Hi friends in this video we are going to
see one more important parameter of a AC
waveform and that is phase standard
equation is actually given as small e
equal to e M sine Omega T plus Phi so we
have miss out this phi earlier or 
consider Phi equal to zero but if this
Phi is not zero then what about so Phi
is nothing but a phase of AC quantity so
let us define what phase is the phase of
an alternating quantity at any instant
is the angle theta travelled by the
phase representing that alternating
quantity up to the instant of
consideration measured from the
reference so suppose this waveform I
have considered and this waveform is
nothing but a straight line like this
this waveform is traveling like this
from 0 to 2pi same way this line is
traveling from 0 to 2pi like this
completing a circle so at one particular point of
instant where this quantity is making
angle theta with this x-axis
correspondingly the waveform is travel
like this from here to here so at this
particular instant the phase of the
quantity is theta same way I represented
theta over here so what is the phase
phase is nothing but from the reference
how much the AC quantity is traveled it
may travel before reference or it may
start travel after reference reference
we consider normally theta equal to 0 so
lets see a phase difference concept so
actually phase and phase difference are
nothing but one and the same meaning
suppose I will consider a reference VM
sine theta because the phase over here
is zero one
more quantity I consider here i equal to
IM sine theta so here also it is IM sine
theta i am not considering any phase
meaning this two quantities V and I both
are having Phi 0 that means V is
starting from 0 same way I is also
starting from 0 so the phase difference
between them is actually 0 same way I
consider one more diagram where I can
say a lagging phase what does that mean
reference I am considering again a VM
sine theta like this starting from 0
second quantities I is starting from
here it is not a zero okay so it start
late so it will start late by angle Phi
so this angle Phi is over here that means
this quantity is started from here where
the reference is saying it is starting
from zero so there is a difference
between the phases of this two quantity
and that difference is phi and the current
I is starting late late means it is a
lagging phase so for a lagging phase
quantities the wave form will start
after 0 0 is our reference suppose I
consider a leading phase once again this
quantity is the reference starting from
0 i denoted as v equal to VM sine theta
whenever it start the other quantity
which is I already started so it has
already started and the starting point
for this quantity is somewhere here
before 0 so since it has already started
that means it is leading it has taken a
lead over a reference so that quantity
is leading so for a leading phase I am
having this diagram so what we have seen
over here phase difference is nothing
but difference between the phases of two
quantities and phase of a one particular
AC quantity is nothing but a difference
of reference and that particular Ac
quantity so here we have seen that there
are three types of phase differences
possible one if two quantities are
starting from the same instant and
ending on the same instant then I can
say those quantities are in phase one
quantity is late compared to the other
quantity then we can say that quantity
is lagging third whenever quantity has
already started which has already
started its journey before second
quantity appearing then that quantity we
can say a leading quantity so here we
have discussed phase and phase
differences which are very much
essential in order to know the concept
called as phasor diagram thank you
