Current and Resistance (Physics)

Recall cards on electric current, resistance, and superconductors. Electrical current: current as the rate of charge flow, I = dQ/dt, the ampere and its name, the need for a complete circuit, conventional current, electron carriers in metals, carriers in ionic solutions, and drift velocity. Model of conduction in metals: the free-electron sea, the zig-zag drift path, signal speed versus drift speed, I = nqAv_d, current as a scalar, current density J as a vector, J = nqv_d, E = rho J, and why good electrical conductors conduct heat. Resistivity and resistance: resistivity rho, its reciprocal conductivity, the ohm-meter, conductor versus insulator versus semiconductor, R = V/I, R = rho L/A, the ohm, the temperature dependence rho = rho_0[1 + alpha(T - T_0)], and the temperature coefficient alpha. Ohm's law: V = IR, Ohm's experiment, its empirical nature, ohmic versus nonohmic devices, and the diode. Electrical energy and power: P = IV, P = I^2 R, P = V^2/R, the watt, resistive heating, E = Pt, and the kilowatt-hour. Superconductors: zero resistance below a critical temperature, Onnes's discovery, mercury, the Meissner effect, Type I and Type II, YBCO, BCS theory and Cooper pairs, persistent currents, MRI magnets, SQUIDs, and the Josephson effect.