O-Level and SEC G3 Physics K323
P15: D.C. Circuits
Apply series and parallel rules, circuit measurements, resistance, power, and energy to a clearly labelled circuit.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
D.C. circuit questions combine standard symbols, series and parallel rules, equivalent resistance, whole-circuit reasoning, potential dividers and input transducers.
Series and parallel rules
Current is the same through every component in one series path. Potential differences across series components add to the supply. In parallel, branch currents add to the source current and every branch has the same potential difference.
Apply the rules to labelled junctions and loops rather than memorising a phrase without identifying the topology. A drawing that looks side by side is not necessarily an electrical parallel connection.
Equivalent resistance and whole circuits
Series resistances add. Parallel branches provide extra paths, so equivalent resistance is smaller than the smallest branch resistance. Use this plausibility check before accepting a result.
For a whole circuit, combine equivalent resistance with , then return to branch rules. Keep the supply potential difference separate from potential differences across individual components.
Potential dividers and transducers
A potential divider shares a supply voltage between series components. The output across one component is a fraction determined by its resistance relative to total series resistance.
An NTC thermistor decreases resistance as temperature increases. An LDR decreases resistance as light intensity increases. In a potential divider, whether output rises or falls depends on which component the output is taken across.
Formulae and relationships
| Relationship | Use |
|---|---|
| Find series resistance. | |
| Find parallel resistance. | |
| Analyse an unloaded two-resistor potential divider. |
Worked examples
Example 1: Two resistors, and , are connected in parallel across . Find total current.
- Find equivalent resistance: , so .
- Use .
- Check: the equivalent resistance is below (3.0,Ω), as expected.
Answer: .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Apply current and potential-difference rules to clearly labelled series and parallel branches.
- Calculate equivalent resistance and check whether its size is physically plausible.
- Use electrical power and energy relationships without confusing ratings with energy consumed.
Official outcome coverage
K323 P15: 10 mapped outcomes, references P15(a), P15(b), P15(c), P15(d), P15(e), P15(f), P15(g), P15(h), P15(i), P15(j). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Build and troubleshoot a circuit from a standard symbol diagram, checking meter placement, polarity, and safe component limits before switching on.
Exam traps and retrieval check
Avoid these traps
- Adding parallel resistances directly.
- Assuming current splits equally through unequal branches.
- Predicting a potential-divider output without stating which component it is measured across.
Check from memory
What is shared by parallel branches?
Potential difference.
How does LDR resistance change in brighter light?
It decreases.
What must a parallel equivalent resistance be smaller than?
The smallest individual branch resistance.
Pure versus Combined scope
Combined Physics revisits part of this core under CP13 D.C. Circuits, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.