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Current Electricity - JEE Notes



1. Electric Current (I)
- Definition: Charge flow per unit time.
- Formula: I = Q / t, where Q is charge, t is time.
- SI Unit: Ampere (A).
- Conventional Current: Flow of positive charges.
- Electron Flow: Opposite to current direction.


2. Drift Velocity (v_d) & Relation with Current
- Drift Velocity: Average velocity of electrons due to an electric field.
- Formula: v_d = I / (nAe), where:
I = Current, n = Electron density, A = Area, e = Charge of electron.
- Mobility: mu = v_d / E (E = Electric Field).


3. Ohm's Law
- Statement: V = IR.
- Graph: Straight line (for ohmic conductors).
- Non-Ohmic Conductors: Diodes, Filament Bulbs, etc.


4. Resistance (R)
- Formula: R = rho * (L / A), where rho = resistivity, L = length, A = area.
- Factors Affecting Resistance:
- Material (higher resistivity -> higher resistance).
- Length (longer wire -> more resistance).
- Cross-sectional Area (thicker wire -> less resistance).
- Temperature: R = R_0 (1 + alpha * Delta T).


5. Kirchhoff's Laws
(i) Kirchhoff's Current Law (KCL)
- Statement: "Sum of currents entering a junction = sum of currents leaving it."
- Formula: Sum of I_in = Sum of I_out.