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A sinusoidal source of voltage V and frequency f is connected to a series circuit of variable resistance, R and a fixed reactance, X. the locus of the tip of the current-phasor I, as R is varied from 0 to ∞ is:
A semicircle with a diameter of 
A straight line with a slop of 
An ellipse with 
as major axis
A circle of radius 
and origin at 
Current 
Since ‘X’ is fixed and ‘R’ is variable
When R = 0; 
When R = X; 
When 
; 
So the locus of I will ok like as shown below
So the locus of I is a semicircle with diameter 
Note: In such type of questions, the process followed is similar to drawing Polar Plot in Control Systems.