Differences between alternating current (ac) and direct current (dc)
Contents::
 Direct Current (dc)
 Frequency of dc Current.
 Alternating current (ac)
 unidirectional current
 Mixture of ac and dc.
 Differences between ac and dc current?
 Benefits of ac Current over dc current . Why AC current is supplied in commercial power supply?
Direct Current (DC)::
 The current which do not change its magnitude and direction with respect to time is called a direct Current or DC.

Fig1: direct current 
Here in Fig1 we can see that the magnitude of the direct current is constant which is shown in yellow line and it's direction doesn't alter , always along Positive Y direction.
 Frequency of dc: since dc Current don't change its magnitude and direction then there is no question on repeating a cycle. So, ideally we can say the time period of dc is Infinite (it never repeats , so we can say that it will take infinite time to repeat a cycle). So, time period of dc , T = infinite. Hence the Linear frequency, f = 1/T = zero (0). Angular frequency, ω = 2π/T = zero.
Alternating current (ac) ::  The current which alters or changes it's direction from Positive to negative and negative to Positive repeatedly , is called an alternating current.
 Note that : an ac may or may not change its magnitude, but always changes it's direction. Some examples of alternating current are shown below.

Fig2: Sine wavelike AC 

Fig3: Triangular wavelike AC


Fig4: Rectangular wavelike AC

 In Fig2 and Fig3 i.e. for Sinusoidal AC and Triangular AC , both of the magnitude and direction changes with time . But for Rectangular ac which is shown in Fig4 , the magnitude do not change with time . However the direction alters. At time interval 0<t<T/2, the magnitude of Rectangular ac was "I" along Positive Y axis and at the interval T/2<t<T , the magnitude also "I" ,but switched it's direction to negative Y axis. Thus, Rectangular ac changes it directions but don't change its magnitude. (Here T is the time period).
Unidirectional current::
 The current which doesn't have a constant magnitude and also don't alter its direction, is neither dc nor ac . This type of current is called Unidirectional current.

Fig5: Unidirectional current

 Here in Fig5 , we can see that the magnitude of the current is not constant and the direction is always along Positive Y axis i.e. do not alter its direction from Positive to negative or negative to Positive.
 This type of current is actually the rectified current which can be generated by passing alternating current (ac) through a rectifier circuit.
 Any type of Unidirectional current is considered to be direct current (dc) , because we can get a pure direct current from this Unidirectional current by passing it through Filter circuit and then through regulator circuit.
Mixture of ac and dc ::  Sometimes ac noise or distortions get mixed with dc and produces distorted dc or mixture of ac and dc, as shown below

Fig6: Mixture of ac and dc 
Differences between AC and DC current::
 Alternating current always alters it's direction. But DC current do not alter its direction.
 Alternating current may have or may not have the same magnitude at every time. But direct current always have same magnitude at any time.
 Alternating current has a finite frequency and time period. But in case of direct current, time period is Infinite and frequency is zero (always).
 Strength of AC current is expressed by it's rms value or effective value which is equal to I/√2 for Sinusoidal AC, i(t)=I Sin(ωt) . But strength of dc is expressed by the value of direct current.
Benefits of AC over DC ::
 AC can be produced easily and it's magnitude can be easily increased or decreased (as required) by using Transformer.
 AC can be transmitted to a long distance.
 Average power loss while transmitting AC is much lower than that of DC . This made AC production and transmission very cheap (not so costly).
These are the benefits of AC over DC . That's why in commercial power supply AC current is supplied instead of direct current.
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