MH's ideal coil and voltage question

[tinman]

The instant 4V is applied to the inductor, the cemf goes to -4V and current begins to flow.

In the case of an ideal inductor there is no trade-off with a resistor, therefore the cemf remains constant (and equal to Vin) as does the rate of rise of the current.

Are we all in agreement?

I think we are going around in circles  here.
The question tryi g to be answered  here,is how can it be,that the CEMF value,and the EMF value can be the same,but current still flows?.

What i thi k is being forgotten here is-there is no R value for the ideal coil,and there for,no loss or reduction in current value that is induced by the CEMF.
The very same holds true for the EMF induced current.

So,lets think about this.
If there was no CEMF,and there for no self induced current that apposes that which created it,and no resistance-as our coil is ideal,then when the 4 volts is placed across the coil,the current would rise instantly,to an infinite amount.
So now ,ask your self this-->what value would the self induced current have to be to limit an infinite amount of current  flow?

You might think that was a stupid thing to ask,but this can be answered correctly and accurately  with any real world inductor,at any point in time. But simply removing the coils winding resistance,throws a spanner in the works.

The reason that the self induced current value is always less than the EMF induced current in real world inductors,is because of the conducting wires resistance.
You take that resistance away,then there is no way any of the self induced currents energy can be dissipated,and there for it is equal and opposite to that which created it.

It is an assumption that the current is going to rise at .8 amps a second,because as you stated Poynt-the EMF and CEMF values are the same, and the mechanism  that allows current to flow,is yet unknown.

How was a definitive current value given,when unknows exist?.


Brad

[picowatt]

I disagree PW.

The instant current begins to flow and rise, it is already rising at 0.8A/s (there is no gradual or "reaching" rise in current to the level of 0.8A/s). But we may be saying the same thing.

As far as the polarity of the cemf, yes it is the same as Vin.

I was just trying to clarify that the CEMF did not exist prior to, or reach 4 volts, until the rate of change was .8 amps per second, which for all practical intents and purposes, can be considered instantaneous. 

Any rate of change faster or slower than .8 amps per second would generate a CEMF that is more than of less than 4 volts.   

PW

[picowatt]

I think we are going around in circles  here.
The question tryi g to be answered  here,is how can it be,that the CEMF value,and the EMF value can be the same,but current still flows?.

Technically speaking, current would not flow if the CEMF and EMF were exactly equal.  But then what would the rate of change of the current be?  Zero volts per second, so no CEMF would be generated.  With no CEMF, current would again flow until it reached .8 amps per second, at which time the CEMF would again equal the EMF, and the cycle would repeat.  But it does not happen in this step-wise fashion, it is a smooth feedback type process where the CEMF is maintained as equal to the level of the applied EMF (or at the least, very, very close to it, if that helps with the visualization) so that the rate of change is .8 amps per second.

What i thi k is being forgotten here is-there is no R value for the ideal coil,and there for,no loss or reduction in current value that is induced by the CEMF.
The very same holds true for the EMF induced current.

No, the R value is not being forgotten, it just does not come into play with regard to the CEMF.  The CEMF is an induced voltage.  The CEMF equals 4 volts when the RATE OF CHANGE of the current flow is .8 amps per second (4V applied to 5H).  Do not confuse the CEMF, which is a mechanism that determines the rate at which energy can be stored, with the actual amount of energy that is stored.  And do not forget that the inductor is storing energy. 

So,lets think about this.
If there was no CEMF,and there for no self induced current that apposes that which created it,and no resistance-as our coil is ideal,then when the 4 volts is placed across the coil,the current would rise instantly,to an infinite amount.
So now ,ask your self this-->what value would the self induced current have to be to limit an infinite amount of current  flow?

As soon as the RATE OF CHANGE reached .8 amps per second, 4 volts of CEMF would be generated and effectively regulate/limit the rate of the current's rise to that .8 amps per second.

You might think that was a stupid thing to ask,but this can be answered correctly and accurately  with any real world inductor,at any point in time. But simply removing the coils winding resistance,throws a spanner in the works.

The reason that the self induced current value is always less than the EMF induced current in real world inductors,is because of the conducting wires resistance.
You take that resistance away,then there is no way any of the self induced currents energy can be dissipated,and there for it is equal and opposite to that which created it.

