Number density (n)
Charge & Current - OCR A-Level Physics
Key Definition
Number density (n)
The number of free charge carriers per unit volume of a material. Measured in m^-3.
The number of free charge carriers per unit volume of a material. Measured in m^-3.
- ConductorsMaterials with a high number density of free charge carriers (e.g. metals), allowing easy flow of current. (metals) have high n. Copper: n \(\sim 8.5 \times 10^{2}\)8 m^-3.
- SemiconductorsMaterials with resistivityA material property that quantifies how strongly it resists currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohm-metres (Ω m). between conductors and insulators. Their number densityMass per unit volume of a material. Measured in kg m⁻³.The number of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carriers per unit volume in a material. Measured in m⁻³. of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carriers increases with temperature. have medium n. Silicon: n \(\sim 10^{1}\)5 m^-3 at room temperature.
- InsulatorsMaterials with very few free charge carriers, so they do not conduct electricity under normal conditions. have very low n. Almost no free charge carriers.
- For a semiconductor, n increases as temperature increases.
- This is why semiconductorsMaterials with resistivityA material property that quantifies how strongly it resists currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohm-metres (Ω m). between conductors and insulators. Their number densityMass per unit volume of a material. Measured in kg m⁻³. of charge carriers increases with temperature. conduct more at higher temperatures.
Examiner Tips and Tricks
- Number densityMass per unit volume of a material. Measured in kg m⁻³.The number of charge carriers per unit volume in a material. Measured in m⁻³. is an OCR-specific concept not tested in AQA.
- You must be able to explain why conductors, semiconductorsMaterials with resistivityA material property that quantifies how strongly it resists currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohm-metres (Ω m). between conductors and insulators. Their number density of charge carriers increases with temperature., and insulators differ in terms of n - not just state it.
- Link n to the material's ability to conduct.
Common Mistake
MEDIUM
Wrong: Confusing number densityThe number of charge carriers per unit volume in a material. Measured in m⁻³. n with the number of electrons n in $Q = ne$.
Right: In Q = ne, n is a simple count (no units). In I = Anev, n is number density with units m^-3. Context tells you which n is meant.
Right: In Q = ne, n is a simple count (no units). In I = Anev, n is number density with units m^-3. Context tells you which n is meant.