Asked by Emma
what causes an object to be a conductor
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An object is a conductor if it contains charged particles that are free to move when an electric field is applied. In other words, conduction requires both charge carriers and a way for them to move.
Key points:
- Mobile charge carriers: In metals these are delocalized (free) electrons; in ionic solutions and molten salts they are ions; in semiconductors carriers are electrons and holes that can be generated thermally or by doping.
- Band structure (quantum picture): A conductor has a partially filled conduction band or overlapping valence and conduction bands so electrons can move to slightly different energy states without a large energy input. Insulators have a large band gap that prevents this.
- Carrier density and mobility determine conductivity: conductivity σ ≈ n q μ (n = carrier density, q = charge magnitude, μ = mobility). In the Drude model σ = n e^2 τ / m (τ = average time between scattering events).
- Scattering and lattice structure limit mobility: collisions with phonons (thermal vibrations), impurities, and defects reduce mobility and raise resistivity.
- Temperature behavior differs: metals usually conduct worse at higher temperatures (more scattering); intrinsic semiconductors conduct better at higher temperatures (more carriers).
- Special cases: superconductors have zero resistivity below a critical temperature due to Cooper pairing of electrons.
Examples: copper is a good conductor because it has many delocalized electrons; pure water is a poor conductor but saltwater conducts because dissolved ions are mobile.
In short: an object is a conductor when it has sufficient mobile charged particles and a band structure or medium that lets them move under an applied field.
Key points:
- Mobile charge carriers: In metals these are delocalized (free) electrons; in ionic solutions and molten salts they are ions; in semiconductors carriers are electrons and holes that can be generated thermally or by doping.
- Band structure (quantum picture): A conductor has a partially filled conduction band or overlapping valence and conduction bands so electrons can move to slightly different energy states without a large energy input. Insulators have a large band gap that prevents this.
- Carrier density and mobility determine conductivity: conductivity σ ≈ n q μ (n = carrier density, q = charge magnitude, μ = mobility). In the Drude model σ = n e^2 τ / m (τ = average time between scattering events).
- Scattering and lattice structure limit mobility: collisions with phonons (thermal vibrations), impurities, and defects reduce mobility and raise resistivity.
- Temperature behavior differs: metals usually conduct worse at higher temperatures (more scattering); intrinsic semiconductors conduct better at higher temperatures (more carriers).
- Special cases: superconductors have zero resistivity below a critical temperature due to Cooper pairing of electrons.
Examples: copper is a good conductor because it has many delocalized electrons; pure water is a poor conductor but saltwater conducts because dissolved ions are mobile.
In short: an object is a conductor when it has sufficient mobile charged particles and a band structure or medium that lets them move under an applied field.
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