# Current of electricity

> CIE A-Level Physics · CIE 9702
> Source: https://www.owlsprep.com/study/cie-9702-u9-overview/
> Weight: n/a

This unit introduces all core concepts of direct current electricity, from the definition of current to the behaviour of resistors and common components. It forms the essential foundation for all further electricity and circuit units in your A-Level course.

**Prerequisites:** Basic understanding of electric charge and energy from GCSE Physics

## Learning objectives

- Define core electrical quantities (current, p.d., e.m.f., resistance, resistivity) correctly
- Apply key formulas relating current, drift velocity, resistance and temperature to solve standard problems
- Interpret and compare I-V characteristics of common electrical components
- Explain the effect of temperature on resistance for different types of materials

## 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](https://www.owlsprep.com/study/cie-9702-u9-electric-current/) — Learn the formal definition of electric current and calculate current from charge flow.
- [Charge carriers and drift velocity](https://www.owlsprep.com/study/cie-9702-u9-charge-carriers-and-drift-velocity/) — Explore the microscopic model of current and work with the relationship $I = nAqv$.
- [Potential difference and e.m.f.](https://www.owlsprep.com/study/cie-9702-u9-potential-difference-and-e-m/) — Distinguish between p.d. and e.m.f. and calculate energy transfer in electrical circuits.
- [Resistance and resistivity](https://www.owlsprep.com/study/cie-9702-u9-resistance-and-resistivity/) — Define resistance and resistivity, and calculate how sample dimensions affect total resistance.
- [I-V characteristics](https://www.owlsprep.com/study/cie-9702-u9-i-v-characteristics/) — Analyse and compare I-V behaviour for ohmic conductors, filament lamps and diodes.
- [Temperature dependence of resistance](https://www.owlsprep.com/study/cie-9702-u9-temperature-dependence-of-resistance/) — Explain how temperature changes affect resistance in metals and thermistors.

## Common pitfalls

- **Wrong:** Confusing e.m.f. and potential difference
  - Why it fails: Students often mix up these two similar energy-related quantities
  - Correct: E.m.f. is energy supplied to charge by a source; p.d. is energy transferred from charge to components per unit charge
- **Wrong:** Mixing up resistance and resistivity
  - Why it fails: Resistance depends on the size and shape of a material sample, while resistivity is a property of the material itself
  - Correct: Resistivity is constant for a material at a given temperature, while resistance changes with length and cross-sectional area
- **Wrong:** Confusing resistance behaviour for NTC thermistors and metals
  - Why it fails: Both materials respond differently to temperature increases
  - Correct: Metals: resistance increases with temperature; NTC thermistors: resistance decreases with temperature

## Cheatsheet

| Concept | Key Relationship |
| --- | --- |
| Electric current | $I = \frac{\Delta Q}{\Delta t}$ |
| Drift velocity model | $I = nAqv$ (n = number density, A = area, q = carrier charge, v = drift velocity) |
| Resistance (Ohm's law) | $R = \frac{V}{I}$ |
| Resistivity | $\rho = \frac{RA}{L}$ |
| Temperature dependence of resistance | $R = R_0(1 + \alpha \Delta T)$ (α = 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.

- [Electric current](https://www.owlsprep.com/study/cie-9702-u9-electric-current/)
- [Charge carriers and drift velocity](https://www.owlsprep.com/study/cie-9702-u9-charge-carriers-and-drift-velocity/)
- [Potential difference and e.m.f.](https://www.owlsprep.com/study/cie-9702-u9-potential-difference-and-e-m/)

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