# Homework Help: LR circuit analysis

1. Apr 22, 2014

### utkarshakash

1. The problem statement, all variables and given/known data
Switch S is closed for long time in the figure shown. Find the current i as soon as the switch is opened.

3. The attempt at a solution

At t=∞ inductor behaves as closed circuit. So, I can simply replace it with a wire and calculate the current which comes out to be 9A. But I can't understand what will happen when the switch is opened. Will the current die out instantly? Will it be the same as before?

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2. Apr 22, 2014

### SammyS

Staff Emeritus
First of all, how did you get 9 A? Is that what you get for the current through the inductor ? (Yes, I assume you're getting this while the switch is still closed.) ... but how did you get that?

3. Apr 23, 2014

### utkarshakash

Sorry. I misunderstood the question. It asks for the current through the inductor. I calculated the current flowing through the battery instead. btw I got the correct answer.

4. Apr 23, 2014

### ehild

So what current flows through the inductor?

The current through the battery ceases immediately when the switch is opened, but the current through the inductor does not. Think of Lenz's Law.

5. Apr 23, 2014

### rude man

Can current EVER die out instantly in an inductor? Can current change in an inductor if you don't give it any time to change?