SEC G3 Combined Science Physics component K326/K327
CP13: D.C. Circuits
Apply symbols, measurements, series and parallel rules, and equivalent resistance to labelled circuits.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
A circuit solution starts with a correct diagram, identifies which quantities are shared or divided, and then applies component and whole-circuit relationships consistently.
Circuit diagrams and meters
Use standard symbols for cells, batteries, switches, lamps, LEDs, fixed and variable resistors, fuses, ammeters, and voltmeters.
An ammeter belongs in series with the path whose current is measured. A voltmeter belongs across the component whose potential difference is measured.
Series and parallel rules
In one series path, current has one value throughout and component potential differences add to the supply value.
At a parallel junction, branch currents add to the source current, while every branch connected between the same two nodes has the same potential difference.
Effective resistance and whole circuits
Series resistances add directly. For parallel resistors, add reciprocals to obtain the reciprocal of effective resistance.
Reduce a network in stages, then use and the current and potential-difference rules to recover individual component values.
Assessed representations
Standard circuit symbols
Ammeter and voltmeter placement
Formulae and relationships
| Relationship | Use |
|---|---|
| Combine series resistors. | |
| Combine parallel resistors. |
Worked examples
Example 1: A and resistor are in parallel across . Find total current.
- Find reciprocal resistance: .
- Therefore .
- Use .
Answer: .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Translate the circuit into standard symbols and place ammeters in series and voltmeters in parallel.
- Apply current and potential-difference rules to clearly labelled series and parallel branches.
- Calculate equivalent resistance and check that a parallel result is below the smallest branch resistance.
Official outcome coverage
K326 CP13: 7 mapped outcomes, references CP13(a), CP13(b), CP13(c), CP13(d), CP13(e), CP13(f), CP13(g). Check the official K326 syllabus.
K327 CP13: 7 mapped outcomes, references CP13(a), CP13(b), CP13(c), CP13(d), CP13(e), CP13(f), CP13(g). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Build a low-voltage circuit from a standard-symbol diagram. Check meter position, polarity, branch connections and component ratings before switching on, then compare measured series or parallel values with the circuit rules and investigate any discrepancy.
Exam traps and retrieval check
Avoid these traps
- Connecting an ammeter in parallel.
- Adding parallel resistances directly.
- Using a branch current as though it were the source current.
Check from memory
What stays equal in series?
Current.
What stays equal across parallel branches?
Potential difference.
Where is a voltmeter connected?
Across the component.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Circuit symbols, series and parallel rules, effective resistance, and whole-circuit calculations.