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

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

  1. The ratio of the currents through L1 and L2 is fixed at all times t>0
  2. After a long time, the current through L1 will be VRL2L1+L2
  3. After a long time, the current through L2 will be VRL1L1+L2
  4. At t=0 , the current through the resistance R is VR

Solution

 

As L1 and L2 are in parallel so equivalent  

inductance, L=L1L2L1+L2

Here, L1 and L2 are the inductance (shown in circuit)

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

I= VR1-e-RtL

At t=0,

I=VR1-e0=0

After a longtime i.e. at t=

I=VR1-e-

I=VR

Let currents through L1 and L2 are I1 and I2 respectively. As L1 and L2 are joined in parallel

Voltage across L1= Voltage across L2

VL1=VL2

L1dI1dt=L2dI2dt

L1I1=L2I2

Thus, I1I2=L2L1

By current division rule,

Current I1 across L1 is 

I1=L2L1+L2×I

I1=VRL2L1+L2

Current I2 across L2 is

I2=L1L1+l2×I

I2=VRL1L1+L2

Asked in: JEE Advanced 2017 (Paper 2)

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