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If we allow the AC current represented by i = i 0 sin ( ωt + φ ) to pass through a resistor of resistance R , the power dissipated due to flow of current would be P = i 2 R Since magnitude of current changes with time , the power dissipation in circuit also changes The average Power dissipated over one complete current cycle would be
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If we pass direct current of magnitude i rms through the resistor , the power dissipate or rate of production of heat in this case would be P =( i rms ) 2 R Thus rms value of AC is that value of steady current which would dissipate the same amount of power in a given resistance in a given tine as would gave been dissipated by alternating current This is why rms value of AC is also known as virtual value of current
Phasor diagram
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Phasor diagrams are diagram representing alternating current and voltage of same frequency as vectors or phasors with the phase angle between them
Phasors are the arrows rotating in the anti-clockwise direction i . e . they are rotating vectors but they represents scalar quantities
Thus a sinusoidal alternating current and voltage can be represented by anticlockwise rotating vectors if they satisfy following conditions
Length of the vector must be equal to the peak value of alternating voltage or current
Vector representing alternating current and voltage would be at horizontal position at the instant when alternating quantity is zero