resistance of each component.
In a series circuit there is only one path and charge is not used up, so the current is the same at every point: an ammeter reads the same wherever you place it. Each coulomb passes through every component in turn and gives up some energy at each one, so the potential differences across the components add up to the potential difference of the supply. Because the charge has to push through both resistances one after the other, the total resistance is the sum, R = R₁ + R₂. Put a 4 Ω and a 2 Ω resistor across a 6 V supply and the total resistance is 6 Ω, so the current is 6 ÷ 6 = 1 A everywhere, and the potential differences are 4 V and 2 V, which add back to 6 V. The larger resistor always takes the larger share of the supply potential difference.
Students should be able to: use circuit diagrams to construct and check series and parallel circuits that include a variety of common circuit components describe the difference between series and parallel circuits explain qualitatively why adding resistors in series increases the total resistance whilst adding resistors in parallel decreases the total resistance explain the design and use of dc series circuits for measurement and testing purposes calculate the currents, potential differences and resistances in dc series circuits solve problems for circuits which include resistors in series using the concept of equivalent resistance.
Adding a resistor in series puts more opposition into the one path the charge must follow, so the total resistance rises and, for a fixed supply, the current falls. Adding a resistor in parallel does the opposite: it opens an extra route between the same two junctions, so the same potential difference now drives current down both branches. The total current from the supply rises, and since R = V ÷ I a bigger current at an unchanged potential difference means a smaller total resistance, always smaller than the smallest single branch. That is why a hand warmer draws more power when a second heating element is switched in. Series circuits suit testing because one ammeter reading applies to the whole loop and a variable resistor placed in series controls the current everywhere at once.
Your focus
- Trace a circuit diagram from one cell terminal to the other and count the separate paths open to the charge.
- Draw a test circuit with the cell, ammeter and component in one loop and the voltmeter across the component.
- Identify which part of a mixed circuit is in series and which is in parallel, such as a switch feeding two lamps.
Show all 9 objectives
- Calculate the total resistance of two series resistors and use it to find the single current in the loop.
- Predict the potential difference across each of two unequal series resistors and check that they sum to the supply value.
- Explain, using a single path and conserved charge, why an ammeter reads the same at every point in a series loop.
- Explain why a second parallel branch lowers total resistance, naming the extra path and the unchanged supply potential difference.
- Calculate the current, potential difference and resistance at each point of a dc series circuit of two or three resistors.
- Justify placing a variable resistor in series when a test circuit needs its current varied.
Series and parallel circuits exam tips
Marking Points
- one mark for a single unbroken loop containing the cell, the component and the ammeter in series
- one mark for the voltmeter drawn in parallel across the component being tested
- one mark for identifying a junction as the point at which a parallel branch begins
- one mark for stating that each parallel branch is a separate complete path back to the supply
- one mark for the current being the same through every component in the series loop
- one mark for adding the resistances, R = R₁ + R₂, to find the total resistance
- one mark for using the total resistance in I = V ÷ R to find the single circuit current
- one mark for a potential difference found from V = I × R, with the individual values adding to the supply value
- one mark for stating that a resistor added in parallel provides another path for the charge
- one mark for the total resistance of the circuit decreasing when that branch is added
- one mark for the current increasing because the potential difference across the battery is unchanged
- one mark for series resistances adding, so that the current falls when a resistor is added in series
Examiner Tips
- 💡Trace the circuit with a finger from one terminal of the cell and count how many separate routes reach the other terminal.
- 💡Put a dot at every junction you draw; examiners look for a dot where three or more wires meet.
- 💡When asked for the difference, name the number of paths available to the charge, not just that components are in a line.
- 💡Work in the order total resistance, then current, then each individual potential difference; that route collects the marks.
- 💡Check your answer by adding the individual potential differences, which must total the supply value.
- 💡If two potential differences are given and a third is asked for, subtract from the supply rather than starting a calculation.
- 💡For a qualitative explanation, write the chain: extra path, lower total resistance, larger current from the supply.
- 💡'Explain qualitatively' signals that no parallel resistance calculation is ever required here.
- 💡Quote R = V ÷ I when arguing that a larger total current at the same potential difference means a smaller total resistance.
Common Mistakes
- drawing the ammeter across the lamp and the voltmeter in the loop, swapping the two connections
- calling two lamps parallel because they are drawn side by side rather than because the wire splits at a junction
- treating a mixed circuit as entirely parallel when the supply and switch are still in series
- leaving a gap in the loop so the circuit is incomplete, then claiming there is a current
- sharing the current between series components in the way that potential difference is shared
- giving individual potential differences that add to more than the supply potential difference
- using the full supply potential difference across one resistor when calculating the current
- assuming the component nearest the cell receives more current because the charge reaches it first
- assuming more resistors must always mean more resistance, so predicting a smaller current when a parallel branch is added
- giving a combined parallel resistance larger than one of the individual branch resistances
- explaining the parallel case by saying the potential difference splits between the branches
- allowing the supply potential difference to change during the explanation instead of holding it constant