NEET Physics Notes Alternating Current-Concept of Alternating Current
Concept of Alternating Current
Concept of Alternating Current
An alternating current is the current (or voltage) whose magnitude keeps on changing continuously with time, between zero and a maximum value and the direction also reverses periodically.
Peak and RMS Values of Alternating Current / Voltage
The common alternating current varying as a sine function of time, is given by I
Here, I0 and V0 are the maximum or peak values of current and voltage, ω the angular frequency, v the frequency and T the period of the given AC. In our domestic circuits, the frequency of AC is 50 Hz. Thus, it changes its direction after every 
The average value of an AC, for one complete cycle is zero.
Generally, average or mean value of an AC means the average value of the given AC over one half cycle, i.e
Reactance and Impedance
The opposition offered by a pure inductor or capacitor or both to the flow of AC, through it, is called reactance X. Its unit is ohm 
and dimensional formula is ML2T-3A-2].
Inductive reactance, 
Capacitive reactance, 
Reciprocal of reactance is known as susceptance.
Thus, 
Total opposition offered by an AC circuit to the flow of current through it, is called its impedance Z. Its unit is ohm and dimensional formula is [ML2T-3A-2]. For an AC circuit
Reciprocal of impedance is known as admittance.
Thus, 
Its unit is Siemens (S).
AC Circuit Elements
The circuit consists of resistor, capacitor and inductor are called pure resistive, pure inductive and pure capacitive circuit. There circuits with their respective phasor diagram are given below.
- Pure Resistive Circuit
Let an alternating voltage 
be applied across a pure resistance R. Then,
Current and voltage are in the same phase, i.e. current is given by 
- Pure Inductive Circuit
Let an alternating voltage 
be applied across a pure inductance L. Then, the average
The inductance offers some opposition to the flow of AC. It is known as “inductive reactance”
Thus, a pure inductance does not oppose the flow of DC ω = 0 but opposes the flow of AC.
- Pure Capacitive Circuit
Let an alternating voltage be applied across a pure capacitance C. Then,
the capacitance offers some opposition to the flow of current but allows AC to pass through it. The opposition offered is known as the capacitive reactance.
