
- AAfter a long time, the current through L1 will be
- BAfter a long time, the current through L2 will be
- CThe ratio of the currents through L1 and L2 is fixed at all times (t > 0)
- DAt t = 0, the current through the resistance R is
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 ), the current through both inductors and is zero.
-
Therefore, immediately after the switch is closed (at ), the currents through the inductors must still be zero.
-
The current through the resistor
R, denoted as , is the sum of the currents through the parallel branches containing and (by Kirchhoff's Current Law). -
At
t=0, the current through the resistor is: -
Evaluation of Option D: Option D states that at
t = 0, the current through the resistanceRisV/R. Our analysis shows that . Thus, Option D is incorrect.
2. Analysis for any time t > 0
-
The two inductors and are connected in parallel. This means the voltage across them is the same at all times
t > 0. Let's call this voltage . -
The voltage across an inductor is given by . Therefore:
-
Equating the two expressions for :
-
We can integrate this equation with respect to time from
t=0to any timet: -
Using the initial conditions and :
-
This relation holds for all times
t > 0. From this, we can find the ratio of the currents: -
Evaluation of Option C: Since and are constants, the ratio is also a constant. This means the ratio of the currents through and 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, , becomes zero as
di/dt = 0. -
So, the parallel combination of and acts as a short circuit, meaning the voltage across this parallel branch is zero.
-
Applying Kirchhoff's Voltage Law to the main loop: Since , we have:
-
This total steady-state current splits between the two inductors: (Equation 1)
-
The relationship derived in step 2 is valid for all
t, including : (Equation 2) -
Now we solve the system of two equations for and . From Equation 2:
-
Substitute this into Equation 1:
-
Evaluation of Option A: This matches the expression given in Option A. Thus, Option A is correct.
-
Now, let's find :
-
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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