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How does the current divide in parallel inductors?

  • #1
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3

Homework Statement:

In the circuit of Fig. 63.82 io(0)=2mA. Determine io(t) and Vo(t) for t>0

Relevant Equations:

$$V_O = L*\frac{di}{dt}$$
6.60.png



I apologize using English fluently because I am not an Enlgish speaker.

When I tried to solve this problem, I used current divider rule.

So, $$i_o(t) = \frac{3}{3+5}*4e^{-2t} = 1.5*e^{-2t} A$$

However, This was wrong.

The answer is $$ 1.5*e^{-2t} + 0.5 A$$

If I use $$V_O = L*\frac{di}{dt}$$ , I can get right answer.

I wonder why there is a difference
between using current divider rule and using $$V_O = L*\frac{di}{dt}$$.
 
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Answers and Replies

  • #2
605
436
The current divider rule assumes a steady state solution. This is a transient problem. The current divider rule doesn't properly account for the initial conditions. At t=0 each inductor has 2A, which isn't consistent with the 3:5 ratio.
 
  • #3
ehild
Homework Helper
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1,874
The answer is $$ 1.5*e^{-2t} + 0.5 A$$

If I use $$V_O = L*\frac{di}{dt}$$ , I can get right answer.

I wonder why there is a difference
between using current divider rule and using $$V_O = L*\frac{di}{dt}$$.
The voltage divider rule is valid for resistors, when the voltage is proportional to the current, U=RI. In case of inductors, this is not true, U=LdI/dt instead.
 
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  • #4
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3
The current divider rule assumes a steady state solution. This is a transient problem. The current divider rule doesn't properly account for the initial conditions. At t=0 each inductor has 2A, which isn't consistent with the 3:5 ratio.
Thank you for your advise! I got what is a steady state right away.
 
  • #5
12
3
The voltage divider rule is valid for resistors, when the voltage is proportional to the current, U=RI. In case of inductors, this is not true, U=LdI/dt instead.
Oh, I thought current divider was also applied to the inductor. Thank you for your help!!
 

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