Voltage, current, and phase angle. In general, the instantaneous voltage between two points in an ac circuit is not in phase with the instantaneous current passing through those points.
Resistance and reactance. The voltage across a resistor is in phase with the current, . The voltage across an inductor leads the the current by , , inductive reactance . The voltage across a capacitor lags the the current by , , capacitive reactance .
Impedance and the L-R-C series circuit. In general ac circuit, the voltage and current amplitutes are related by the circuit impedance , . In an L-R-C series circuit, , .
Power in ac circuits. The average power input to an ac circuit: , where is the phase angle of the voltage relative to the current. The factor is called the power factor of the circuit.
Resonance angular frequencey. .
Transformers. , .