What is the difference between AC and DC?

DIRECT CURRENT (DC): electrons flow in one direction only. Constant polarity. Examples: batteries (chemical reactions provide constant voltage), solar cells, DC generators, USB power, automotive 12V system, electronic devices internally. ALTERNATING CURRENT (AC): electrons reverse direction periodically. Voltage cycles between positive and negative peaks. In US household wiring, AC alternates 60 times per second (60 Hz). Most of the world uses 50 Hz. Why AC dominates electrical distribution: (1) TRANSFORMERS work with AC, allowing efficient voltage transformation; (2) HIGH VOLTAGE TRANSMISSION at low current minimizes I²R losses in wires; (3) Stepping down to safer voltages near use; (4) Easier to generate with rotating machinery (generators naturally produce AC). 'War of currents' (1880s-90s): Edison championed DC; Tesla/Westinghouse championed AC; AC won due to transformer-enabled distribution. Modern situation: AC for transmission and distribution; DC for electronics and increasingly for HVDC long-distance transmission (less loss, easier underwater/underground). Conversion: RECTIFIER (diodes) converts AC to DC — used in power supplies, chargers; INVERTER converts DC to AC — used in solar systems (panels produce DC, grid uses AC), UPS, electric vehicle drives. AC waveforms: typically SINUSOIDAL (smooth sine wave); also SQUARE WAVE (digital, switches between high and low); SAWTOOTH; triangular. AC voltage specifications: PEAK voltage (maximum positive or negative); PEAK-TO-PEAK (peak positive to peak negative); RMS (root mean square — equivalent DC voltage that delivers same power; for sine wave, RMS = peak / √2 ≈ 0.707 × peak); 120V US household is RMS — peak is about 170V. AC frequency in different countries: 60 Hz (US, Canada, much of Americas, parts of Asia); 50 Hz (Europe, Africa, most of Asia, Australia). Voltage: 120V (US, Canada, Mexico); 230V (Europe, much of world); 100V (Japan). International travelers need adapters and possibly voltage converters.