Current

Definition – Current is the rate at which charge flows. It is measured in Amperes (A). It is denoted by letterI. Voltage is the driving force for current to flow in the circuit. Current flows from higher potential to lower potential.

(Note:-When we say current flows in a circuit, electrons flow from lower/negative potential to higher potential while the direction of current is kept from higher to lower/negative potential.)

Types of current

There are two types of current – AC current & DC current.

AC (Alternating Current) – It varies with respect to timeand formssinusoidal waves.Same as ac voltage it periodically reverses its direction. AC current flows in the circuit when AC voltage is applied to it.

DC (Direct Current) – DC current doesn’t vary with time so it is represented by a straight line. DC current flows in the circuit when DC voltage is applied to it.

 

RMS value, Peak value & Average value

There are some terms associated with AC Voltage & Current which we must understand like RMS value, Peak value & Average value.

Peak Value of Voltage & Current –It is the maximum value of voltage/current. Peak value is positive in first half of cycle & negative in another half cycle. In other words, in a complete cycle, voltage/current with respect to time varies from zero value to peak (+ve) value then again becomes zero & then goes to (-ve) peak value & then again it becomes zero value& this cycle is repeated.

 

Average value of Voltage & Current –Average value of a voltage for one complete cycle is 0 as two half values (one positive & one negative) cancels each other out. Hence average value is taken from half cycle whose value is 0.637 times the peak value of voltage.

Vav = 2/π X Vpk = 0.637 Vpeak

Similarly, Average value of Current Iav=  0.637Vpeak

Rms value of Voltage & Current – RMS (Root Mean Square) value of voltage & current is the average effect in ac circuit. Only RMS value is used for measuring & calculation purpose. Rms value of AC voltage & Current is equivalent to DC value that would produce the same output voltage.The RMS value of voltage (or current) is 0.707 times of peak voltage and 1.11 times of average value.

(Note –Multi-meters & other AC meters (voltage & current) giverms value of voltage & current.)

Vrms=  0.707Vpeak = 1.11Vav,

Irms  = 0.707Vpeak = 1.11Iav

PHASOR DIAGRAM (VECTOR DIAGRAM)

AC voltage & Current can also be analyzed by phasor diagram when frequencies are same but amplitude need not be same.Diagram given below shows a sine wave represented by phasor.

In above diagram, two voltages have the same amplitude; therefore the length of arrow of both voltages is same. The saffron current leads the blue current by 20 deg.