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JEE Advanced 2017Paper 2
Exam
JEE Advanced
Year
2017
Paper
Paper 2
Subject
Physics
Chapter
Electromagnetic Induction

JEE Advanced 2017 · Paper 2 · Physics · Electromagnetic Induction

A source of constant voltage V is connected to a resistance R and...

A source of constant voltage VV is connected to a resistanceRR and two ideal inductors L1{L}_{1} and L2{L}_{2} through a switch SS as shown. There is no mutual inductance between the two inductors. The switch SS is initially open. At t=0t = 0 , the switch is closed and current begins to flow. Which of the following options is/are correct?
A source of constant voltage V is connected to a resistance R and two ideal inductors L _ 1 and L _ 2 through a switch

Options

  1. AThe ratio of the currents through L1{L}_{1} and L2{L}_{2} is fixed at all times (t>0)\left(t > 0\right)
  2. BAfter a long time, the current through L1{L}_{1} will be VRL2L1+L2\dfrac{V}{R}\dfrac{{L}_{2}}{{L}_{1} + {L}_{2}}
  3. CAfter a long time, the current through L2{L}_{2} will be VRL1L1+L2\dfrac{V}{R}\dfrac{{L}_{1}}{{L}_{1} + {L}_{2}}
  4. DAt t=0t = 0 , the current through the resistance R is VR\dfrac{V}{R}

Correct answer

A. The ratio of the currents through L1{L}_{1} and L2{L}_{2} is fixed at all times (t>0)\left(t > 0\right)

Step-by-step solution

A source of constant voltage V is connected to a resistance R and two ideal inductors L _ 1 and L _ 2 through a switch 

As L1{L}_{1} and L2{L}_{2} are in parallel so equivalent

inductance, L=L1L2L1+L2L = \dfrac{{L}_{1}{L}_{2}}{{L}_{1} + {L}_{2}}

Here, L1{L}_{1} and L2{L}_{2} are the inductance (shown in circuit)

The current through the RLRL series circuit at anytime is given as 

I=VR(1−e−RtL)I = \dfrac{V}{R}\left(1 - e\dfrac{- Rt}{L}\right)

At t=0t = 0,

I=VR(1−e0)=0I = \dfrac{V}{R}\left(1 - {e}^{0}\right) = 0

After a longtime i.e. at t=∞t = \infty

I=VR(1−e−∞)I = \dfrac{V}{R}\left(1 - {e}^{- \infty}\right)

⇒I=VR⇒ I = \dfrac{V}{R}

Let currents through L1{L}_{1} and L2{L}_{2} are I1{I}_{1} and I2{I}_{2} respectively. As L1{L}_{1} and L2{L}_{2} are joined in parallel

Voltage across L1={L}_{1} = Voltage across L2{L}_{2}

⇒VL1=VL2⇒ {V}_{{L}_{1}} = {V}_{{L}_{2}}

⇒L1dI1dt=L2dI2dt⇒ {L}_{1}\dfrac{d {I}_{1}}{d t} = {L}_{2}\dfrac{d {I}_{2}}{d t}

⇒L1I1=L2I2⇒ {L}_{1}{I}_{1} = {L}_{2}{I}_{2}

Thus, I1I2=L2L1\dfrac{{I}_{1}}{{I}_{2}} = \dfrac{{L}_{2}}{{L}_{1}}

By current division rule,

Current I1{I}_{1} across L1{L}_{1} is 

I1=L2L1+L2×I{I}_{1} = \dfrac{{L}_{2}}{{L}_{1} + {L}_{2}} \times I

⇒I1=VR(L2L1+L2)⇒ {I}_{1} = \dfrac{V}{R}\left(\dfrac{{L}_{2}}{{L}_{1} + {L}_{2}}\right)

Current I2{I}_{2} across L2{L}_{2} is

I2=L1L1+l2×I{I}_{2} = \dfrac{{L}_{1}}{{L}_{1} + {l}_{2}} \times I

I2=VR(L1L1+L2){I}_{2} = \dfrac{V}{R}\left(\dfrac{{L}_{1}}{{L}_{1} + {L}_{2}}\right)

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