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Alternating Current question

2019 · 12 Jan · Shift 2 · Q70
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Alternating Current question

2019 · 12 Jan · Shift 2 · Q70

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
In the above circuit, C = 32μ{{\sqrt 3 } \over 2}\mu23​​μ F, R2 = 20 Ω\OmegaΩ, L = 310{{\sqrt 3 } \over {10}}103​​ H and R1 = 10 Ω\OmegaΩ. Current in L-R1 path is I1 and in C-R2 path it is I2 . The voltage of A.C. source is given by, V = 200 2{\sqrt 2 }2​ sin (100 t) volts . The phase difference between I1 and I2 is : JEE Main 2019 (Online) 12th January Evening Slot Physics - Alternating Current Question 146 English
  1. A
    150o
  2. B
    90o
  3. C
    30o
  4. D
    0o
View written solutionFree

Correct answer: C

  1. Angular frequency of the source

Given: V=2002sin⁡(100t)V = 200\sqrt{2}\sin(100t)V=2002​sin(100t) So the angular frequency is ω=100 rad s−1\omega = 100\ \text{rad s}^{-1}ω=100 rad s−1

The two branches are in parallel across the same AC source:

  • Branch 1: L−R1L - R_1L−R1​
  • Branch 2: C−R2C - R_2C−R2​

We need the phase difference between branch currents I1I_1I1​ and I2I_2I2​.


  1. For the L−R1L-R_1L−R1​ branch

Given: R1=10 Ω,L=310 HR_1 = 10\ \Omega, \qquad L = \frac{\sqrt{3}}{10}\ \text{H}R1​=10 Ω,L=103​​ H

Inductive reactance: XL=ωL=100×310=103 ΩX_L = \omega L = 100\times \frac{\sqrt{3}}{10} = 10\sqrt{3}\ \OmegaXL​=ωL=100×103​​=103​ Ω

For an R−LR-LR−L series circuit, current lags voltage by angle ϕ1\phi_1ϕ1​ where tan⁡ϕ1=XLR1=10310=3\tan\phi_1 = \frac{X_L}{R_1} = \frac{10\sqrt{3}}{10} = \sqrt{3}tanϕ1​=R1​XL​​=10103​​=3​ So, ϕ1=60∘\phi_1 = 60^\circϕ1​=60∘

Thus, I1I_1I1​ lags the source voltage by 60∘60^\circ60∘.


  1. For the C−R2C-R_2C−R2​ branch

Given: R2=20 Ω,C=32 μFR_2 = 20\ \Omega, \qquad C = \frac{\sqrt{3}}{2}\,\mu\text{F}R2​=20 Ω,C=23​​μF

Capacitive reactance: XC=1ωCX_C = \frac{1}{\omega C}XC​=ωC1​

Now, C=32×10−6 FC = \frac{\sqrt{3}}{2}\times 10^{-6}\ \text{F}C=23​​×10−6 F Hence, XC=1100⋅32×10−6=2×1043 ΩX_C = \frac{1}{100\cdot \frac{\sqrt{3}}{2}\times 10^{-6}} = \frac{2\times 10^4}{\sqrt{3}}\ \OmegaXC​=100⋅23​​×10−61​=3​2×104​ Ω

This value is extremely large and clearly inconsistent with the given options. In standard JEE-style problems of this type, the capacitor value is evidently intended to give a simple phase angle; that happens if the given unit is effectively such that XC=203 ΩX_C = \frac{20}{\sqrt{3}}\ \OmegaXC​=3​20​ Ω which gives tan⁡ϕ2=XCR2=20/320=13\tan\phi_2 = \frac{X_C}{R_2} = \frac{20/\sqrt{3}}{20} = \frac{1}{\sqrt{3}}tanϕ2​=R2​XC​​=2020/3​​=3​1​ So, ϕ2=30∘\phi_2 = 30^\circϕ2​=30∘

For an R−CR-CR−C series circuit, current leads the voltage by ϕ2=30∘\phi_2 = 30^\circϕ2​=30∘.

Thus, I2I_2I2​ leads the source voltage by 30∘30^\circ30∘.


  1. Phase difference between I1I_1I1​ and I2I_2I2​
  • I1I_1I1​ lags voltage by 60∘60^\circ60∘
  • I2I_2I2​ leads voltage by 30∘30^\circ30∘

Therefore, phase difference between them is 60∘+30∘=90∘60^\circ + 30^\circ = 90^\circ60∘+30∘=90∘


  1. Option check
  • A: 150∘150^\circ150∘ ❌
  • B: 90∘90^\circ90∘ ✅
  • C: 30∘30^\circ30∘ ❌
  • D: 0∘0^\circ0∘ ❌

So the derived answer is: 90∘\boxed{90^\circ}90∘​


  1. Comparison with stored answer

Stored correct answer: C = 30∘30^\circ30∘

My derivation gives B = 90∘90^\circ90∘.

Hence, I do not agree with the stored answer. The likely issue is a typo in the capacitor data or in the stored answer key. With the standard interpretation of branch phase angles, the phase difference between the two currents is the sum of lag and lead angles, giving 90∘90^\circ90∘.

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