Has anyone else any trouble with this?

Not me.

Not me.

Are you saying that an ideal inductor has no impedance in reference to MHs question?.
I would think that anything that acts against a current flow,could be seen as an impedance to that current flow. Impedance is just a form of resistance -is it not?.




Brad

The inductance value is being used to calculate the peak current at T=7 seconds,but the induction process to make this calculation is not correct between the time period T=5 seconds to T=7 seconds. The inductance value being used to make this calculation is not correct. We have started at T=5 seconds,and ended at T=7 seconds to calculate the peak current reached at T=7 seconds. This is on the understanding that the induction process taking place at this time will start with no current flowing through the coil,and no existing magnetic field. We apply our voltage,and current starts to flow,and a magnetic field begins to build(induction). We calculate that using a 5H coil,the peak current value reached at T= 7 seconds will be 1.2 amp's. But as i said,this assumes that there is no current flowing through the coil. This assumes that a current will begin to flow,a magnetic field will begin to form,and the CEMF value will fall as the magnetic fields change in time reduces.

Where and how did you arrive at the notion that at T=5s or T=7s, that no current is flowing?

See if this help's.
Lets say there is no current flowing through the inductor.
We apply our negative 3 volts for 2 seconds. That gives us a peak current at the end of the 2 seconds of 1.2 amps.

Now we do the same,only this time there is 2.4 amps flowing through the coil loop.
We once again place our negative 3 volts across the coil for 2 second's. We now have the very same answer as far as current value go's ,of 1.2 amps peak at the end of the 2 seconds.

How can it be the same value,when in one case there is no current flowing through the coil,and in the other case there is 2.4 amps flowing through the coil?.

Brad,

First, understand this; the equation tells us what the final value of current will be, not the "peak" current as you have been calling it. That may be part of your confusion.

Ok, with no current flowing (the beginning of the test) if we apply a -3V supply for 2s, the final current will be -1.2A, not +1.2A as you have stated. The current will ramp down from 0A to -1.2A over 2s.

Now, if we do the same and apply our -3V for 2s but this time there is already +2.4A of current flowing, then the current will ramp down from +2.4A to +1.2A, again a change of current (of 1.2A) in the negative direction.

You will note that the final current in these two cases is not nearly the same; one is -1.2A, and the other is +1.2A.

Brad,



Ok, with no current flowing (the beginning of the test) if we apply a -3V supply for 2s, the final current will be -1.2A, not +1.2A as you have stated. The current will ramp down from 0A to -1.2A over 2s.



You will note that the final current in these two cases is not nearly the same; one is -1.2A, and the other is +1.2A.

First, understand this; the equation tells us what the final value of current will be, not the "peak" current as you have been calling it. That may be part of your confusion.

No,it's not my confusion at all. As i have stated a time period,then the peak current reached at the end of that time period is the final current value.

Now, if we do the same and apply our -3V for 2s but this time there is already +2.4A of current flowing, then the current will ramp down from +2.4A to +1.2A, again a change of current (of 1.2A) in the negative direction.

And that there is what i think is the problem.
Is it correct to use the same equation when the coil has a positive current already running through it,to make a calculation of the negative current value at the end of the 2 seconds,as to when that same equation is used when the coil has no current flowing through it,to make a calculation of the negative current at the end of the two seconds.

The coil is in a different state when it has current already flowing through it to that if it has no current flowing through it,but yet we use the equation to calculate the negative current value as if there is no current flowing through the coil.

Example 1.
The coil is open.
We apply a voltage of -3 volts(negative to keep in line with MH question only,and keep things clear)across the 5H coil for 2 seconds. At the end of the 2 seconds,the peak(final) current value will be 1.2 amps. That is 3/5 x 2=1.2. At T=0,the voltage is connected across the coil. First a voltage appears across the coil,and then a short time after,a current starts to flow,and a magnetic field starts to build. This increasing magnetic field causes a CEMF to be developed,and this CEMF is what slows the rise time of the induced current. As the magnetic fields change in time is reduced in value,the CEMF is also reduced,and so the current continues to increase over time. If there was no CEMF,then the current would go straight to it's maximum value. So this CEMF can be seen as the impedance to the current flow.

Example 2.
We are now going through the phases of MHs question.
At T=5 seconds,we apply -3 volts across the 5H coil for 2 seconds.
We once again use the same math(equation) 3/5 x 2=1.2 amps.
The difference this time being that there is already a positive current flowing through the coil,with a value of 2.4 amps. This time a magnetic field already exist,that is opposite to that of what the applied EMF wants to create. This time there will be no CEMF as the current value increases,as the current value in the coil is decreasing from T=5 seconds to T=7 seconds,not increasing. As the current is decreasing in value,the EMF across the coil during this decrease is the same as the applied EMF-->no counter EMF during this 2 second time period.
If there is no CEMF,then what stops the current going straight to it's maximum value when the -3 volt EMF is applied across that coil?.

Brad

Brad,

You now have everything at hand required to arrive at the full and correct understanding of the circuit current in MH's question. It has been explained, illustrated, simulated, and elaborated upon. It's now up to you.

I know you can do better and that you can "get it", if you give it a fair and persistent go.

I have already got it,the way you guys are seeing it--as i said to MH,that is not a problem,and the question was never a problem-->when dealing with generic equation's.
But i question those equation's,and the situation they are being used in.

In that we are using an ideal voltage source,and that voltage is retained under any load,then i would expect to see a large reverse(negative) current spike at T-5 second's-the instant the -3 volts is placed across the coil.

This slow drop in current from T=5 seconds to T=7 seconds,is not what i think should be the case.
We already know that if the coil became open,then we would see a polarity change in voltage,and this polarity change would then be the same as the EMF applied at T=5 seconds.

I can't see how you are "getting it" if you don't see how the equation, and sim works for all cases. So I have to disagree that you get it.

If you are expecting there to be a large reverse current spike, then you're forgetting that the current can't and won't do that in an inductor. By their very nature, inductors don't work that way, and in fact they resist any change in current. As I said, when the -3V step occurs, the inductor current will ramp in a negative direction, no matter what the starting current is. There is no "spike", and the current doesn't instantly go from +2.4A to some negative current value.