An alternating current is that which flows first in one direction in a circuit, which is called positive direction, then flow in reverse direction which is called negative direction ,repeating such cycle of changes continuously.
The graph to a base of time shows the waveform of the current as a succession of instantaneous values.
In general there will be peak values in both positive and negative half cycles where the current is momentarily greatest.
The time occupied by one complete cycle is called the period
The number of cycle completed in one second is called frequency in cycle per second or Hertz ( Hz ).
An alternating voltage produced alternating current. The alternating voltage and current may or may not be in phase.If it is not in phase then they are said to have phase difference .
The term phase difference is only meaningful only in case of alternating quantities and harmonically varying function of the time.
The phase difference may be indicated by a time interval between the zeros of the voltage and current waves at the beginning of a positive half cycle.
The instantaneous value of alternating current is given by
If T is the time period of alternating current and f its frequency then
Instantaneous value of alternating current can be represented by sine equation or cosine equation. Both representation lead to the same result.
Similarly instantaneous value of alternating e.m.f may be represented by
Since alternating current varies continuously with time ,its effect is measured either by mean value ( or average value )of a.c or by defining root mean square value ( or virtual value ) of A.C current
Average and RMS value
Mean or average value of AC;
The steady or direct current which transfer in the circuit the same charge as a transferred by the ac during the alternation is called the mean or average value of alternating wave.
The ac voltage or current at any time is given by
where w is called angular frequency of AC.
Relation Between mean value and peak value
RMS Value of AC
The RMS value of an alternating current is the value of direct current which produced in the same conductor the same amount of heat in the same time.
R.M.S Value of A.C current=Virtual value =Effective current
Relation between virtual value and peak value of a.c
The R.M.S or virtual value of ac is same even for a complete cycle of ac
This fact fact is made use of to construct hot wire instruments for measuring current and voltages.Such ammeter and voltmeter record virtual value of alternating current and voltage
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