A current of 10 A flows in a circuit with a 30° angle of lag when the applied voltage is 100 V. What are the values of resistance and reactance in the circuit respectively ?

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ESE Electrical 2021 Official Paper
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  1. 8.66 Ω and 5 Ω  
  2. 5Ω and 8.66 Ω
  3. 6.66 Ω and 4 Ω
  4. 4 Ω and 6.66 Ω

Answer (Detailed Solution Below)

Option 1 : 8.66 Ω and 5 Ω  
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Detailed Solution

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Concept:

AC circuit with R-L load:

An AC circuit with an R-L load is shown below.

F1 Arpit 1-12-20 Savita D13

VR = Voltage across resistor

VL = Voltage across inductor

V = √(VR2 + VL2

Total impedance is

Z = R + jωL Ω

Z = √(R2 + ω2L2) ∠θ

θ  = tan-1(ωL/R) 

Calculation:

Given 

Current i = 10 A

Phase angle θ = 30°  lag

Applied voltage V = 100 V

⇒ Impedance Z = V / i = 100 / 10 = 10 Ω 

Impedance in polar form = Z ∠θ  = 10 ∠-30° ('-' for lag)

Impedance in rectangular form = R + j XL = 10 cos 30°  + j 10 sin 30° = 8.66 + j 5

⇒ R = 8.66 Ω and XL = 5 Ω

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