You are wrong.  Faraday says nothing about a required resistance or required dissipation.  Resistance does not enter into it.  If the rate of change of the current flowing thru the 5H inductor is .8 amps per second, the generated CEMF will be 4 volts.  If the rate of change is less than .8 amps per second, the generated CEMF will also be less than 4 volts.  And do not forget that with regard to an ideal inductor, we are not dissipating energy, we are storing/retrieving energy. 

It is an assumption that the current is going to rise at .8 amps a second,because as you stated Poynt-the EMF and CEMF values are the same, and the mechanism  that allows current to flow,is yet unknown.

How was a definitive current value given,when unknows exist?.
Brad

Consider reading this whole Wiki, but from the Wiki:

https://en.wikipedia.org/wiki/Inductor

So inductance is also a measure of the amount of electromotive force (voltage) generated for a given rate of change of current. For example, an inductor with an inductance of 1 henry produces an EMF of 1 volt when the current through the inductor changes at the rate of 1 ampere per second. This is usually taken to be the constitutive relation (defining equation) of the inductor.

PW

[tinman]

Technically speaking, current would not flow if the CEMF and EMF were exactly equal.  But then what would the rate of change of the current be?  Zero volts per second, so no CEMF would be generated.  With no CEMF, current would again flow until it reached .8 amps per second, at which time the CEMF would again equal the EMF, and the cycle would repeat.  But it does not happen in this step-wise fashion, it is a smooth feedback type process where the CEMF is maintained as equal to the level of the applied EMF (or at the least, very, very close to it, if that helps with the visualization) so that the rate of change is .8 amps per second.

No, the R value is not being forgotten, it just does not come into play with regard to the CEMF.  The CEMF is an induced voltage.  The CEMF equals 4 volts when the RATE OF CHANGE of the current flow is .8 amps per second (4V applied to 5H).  Do not confuse the CEMF, which is a mechanism that determines the rate at which energy can be stored, with the actual amount of energy that is stored.  And do not forget that the inductor is storing energy. 

As soon as the RATE OF CHANGE reached .8 amps per second, 4 volts of CEMF would be generated and effectively regulate/limit the rate of the current's rise to that .8 amps per second.

You are wrong.  Faraday says nothing about a required resistance or required dissipation.  Resistance does not enter into it.  If the rate of change of the current flowing thru the 5H inductor is .8 amps per second, the generated CEMF will be 4 volts.  If the rate of change is less than .8 amps per second, the generated CEMF will also be less than 4 volts.

Consider reading this whole Wiki, but from the Wiki:

https://en.wikipedia.org/wiki/Inductor

PW

The assumption is still being made , that the current value will continue to rise at 8 amps per second.
You seem to keep missi g the point i am making,and one that Poynt hast stated is yet to be understood.
At T=1 second,the EMF and CEMF are the same value. If at this point the current continues to rise,then it must be stated as to why the self induced current is of a lesser value than that of the EMF induced current,to allow current to continue to flow, when there is no potential difference across the coil

It would seem to me that making a claim to have a definitive answer,is very premature, when the mechanism is not yet known as to why the self induced current should be less than that of the EMF induced current.


Brad

[picowatt]

The assumption is still being made , that the current value will continue to rise at 8 amps per second.
You seem to keep missi g the point i am making,and one that Poynt hast stated is yet to be understood.
At T=1 second,the EMF and CEMF are the same value. If at this point the current continues to rise,then it must be stated as to why the self induced current is of a lesser value than that of the EMF induced current,to allow current to continue to flow, when there is no potential difference across the coil

It would seem to me that making a claim to have a definitive answer,is very premature, when the mechanism is not yet known as to why the self induced current should be less than that of the EMF induced current.


Brad

The CEMF is only less than the applied EMF of 4 volts when the rate of change of the current flowing thru the 5H inductor is less than .8 amps per second.  At .8 amps per second the CEMF would equal the applied EMF.  The generated CEMF is what limits the rate at which the current can rise to .8 amps per second.

It is not an assumption, it is the how and why inductors work like they do.  Have you read the Wiki?  Perhaps that will help.

I do not know how to explain the action of the inductor's CEMF any better than I already have several times now. 

PW