Current Electricity
Class 12 · Current Electricity
R(T) — Materials Compared
Cu, Fe, W (positive α) vs Si (negative α). Linear plot.
Key Notes
01
Metals: R(T) = R₀(1 + αΔT). α > 0 — resistance increases with T.
02
Semiconductors and insulators: α < 0 — resistance DECREASES with T (more carriers thermally generated).
03
Superconductors: R = 0 below critical temperature T_c.
Formulas
Linear approximation
Valid for small ΔT.
Exponential (semiconductor)
More accurate for non-metals.
Important Points
Cu α ≈ 4 × 10⁻³/K. Si α negative and large in magnitude.
Light bulbs draw a surge current at switch-on (cold filament has lower R).
R(T) — Materials Compared notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.