Cornell Note — Main Concepts (Pinned) — Notes

Cue / Key Question	Notes
What is the formula for electric current?	I = $\frac{Q}{t}$ = neAvd; SI unit: A; [A]
What is drift velocity?	vd = eEτ/m = $\frac{I}{neA}$ ; ~10^{-4} m/s
What is Ohm's law?	V = IR; R = ρl/A
Series vs parallel?	Series: $R_{eq}$ = ΣRᵢ; Parallel: 1/ $R_{eq}$ = Σ(1/Rᵢ)
EMF and terminal voltage?	V = ε − Ir (discharging); V = ε + Ir (charging)
Kirchhoff's laws?	KCL: ΣI = 0 (charge cons.); KVL: Σ $\Delta V$ = 0 (energy cons.)
Wheatstone bridge balance?	P/Q = $\frac{R}{S}$ → Ig = 0
Metre bridge formula?	R/S = $\frac{l}{100 − l}$
Potentiometer principle?	$V \propto l$ ; at null: ε = kl (no current → true EMF)
Temperature effect?	Metals: α > 0 (R↑); Semiconductors: α < 0 (R↓)

Apparatus Reference: Potentiometer — measuring EMF and internal resistance of cells Wikimedia Commons: Potentiometer experimental setup

Summary Box: Current Electricity covers the flow of charge in conductors. The microscopic picture (drift velocity, electron density) connects to macroscopic laws (Ohm, Kirchhoff). Precision null methods (Wheatstone bridge, potentiometer) are the main NEET numerical targets.