Conductors, Insulators & Semiconductors
Free electrons make the difference, and they empty a conductor of field.
Lesson at a glanceShow the key points
- Conductors: free (conduction) electrons drift against an applied field. Insulators/dielectrics: electrons bound, only shift slightly.
- Semiconductors: insulating at 0 K; a few electrons (and their vacancies) conduct at higher temperatures.
- In a field, a conductor's electrons redistribute to the surface until the internal field cancels: E = 0 inside in electrostatics.
Put a material in an electric field and what happens depends on whether its electrons are free to roam. That single fact divides materials into conductors, insulators and semiconductors.
Three kinds of material
Outer electrons are only loosely bound and roam freely through the material (free or conduction electrons). In a field they drift opposite to it. Metals, mercury, ionized gases.
All electrons are tightly bound to their atoms or molecules. In a field they shift slightly but can’t travel. Most non-metals: glass, plastic, rubber.
Semiconductors behave as insulators at 0 K, but at higher temperatures a few electrons break free and respond to a field; the vacancies they leave behind also help conduction. They sit between conductors and insulators (a later chapter covers them).
No field inside a conductor
A neutral metal plate has free electrons spread evenly. Switch on a field from left to right: the electrons drift left, the left face becomes negative and the right face positive. These charges make an internal field pointing right to left, which grows as more electrons drift. The drift stops when it exactly cancels the applied field. This happens in well under a millisecond.
So, in electrostatics, the electric field inside a conductor is zero, and any extra charge on a conductor sits on its surface. This result will be used again and again in the next chapters.
- 1Conductors have free electrons; insulators (dielectrics) don't; semiconductors have a few, increasing with temperature.
- 2In a field, a conductor's electrons redistribute until the field inside is zero.
- 3In electrostatics E = 0 inside a conductor; charge resides on the surface.
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Keep exploring
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