Current Electricity
Class 12 · Current Electricity
R = ρL/A
Wire stretches/thins live; resistance updates accordingly.
Key Notes
01
R = ρL/A — proportional to length, inversely proportional to cross-section.
02
Resistivity ρ is material-dependent: copper ~1.7 × 10⁻⁸ Ω·m, nichrome ~1.1 × 10⁻⁶ Ω·m.
03
Stretching a wire to length nL keeps mass constant → A drops to A/n → R becomes n²R.
Formulas
Resistance
Geometric × material.
Important Points
Be careful with stretching: ρ doesn't change but L and A both change.
Conductivity σ = 1/ρ.
R = ρL/A 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.