# Notes And Questions NCERT Class 12 Physics Chapter 7 Alternating Current

Please refer to Alternating Current Class 12 Physics Notes and important questions below. The Class 12 Physics Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 12 Physics examination questions given below to obtain better marks in exams

## Alternating Current Class 12 Physics Notes and Questions

The below Class 12 Alternating Current notes have been designed by expert Physics teachers. These will help you a lot to understand all the important topics given in your NCERT Class 12 Physics textbook. Refer to Chapter 7 Alternating Current Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.

• Alternating Current:
The current whose magnitude changes with time and direction reverses periodically is called alternating current. a) Alternating emf E and current I at any time am given by:
E = E0 sin ωt
E0 = NBAω
where l = l0 sinωt – ø)

Where T is the time period.

• Values of Alternating Current and Voltage a) Instantaneous value:
It is the value of alternating current and voltage at an instant t.
b) Peak value:
Maximum values of voltage Eo and current Io in a cycle are called peak values.
c) Mean value:
For complete cycle,

RMS values are also called apparent or effective values.

• Phase difference Between the EMF (Voltage] and the Current in an AC Circuit
a) For pure resistance:
The voltage and the current are in same phase i.e. phase difference9} = 0
b) For pure inductance:
The voltage is ahead of current by  π/2 i.e. phase difference ø =  – π / 2
c) For pure capacitance:
The voltage lags behind the current by π/2 i.e. phase difference ø =  – π / 2

• Reactance:
Reactance

• Impedance:
Impedance is defined as,

Where (jJ is the phase difference of the voltage E relative to the current I.

a) For L- R series circuit:

•   Conductance:
Reciprocal of resistance is called conductance.

• Power in and AC Circuit:
a) Electric power= (current in circuit) x (voltage in circuit) P= IE
b) Instantaneous power:
Pinst = Einst X hnst
c) Average power:

d) Virtual power (apparent power):

• Power Factor:
a) Power factor

b) For pure inductance
Power factor, cos ø = 1
c) For pure capacitance
Power factor, cos ø = 0
d) For LCR circuit

• Watt less Current:
The component of current differing in phase by  π/2 relative to the voltage, is called wattles current.

• The rms value of watt less current

• Choke Coil:
a) An inductive coil used for controlling alternating current whose self- inductance is high and resistance in negligible, is called choke coil.
b) The power factor of this coil is approximately zero.

• Series Resonant Circuit
a) When the inductive reactance (XL) becomes equal to the capacitive reactance (XC) in the circuit, the total impedance becomes purely resistive (Z=R).
b) In this state, the voltage and current are in same phase (ø = 0), the current and power are maximum and impedance is minimum. This state is called resonance. c) At resonance

d) In resonance, the power factor of the circuit is one.

• Half- Power Frequencies:
Those frequencies f1 and f2 at which the power is half of the maximum power (power at resonance), i.e., f1 and f2 are called half- power frequencies.

• Band – Width:
a) The frequency interval between half- power frequencies is called band- width.
:. Bandwidth △f = f2 – f1
b) For a series LCR resonant circuit,