MH's ideal coil and voltage question

[poynt99]

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?

[tinman]

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

Ok,i give up.

I thought i had explained it quite clearly,but apparently not.

Im not saying that there was no current flowing at T=5 second's--im saying the exact opposite.
Because there is a current flowing at T=5 second's,the inductance value that the EMF see's at T=5 seconds is not correct,due to this already flowing current.

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?.

In one case ,we have a rising magnetic field that will produce a counter EMF,and in the other case,we have a falling magnetic field that will produce an EMF that is not !counter! against the applied EMF.

How is it that the same formula can be used for two very different situations?.


Brad

[poynt99]

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.

[verpies]

Are you saying that an ideal inductor has no impedance in reference to MHs question?.

No. I think that an ideal inductor has nonzero reactance and nonzero impedance but it has zero resistance.

[tinman]

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