NEET Physics Notes Alternating Current-Power in an AC Circuit
Power in an AC Circuit
Power in an AC Circuit
Let a voltage 
be applied across an AC and consequently a current 
flows through the circuit. Then,
- Instantaneous power = VI
- Average power over a full cycle of AC is
Wattless Current
Average power is given by 
The phase difference between E rms and I rms is Φ We can resolve lrms into two components
Here, the component 
contributes towards power dissipation and the component 
does not contribute towards power dissipation. Therefore, it is called Wattless current.
L-C Oscillations
An L-C circuit also called a resonant circuit, tank circuit or tuned circuit consists of an inductor and a capacitor when connected together they can act as an electrical resonator, storing energy oscillating at the circuits resonant frequency.
The energy oscillates back and forth between the capacitor and inductor until internal resistance makes the oscillations die out. The oscillation frequency is determined by the capacitance and inductance values.
Choke Coil
Let us consider a choke coil (used in tube lights) of large inductance L and low resistance R. The power factor for such a coil is given by
the current passing through the coil is very small. Such a coil is used in AC circuits for the purpose of adjusting current to any required value without waste of energy. The only loss of energy is due to hystersis in the iron core, which is much less than the loss of energy in the resistance that can also reduce the current, if placed instead of the choke coil.