Current Electricity

1. 1Electric current is the rate of flow of charge; in metals free electrons move opposite to conventional current.
2. 2Drift velocity v_d is the average speed of electrons; I = n e A v_d gives the relation.
3. 3Mobility μ equals v_d divided by the electric field E; it shows why signals travel fast while electrons crawl.
4. 4Ohm's law V=IR holds at constant temperature; a metal gives a straight V‑I line, a diode or filament does not.
5. 5Resistivity ρ = R A / l is a material property; conductivity σ = 1/ρ. For metals ρ rises with temperature, for semiconductors it falls.
6. 6A cell has emf ε and internal resistance r; terminal voltage V = ε – I r drops as current increases.
7. 7TRY IT: Measure a cell’s open‑circuit voltage, then connect a bulb and record the drop; also compare a thin long wire vs a short thick one, and note ammeter‑in‑series and voltmeter‑in‑parallel placements.
8. 8Series cells add ε and r; parallel identical cells keep ε same and split r. Resistors in series share current, in parallel share voltage. Kirchhoff’s junction rule conserves charge; loop rule conserves energy (Gustav Kirchhoff’s 1845 discovery). Wheatstone bridge balances when P/Q = R/S, letting us find an unknown R without current. Power P = VI = I²R = V²/R; heat generated is I²Rt.
9. 9Recap: current direction, drift speed, mobility, Ohm’s law, resistivity, internal resistance, series/parallel rules, Kirchhoff’s laws, Wheatstone bridge, and power formulas.

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