Current Electricity: notes

Basics

• Current: the flow of charge, in amperes (A). I = Q / t. 12 C in 4 s is 3 A.
• Potential difference: energy given to each coulomb, in volts (V).
• Resistance: R = V / I, in ohms. 12 V across 4 Ω gives 3 A.
• Ohm's law: for a metal at steady temperature, current is proportional to voltage.

Circuits

• Series: one path. The current is the same everywhere and the voltages add up. Resistances add: R = R1 + R2.
• Parallel: several paths. The voltages are the same on each branch and the currents add up. Total resistance is less than the smallest branch.
• Meters: an ammeter goes in series; a voltmeter goes in parallel.

Power and safety

• Electrical power: P = IV. A 2 A current at 230 V is 460 W.
• Energy: E = Pt. The kilowatt-hour is 1 kW used for 1 hour.
• Fuse and earth wire: a fuse goes in the live wire and melts when the current is too high. The earth wire carries fault current away from the metal case.