Types of Conductor

Metallic conductors — an enormous number density of free electrons, around 10²⁹ m⁻³, so they conduct well. Resistance rises with temperature because lattice vibrations increase while the number of carriers stays essentially fixed.

Semiconductors — far fewer free charge carriers than a metal, but heating releases many more. This increase in number density outweighs the increased vibration, so resistance falls as temperature rises. The two materials respond to heat in opposite directions, and for the same reason: whichever effect dominates.

Thermistor (NTC) — a semiconductor device whose resistance falls sharply as temperature rises. NTC stands for negative temperature coefficient. Used in temperature sensors and thermostats.

Light-dependent resistor (LDR) — resistance falls as light intensity rises, because incident photons free additional charge carriers. In darkness its resistance may be megohms; in bright light, a few hundred ohms.

Example: both sensors work the same way — something external liberates more charge carriers, so n rises and R falls. Recognising that they share a mechanism makes both easier to reason about than memorising two separate behaviours.

Diode — conducts in one direction only. In forward bias it conducts once the p.d. exceeds a threshold of about 0.6 V for silicon; in reverse bias the resistance is extremely high and effectively no current flows.

Superconductors — some materials, cooled below a critical temperature, lose all electrical resistance. Current then flows with no potential difference and no energy dissipated, so a current once started persists indefinitely.

Why superconductors matter — no resistance means no heating loss, which makes possible very strong electromagnets for MRI scanners and particle accelerators, and lossless power transmission. The obstacle is that critical temperatures are very low, so the cooling costs more energy than the resistive losses it saves except in applications where the strong field is the point.