SEC G3 Combined Science Physics component K326/K327
CP12: Electric Charges and Current of Electricity
Separate static charge from current and relate charge flow, potential difference, and resistance.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Circuit quantities become clear when charge is tracked through time and energy transfer is tracked for each coulomb.
Charge and current
Electric charge may be positive or negative and is measured in coulombs. Like charges repel and unlike charges attract.
Current is charge flow per unit time. Conventional current is defined from positive to negative outside a source, while electrons in a metal move in the opposite direction.
Energy per charge and resistance
The e.m.f. of a source describes energy supplied per charge and is measured in volts. Potential difference across a component describes work done per charge as charges pass through it.
Resistance compares potential difference with current. State the component and operating condition when interpreting a resistance value.
Wire dimensions
For the same material and conditions, a longer wire has greater resistance because charge carriers encounter more of the conductor. A larger cross-sectional area gives lower resistance because more conducting paths are available.
For the same material and temperature, resistance is directly proportional to length and inversely proportional to cross-sectional area, so . In ratio form, . Change only one dimension unless the ratio form accounts for both.
Formulae and relationships
| Relationship | Use |
|---|---|
| Relate charge, current, and time. | |
| Relate resistance, potential difference, and current. | |
| Compare wires of the same material at the same temperature. |
Worked examples
Example 1: A current of flows for . Find the charge transferred.
- Convert to .
- Use .
- Calculate .
Answer: .
Example 2: A wire is replaced by one of the same material with twice the length and three times the cross-sectional area. Compare the new resistance with the original.
- Use resistance proportional to length divided by area.
- Form the ratio .
- Simplify the ratio to .
Answer: The new resistance is two thirds of the original resistance.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Keep positive and negative charge interactions separate from continuous charge flow in a circuit.
- Relate current, charge, potential difference, and resistance without treating charge and energy as the same quantity.
- Compare wire resistance by changing length or cross-sectional area while controlling material and temperature.
Official outcome coverage
K326 CP12: 10 mapped outcomes, references CP12(a), CP12(b), CP12(c), CP12(d), CP12(e), CP12(f), CP12(g), CP12(h), CP12(i), CP12(j). Check the official K326 syllabus.
K327 CP12: 10 mapped outcomes, references CP12(a), CP12(b), CP12(c), CP12(d), CP12(e), CP12(f), CP12(g), CP12(h), CP12(i), CP12(j). 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
Determine the resistance of a low-voltage circuit component from current and potential-difference readings, or compare wires while changing length or cross-sectional area. Place the ammeter in series and voltmeter in parallel, control material and temperature, repeat readings, and disconnect between trials if heating matters.
Exam traps and retrieval check
Avoid these traps
- Using minutes without conversion to seconds.
- Giving electron flow in the conventional-current direction.
- Saying a thicker wire has greater resistance under otherwise equal conditions.
Check from memory
What is current?
Rate of charge flow.
What does one volt describe?
One joule of energy transferred per coulomb.
How does increasing wire area affect resistance?
It decreases resistance.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Charge, current, charge flow, e.m.f., potential difference, resistance, and resistance of a wire.