Alternating Current

• Alternating currents −

  • Peak and RMS value of alternating current/voltage

  • Reactance and impedance

  • LC oscillations (qualitative treatment only)

  • LCR series circuit

  • Resonance

  • Power in AC circuits

  • Wattless current

• AC generator and transformer

SUMMERY

1. In a purely inductive or capacitive circuit, cosφ = 0 and no power is dissipated even though a current is flowing in the circuit. In such cases, current is referred to as a wattless current.

2. The phase relationship between current and voltage in an ac circuit can be shown conveniently by representing voltage and current by rotating vectors called phasors. A phasor is a vector which rotates about the origin with angular speed ω. The magnitude of a phasor represents the amplitude or peak value of the quantity (voltage or current) represented by the phasor. The analysis of an ac circuit is facilitated by the use of a phasor diagram.

3. An interesting characteristic of a series RLC circuit is the phenomenon of resonance.

4. A circuit containing an inductor L and a capacitor C (initially charged) with no ac source and no resistors exhibits free oscillations

5. A transformer consists of an iron core on which are bound a primary coil of Np turns and a secondary coil of Ns turns. If the secondary coil has a greater number of turns than the primary, the voltage is stepped-up (Vs > Vp ). This type of arrangement is called a stepup transformer. If the secondary coil has turns less than the primary, we have a step-down transformer.

6. The average power loss over a complete cycle is given by P = V I cosφ The term cosφ is called the power factor.

7. The current through the capacitor is π/2 ahead of the applied voltage. As in the case of inductor, the average power supplied to a capacitor over one complete cycle is zero.