Math Problem Statement

A non-inductive resistance of 10 Ω is connected in series with an inductive coil across 200V, 50 Hz ac supply. The current drawn by the series combination is 10 A. The resistance of the coil is 2 Ω. Determine (i) Inductance of the coil (ii) Power factor (iii) Voltage across the coil

Solution

To solve this problem, let us approach it step by step.


Given Data:

  1. Resistance of non-inductive resistor, R1=10ΩR_1 = 10 \, \Omega
  2. Resistance of the coil, R2=2ΩR_2 = 2 \, \Omega
  3. Total current, I=10AI = 10 \, \text{A}
  4. Supply voltage, V=200VV = 200 \, \text{V}
  5. Supply frequency, f=50Hzf = 50 \, \text{Hz}

Step 1: Total Impedance (ZZ)

The total impedance is found using Ohm's law: Z=VI=20010=20ΩZ = \frac{V}{I} = \frac{200}{10} = 20 \, \Omega


Step 2: Reactance (XLX_L) and Inductance (LL)

The impedance of the series combination is given by: Z=(Rtotal)2+XL2Z = \sqrt{(R_{\text{total}})^2 + X_L^2} where Rtotal=R1+R2=10+2=12ΩR_{\text{total}} = R_1 + R_2 = 10 + 2 = 12 \, \Omega.

Rearranging to find XLX_L: XL=Z2Rtotal2=202122=400144=256=16ΩX_L = \sqrt{Z^2 - R_{\text{total}}^2} = \sqrt{20^2 - 12^2} = \sqrt{400 - 144} = \sqrt{256} = 16 \, \Omega

The inductive reactance XLX_L is related to inductance LL by: XL=2πfLX_L = 2 \pi f L L=XL2πf=162π(50)=16314.160.0509HL = \frac{X_L}{2 \pi f} = \frac{16}{2 \pi (50)} = \frac{16}{314.16} \approx 0.0509 \, \text{H}

Thus, the inductance of the coil is: L0.0509HL \approx 0.0509 \, \text{H}


Step 3: Power Factor

The power factor (cosϕ\cos \phi) is given by: cosϕ=RtotalZ=1220=0.6\cos \phi = \frac{R_{\text{total}}}{Z} = \frac{12}{20} = 0.6

Thus, the power factor is: cosϕ=0.6\cos \phi = 0.6


Step 4: Voltage Across the Coil

The coil's voltage has two components: resistive (VRV_R) and reactive (VLV_L): VR=IR2=10×2=20VV_R = I R_2 = 10 \times 2 = 20 \, \text{V} VL=IXL=10×16=160VV_L = I X_L = 10 \times 16 = 160 \, \text{V}

The total voltage across the coil is: Vcoil=VR2+VL2=202+1602=400+25600=26000161.2VV_{\text{coil}} = \sqrt{V_R^2 + V_L^2} = \sqrt{20^2 + 160^2} = \sqrt{400 + 25600} = \sqrt{26000} \approx 161.2 \, \text{V}

Thus, the voltage across the coil is: Vcoil161.2VV_{\text{coil}} \approx 161.2 \, \text{V}


Final Answers:

  1. Inductance of the coil: L0.0509HL \approx 0.0509 \, \text{H}
  2. Power factor: cosϕ=0.6\cos \phi = 0.6
  3. Voltage across the coil: Vcoil161.2VV_{\text{coil}} \approx 161.2 \, \text{V}

Would you like further details or have any specific questions? 😊


Related Questions:

  1. How does the power factor affect the efficiency of the circuit?
  2. Can we calculate the power consumed in this circuit? How?
  3. What is the reactive power and apparent power in this circuit?
  4. How would the values change if the frequency were doubled?
  5. What happens to the voltage across the coil if the current increases?

Tip:

In AC circuits, always separate resistive and reactive components when analyzing voltage, current, and power.

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Math Problem Analysis

Mathematical Concepts

AC Circuit Analysis
Impedance
Reactance
Power Factor

Formulas

Z = V / I
Z = sqrt(R_total^2 + X_L^2)
X_L = 2 * π * f * L
cos(ϕ) = R_total / Z
V_coil = sqrt(V_R^2 + V_L^2)

Theorems

Pythagorean Theorem (applied in impedance and voltage calculation)

Suitable Grade Level

Grades 11-12