ALTERNATING CURRENT AND VOLTAGE
Voltage or current is said to be alternating if it is change continously in magnitude and perodically in direction with time. It can be represented by a sine curve or cosine curve
I = I0
sin t or I = I0
cos t
where I = Instantaneous value of current at time t, I
0
= Amplitude or peak value
= Angular frequency = 2
T
= 2f T = time period f = frequency
AMPLITUDE OF AC
The maximum value of current in either direction is called peak value or the amplitude of current. It is represented
by I0
. Peak to peak value = 2I0
PERIODIC TIME
The time taken by alternating current to complete one cycle of variation is called periodic time or time period
of the current.
FREQUENCY
The number of cycle completed by an alternating current in one second is called the frequency of the current.
UNIT : cycle/s ; (Hz)
In India : f = 50 Hz , supply voltage = 220 volt In USA :f = 60 Hz ,supply voltage = 110 volt
GOLDEN KEY POINT
• AC can't be used in
(a) Charging of battery or capacitor (as its average value = 0)
(b) Electrolysis and electroplating (Due to large inertia, ions can not follow frequency of A.C)
• The rate of change of A.C.
Minimum, at that instant when they are near their peak values
Maximum, at that instant when they change their direction.
• For alternating current I0
> Irms > Iav.
• Average value over half cycle is zero if one quarter is positive +
– and the other quarter is negative.
• Average value of symmetrical AC for a cycle is zero thats why average potential difference on any
element in A.C circuit is zero.
• The instrument based on heating effect of current are works on both A.C and D.C supply and also provides
same heating for same value of A.C (rms) and D.C. that's why a bulb bright equally in D.C. and A.C. of same
value.
• If the frequency of AC is f then it becomes zero 2f times in one second and the direction of current changes
2f times in one second. Also it become maximum 2f times in one second.
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