**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.** , .

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