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Electromagnetic Induction question

2017 · Shift 2 · Q50
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Electromagnetic Induction question

2017 · Shift 2 · Q50

JEE AdvancedPhysicsElectromagnetic InductionMultiple correct+4 / −2
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? JEE Advanced 2017 Paper 2 Offline Physics - Electromagnetic Induction Question 10 English
  1. A
    After a long time, the current through L1 will be VRL2L1+L2{V \over R}{{{L_2}} \over {{L_1} + {L_2}}}RV​L1​+L2​L2​​
  2. B
    After a long time, the current through L2 will be VRL1L1+L2{V \over R}{{{L_1}} \over {{L_1} + {L_2}}}RV​L1​+L2​L1​​
  3. C
    The ratio of the currents through L1 and L2 is fixed at all times (t > 0)
  4. D
    At t = 0, the current through the resistance R is VR{V \over R}RV​
View written solutionFree

Correct answer: A, B, C

Step-by-Step Solution:

The problem asks to analyze the currents in an RL circuit just after a switch is closed and after a long time.

1. Analysis at t = 0 (immediately after closing the switch S)

  • An inductor resists an instantaneous change in the current flowing through it. Before the switch is closed (at t=0−t = 0^-t=0−), the current through both inductors L1L_1L1​ and L2L_2L2​ is zero.

  • Therefore, immediately after the switch is closed (at t=0+t = 0^+t=0+), the currents through the inductors must still be zero. IL1(0)=0I_{L1}(0) = 0IL1​(0)=0 IL2(0)=0I_{L2}(0) = 0IL2​(0)=0

  • The current through the resistor R, denoted as IRI_RIR​, is the sum of the currents through the parallel branches containing L1L_1L1​ and L2L_2L2​ (by Kirchhoff's Current Law). IR(t)=IL1(t)+IL2(t)I_R(t) = I_{L1}(t) + I_{L2}(t)IR​(t)=IL1​(t)+IL2​(t)

  • At t=0, the current through the resistor is: IR(0)=IL1(0)+IL2(0)=0+0=0I_R(0) = I_{L1}(0) + I_{L2}(0) = 0 + 0 = 0IR​(0)=IL1​(0)+IL2​(0)=0+0=0

  • Evaluation of Option D: Option D states that at t = 0, the current through the resistance R is V/R. Our analysis shows that IR(0)=0I_R(0) = 0IR​(0)=0. Thus, Option D is incorrect.

2. Analysis for any time t > 0

  • The two inductors L1L_1L1​ and L2L_2L2​ are connected in parallel. This means the voltage across them is the same at all times t > 0. Let's call this voltage VL(t)V_L(t)VL​(t).

  • The voltage across an inductor is given by VL=L(di/dt)V_L = L (di/dt)VL​=L(di/dt). Therefore: VL(t)=L1dIL1dtV_L(t) = L_1 \frac{dI_{L1}}{dt}VL​(t)=L1​dtdIL1​​ VL(t)=L2dIL2dtV_L(t) = L_2 \frac{dI_{L2}}{dt}VL​(t)=L2​dtdIL2​​

  • Equating the two expressions for VL(t)V_L(t)VL​(t): L1dIL1dt=L2dIL2dtL_1 \frac{dI_{L1}}{dt} = L_2 \frac{dI_{L2}}{dt}L1​dtdIL1​​=L2​dtdIL2​​

  • We can integrate this equation with respect to time from t=0 to any time t: ∫0tL1dIL1dt′dt′=∫0tL2dIL2dt′dt′\int_0^t L_1 \frac{dI_{L1}}{dt'} dt' = \int_0^t L_2 \frac{dI_{L2}}{dt'} dt'∫0t​L1​dt′dIL1​​dt′=∫0t​L2​dt′dIL2​​dt′ L1∫IL1(0)IL1(t)dIL1=L2∫IL2(0)IL2(t)dIL2L_1 \int_{I_{L1}(0)}^{I_{L1}(t)} dI_{L1} = L_2 \int_{I_{L2}(0)}^{I_{L2}(t)} dI_{L2}L1​∫IL1​(0)IL1​(t)​dIL1​=L2​∫IL2​(0)IL2​(t)​dIL2​

  • Using the initial conditions IL1(0)=0I_{L1}(0) = 0IL1​(0)=0 and IL2(0)=0I_{L2}(0) = 0IL2​(0)=0: L1[IL1(t)−0]=L2[IL2(t)−0]L_1 [I_{L1}(t) - 0] = L_2 [I_{L2}(t) - 0]L1​[IL1​(t)−0]=L2​[IL2​(t)−0] L1IL1(t)=L2IL2(t)L_1 I_{L1}(t) = L_2 I_{L2}(t)L1​IL1​(t)=L2​IL2​(t)

  • This relation holds for all times t > 0. From this, we can find the ratio of the currents: IL1(t)IL2(t)=L2L1\frac{I_{L1}(t)}{I_{L2}(t)} = \frac{L_2}{L_1}IL2​(t)IL1​(t)​=L1​L2​​

  • Evaluation of Option C: Since L1L_1L1​ and L2L_2L2​ are constants, the ratio L2/L1L_2/L_1L2​/L1​ is also a constant. This means the ratio of the currents through L1L_1L1​ and L2L_2L2​ is fixed at all times t > 0. Thus, Option C is correct.

