# RL circuit after switch has been opened

• Ugnius

#### Ugnius

Homework Statement
Switch is closed when t < 0, switch in the circuit shown has been closed for a
long time and conditions have settled. Switch at the moment t=0 is opened . Calculate the moment of time when the current in R1 is equal to 9.2 mA.
Relevant Equations
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Hi guys , I've been trying to think this one out but I'm drawing a blank.

What I am able to understand that I'm required to calculate the backwards emf when the switch is opened. The current through the resistor R1 is 9mA. I was wondering if 9.2mA is even possible? Can the current exceed Imax at the moment of discarge? I know a lot of current is generated to discharge the magnetic field energy stored in the inductor. As I see the normal current path through the inductor is a - b , and at the moment of discharge it should be b-a and then though the R1 to the switch because it's the shortest path. Is there a possible way to calculate what kind of voltage is going though the resistor when inductor is releasing the energy? Maybe that's the way to calculate Imax in the R1.

Delta2
Your intuition regarding the maximum current not exceeding the initial 9 mA is correct. We also don't expect any big back-emf spikes to occur since the inductor never "sees" any abrupt open circuit conditions during the switching operation -- there's always a continuous circuit for its current to follow.

I would say that this is a poorly constructed question that does not have a physically realistic solution, due to the "desired" current value not being physically realizable.

Is this a multiple choice or online questionnaire, or are you expected to hand in your written work? If the latter, then you can at least show the appropriate math steps to use when the target current value is reasonable.

Ugnius
Your intuition regarding the maximum current not exceeding the initial 9 mA is correct. We also don't expect any big back-emf spikes to occur since the inductor never "sees" any abrupt open circuit conditions during the switching operation -- there's always a continuous circuit for its current to follow.

I would say that this is a poorly constructed question that does not have a physically realistic solution, due to the "desired" current value not being physically realizable.

Is this a multiple choice or online questionnaire, or are you expected to hand in your written work? If the latter, then you can at least show the appropriate math steps to use when the target current value is reasonable.
This is in online questionare , where the answer is in the system and you have a blank square to write the answer to , after submit it's either green or you try again. I fill try to write my proffesor and ask him how is that possible.

gneill
Your intuition regarding the maximum current not exceeding the initial 9 mA is correct. We also don't expect any big back-emf spikes to occur since the inductor never "sees" any abrupt open circuit conditions during the switching operation -- there's always a continuous circuit for its current to follow.

I would say that this is a poorly constructed question that does not have a physically realistic solution, due to the "desired" current value not being physically realizable.

Is this a multiple choice or online questionnaire, or are you expected to hand in your written work? If the latter, then you can at least show the appropriate math steps to use when the target current value is reasonable.
There was a typo after all.

There was a typo after all.
Ah. That's been known to happen occasionally. It's a good thing that you checked with your prof.

Ugnius
Just for fun I checked that, after 0.4 ms, the current Is 1.2 mA. I wonder whether someone wrote 9.2 mA instead of 1.2 mA.

Just for fun I checked that, after 0.4 ms, the current Is 1.2 mA. I wonder whether someone wrote 9.2 mA instead of 1.2 mA.
In our system there is random number generator in range, and he did the range accidentaly 0 - 10 , when he meant 0-9.