MH's ideal coil and voltage question

[MileHigh]

I suggest that you try again Brad because there are several mistakes in each of your examples.  Perhaps if you read more about inductors and how they respond to voltage step functions you will be able to put the pieces of the puzzle together and give it another try.

[poynt99]

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.

[tinman]

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.

The CEMF is no different to that induced by an electric motor,only it is ass about,where the CEMF will increase with motor speed,resulting in a drop in current draw,and with the coil,the CEMF will decrease over time,resulting in a higher current draw. So an increase in CEMF is seen as an impedance to the current flow,by way of reducing the potential voltage difference between the applied EMF,and the CEMF. In the case of the inductor,the CEMF reduces over time,meaning a larger potential difference between applied EMF and CEMF,resulting in a higher current flow value.

We have no CEMF when the -3 volts is applied,due to a collapsing magnetic field that is of the opposite polarity to that of which the applied-3v EMF wishes to build.
We now have an EMF+ an EMF-->CEMF is gone.
Remove the CEMF from an electric motor while it's running,and what will happen to the current draw?.


Brad

[poynt99]

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.

[verpies]

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...

But the value of the CEMF does not depend on the level of magnetic flux and current flowing through an ideal coil. 
The CEMF depends on the rate of change of the flux and current.