3. Analysis after a long time (t → ∞, steady state)

  • In a DC circuit, after a long time, the currents become constant (steady state). In this condition, an ideal inductor behaves like a short circuit (a wire with zero resistance), because the voltage across it, VL=L(di/dt)V_L = L(di/dt)VL​=L(di/dt), becomes zero as di/dt = 0.

  • So, the parallel combination of L1L_1L1​ and L2L_2L2​ acts as a short circuit, meaning the voltage across this parallel branch is zero.

  • Applying Kirchhoff's Voltage Law to the main loop: V−IR(∞)R−VL(∞)=0V - I_R(\infty) R - V_L(\infty) = 0V−IR​(∞)R−VL​(∞)=0 Since VL(∞)=0V_L(\infty) = 0VL​(∞)=0, we have: V−IR(∞)R=0V - I_R(\infty) R = 0V−IR​(∞)R=0 IR(∞)=VRI_R(\infty) = \frac{V}{R}IR​(∞)=RV​

  • This total steady-state current IR(∞)I_R(\infty)IR​(∞) splits between the two inductors: IR(∞)=IL1(∞)+IL2(∞)=VRI_R(\infty) = I_{L1}(\infty) + I_{L2}(\infty) = \frac{V}{R}IR​(∞)=IL1​(∞)+IL2​(∞)=RV​ (Equation 1)

  • The relationship L1IL1(t)=L2IL2(t)L_1 I_{L1}(t) = L_2 I_{L2}(t)L1​IL1​(t)=L2​IL2​(t) derived in step 2 is valid for all t, including t→∞t \to \inftyt→∞: L1IL1(∞)=L2IL2(∞)L_1 I_{L1}(\infty) = L_2 I_{L2}(\infty)L1​IL1​(∞)=L2​IL2​(∞) (Equation 2)

  • Now we solve the system of two equations for IL1(∞)I_{L1}(\infty)IL1​(∞) and IL2(∞)I_{L2}(\infty)IL2​(∞). From Equation 2: IL2(∞)=L1L2IL1(∞)I_{L2}(\infty) = \frac{L_1}{L_2} I_{L1}(\infty)IL2​(∞)=L2​L1​​IL1​(∞)

  • Substitute this into Equation 1: IL1(∞)+L1L2IL1(∞)=VRI_{L1}(\infty) + \frac{L_1}{L_2} I_{L1}(\infty) = \frac{V}{R}IL1​(∞)+L2​L1​​IL1​(∞)=RV​ IL1(∞)(1+L1L2)=VRI_{L1}(\infty) \left(1 + \frac{L_1}{L_2}\right) = \frac{V}{R}IL1​(∞)(1+L2​L1​​)=RV​ IL1(∞)(L2+L1L2)=VRI_{L1}(\infty) \left(\frac{L_2 + L_1}{L_2}\right) = \frac{V}{R}IL1​(∞)(L2​L2​+L1​​)=RV​ IL1(∞)=VRL2L1+L2I_{L1}(\infty) = \frac{V}{R} \frac{L_2}{L_1 + L_2}IL1​(∞)=RV​L1​+L2​L2​​

  • Evaluation of Option A: This matches the expression given in Option A. Thus, Option A is correct.

  • Now, let's find IL2(∞)I_{L2}(\infty)IL2​(∞): IL2(∞)=IR(∞)−IL1(∞)=VR−VRL2L1+L2I_{L2}(\infty) = I_R(\infty) - I_{L1}(\infty) = \frac{V}{R} - \frac{V}{R} \frac{L_2}{L_1 + L_2}IL2​(∞)=IR​(∞)−IL1​(∞)=RV​−RV​L1​+L2​L2​​ IL2(∞)=VR(1−L2L1+L2)=VR(L1+L2−L2L1+L2)I_{L2}(\infty) = \frac{V}{R} \left(1 - \frac{L_2}{L_1 + L_2}\right) = \frac{V}{R} \left(\frac{L_1 + L_2 - L_2}{L_1 + L_2}\right)IL2​(∞)=RV​(1−L1​+L2​L2​​)=RV​(L1​+L2​L1​+L2​−L2​​) IL2(∞)=VRL1L1+L2I_{L2}(\infty) = \frac{V}{R} \frac{L_1}{L_1 + L_2}IL2​(∞)=RV​L1​+L2​L1​​

  • Evaluation of Option B: This matches the expression given in Option B. Thus, Option B is correct.

Conclusion:

Based on the analysis, options A, B, and C are correct, while option D is incorrect.

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