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Inductance and RL circuits help

  • #1
If the switch is closed at t = 0, find the
current in the inductor at t = 0.224 s. Answer
in units of A.
&
Find the current in the switch at that same
time.

I know that i must use i = V/R(1 - e^-t/L/R) but cant seem to get the right answer.

Any help is appreciated


................5 ohms
-------------^^^^^^^------I
I........................................I
I.......10 ohms............1.4 H...I
I------^^^^^^^^---&&&&--I
I........................................I
I.......S.........30 V.....5 ohms.I
I-------/..---I I----^^^^^--I

Homework Statement



What am I doing wrong?

Homework Equations



i = V/R(1 - e^-t/L/R)

The Attempt at a Solution



i = V/R(1 - e^-t/L/R + V/5 = (30 - V)/5

(V/10)( 1 - e^-(0.224*10/1.4) + V/5 = -V/5 + 6

0.1V(0.798)+0.4V = 6

(0.4+0.0798)V = 6

V = 6/0.4798 = 12.5 v

The inductor current at t = 0.224 s

i = V/R(1 - e^-t/L/R)

i = (12.5/10)[ 1 - e^-(0.224(10/1.4) ]

i = 0.9976 A

The switch current:

Is = i + V/5 = 0.9976 + 12.5/5 = 3.497 A
 
Last edited:

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
5
First of all you have to find the voltage across LR circuit.
At t=0 L acts like a conductor. Find the total resistance and current in the circuit. Hence find the voltage across LR circuit.Then use relevant formula to find the current in L.
 

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