Unit Overview
Current of electricity
CIE A-Level PhysicsΒ· 5 min read π n/a
1. Unit at a Glance
This unit builds from the fundamental definition of electric current up to the factors that affect how materials resist current flow. You will start with a microscopic description of charge movement before moving to macroscopic definitions of voltage, resistance and component behaviour.
The concepts you master here are critical for understanding practical circuits, Kirchhoff's laws, and advanced topics like electromagnetism later in your syllabus. The sub-topics follow a logical sequence, building incrementally from basic definitions to complex material behaviour.
This unit is split into the following sequential sub-topics:
Electric current
Learn the formal definition of electric current and calculate current from charge flow.
β β± 3 min
Charge carriers and drift velocity
Explore the microscopic model of current and work with the relationship .
β β β β± 5 min
Potential difference and e.m.f.
Distinguish between p.d. and e.m.f. and calculate energy transfer in electrical circuits.
β β β± 4 min
Resistance and resistivity
Define resistance and resistivity, and calculate how sample dimensions affect total resistance.
β β β± 5 min
I-V characteristics
Analyse and compare I-V behaviour for ohmic conductors, filament lamps and diodes.
β β β± 4 min
Temperature dependence of resistance
Explain how temperature changes affect resistance in metals and thermistors.
β β β β± 4 min
2. Common Pitfalls
Wrong move:
Confusing e.m.f. and potential difference
Why:
Students often mix up these two similar energy-related quantities
Correct move:
E.m.f. is energy supplied to charge by a source; p.d. is energy transferred from charge to components per unit charge
Wrong move:
Mixing up resistance and resistivity
Why:
Resistance depends on the size and shape of a material sample, while resistivity is a property of the material itself
Correct move:
Resistivity is constant for a material at a given temperature, while resistance changes with length and cross-sectional area
Wrong move:
Confusing resistance behaviour for NTC thermistors and metals
Why:
Both materials respond differently to temperature increases
Correct move:
Metals: resistance increases with temperature; NTC thermistors: resistance decreases with temperature
3. Quick Reference Cheatsheet
Concept | Key Relationship |
|---|---|
Electric current | |
Drift velocity model | (n = number density, A = area, q = carrier charge, v = drift velocity) |
Resistance (Ohm's law) | |
Resistivity | |
Temperature dependence of resistance | (Ξ± = temperature coefficient of resistance) |
What's Next
Start with the first sub-topic below to build your foundational understanding of electric current. Once you complete all sub-topics in this unit, you will move on to the next unit covering DC circuits, where you will apply these core concepts to solve problems with combinations of components and cells.