Simulation · Physics · Class 12
Resistivity as temperature changes
From the lesson Resistivity of materials and its temperature dependence in Current Electricity. Change the values and watch what happens.
Resistivity as temperature changesPhysics · Class 12
The idea behind it
NCERT § "Resistivity of Various Materials" and § "Temperature Dependence of Resistivity"
- By resistivity, materials fall into conductors (metals, low ρ), semiconductors (intermediate ρ) and insulators (very high ρ).
- For metals over a limited range, ρ_T = ρ₀[1 + α(T − T₀)], where α is the temperature coefficient of resistivity; α is positive for metals.
- Metals become more resistive when heated mainly because the relaxation time τ falls (more frequent collisions); n hardly changes.
- Alloys such as nichrome, manganin and constantan have high resistivity and very small temperature coefficient, so they are used in heating elements and standard resistors.
- For semiconductors resistivity decreases with rising temperature (negative α), because the number density of charge carriers n increases with temperature.
- For insulators and semiconductors, the rise in n with temperature outweighs the fall in τ, which is why their behaviour is opposite to metals.
- In problems, ignore the change in the wire's dimensions on heating unless told otherwise; then R changes in the same ratio as ρ.