Explanation of inductance needed

[crutschow]

When the magnetic field reaches it maximum density, the inductor [conductor] appears purely resistive.

Eric, this is a nit, but I think when the a magnetic core saturates the inductance to increased current now is roughly equal to the air core inductance of the coil. Of course for practical purposes that is so much lower than the normal inductance as to make the inductor appear essentially resistive.

 

[ericgibbs]

Eric, this is a nit, but I think when the a magnetic core saturates the inductance to increased current now is roughly equal to the air core inductance of the coil. Of course for practical purposes that is so much lower than the normal inductance as to make the inductor appear essentially resistive.

Carl, you are correct in adding that detail, I was trying to keep the explanation as simple as possible.

 

[MrAl]

Hi Eric,

I was trying to make the point of the increasing or decreasing field too. Especially since we have E=-N*d(Phi)/dt which is clearly a voltage appearing due to a changing amplitude flux field.

It is interesting though that a changing amplitude field alone can push an electron. That makes my 'motor action' example a little inappropriate. Perhaps a better example would be a line of electromagnets that have a decreasing or increasing current going through them. Maybe you can think of something better.

 

[carbonzit]

This is not exactly correct, the back EMF is due to the rate of increasing magnetic flux density surrounding the inductor [conductor]

Yes, but you're kind of missing the point. Like others here who simply restate this fact (basically Lenz's law), you're ignoring all that I've tried to describe, which is the mechanism by which this occurs. Do you see that? I really want to know why the rate of increasing magnetic flux density generates the back EMF. In other words, what causes this linkage between the expanding (or contracting) magnetic field and the resulting opposing current flow?

Do you agree or disagree with my explanation?

 

[ericgibbs]

Yes, but you're kind of missing the point. Like others here who simply restate this fact (basically Lenz's law), you're ignoring all that I've tried to describe, which is the mechanism by which this occurs. Do you see that? I really want to know why the rate of increasing magnetic flux density generates the back EMF.
Do you agree or disagree with my explanation?

Simply by the collapsing flux change of the magnetic field moving the free electrons in the conductor in a combined direction,
A conductor in a changing magnetic field is how a transformer and a generator works.

EDIT:
Look at these links and extract,
http://en.wikipedia.org/wiki/Fleming's_left_hand_rule_for_motors

**broken link removed**

IV. Self-Induction
When the current in a conductor varies, the resulting changing magnetic field cuts across the conductor itself and induces a voltage in it. This self-induced voltage is opposite to the applied voltage and tends to limit or reverse the original current. Electric self-induction is thus analogous to mechanical inertia. An inductance coil, or choke, tends to smooth out a varying current, as a flywheel smooths out the rotation of an engine. The amount of self-induction of a coil, its inductance, is measured by the electrical unit called the henry, named after the American physicist Joseph Henry, who discovered the effect. The inductance is independent of current or voltage; it is determined only by the geometry of the coil and the magnetic properties of its core.

 

[crutschow]

Yes, but you're kind of missing the point. Like others here who simply restate this fact (basically Lenz's law), you're ignoring all that I've tried to describe, which is the mechanism by which this occurs. Do you see that? I really want to know why the rate of increasing magnetic flux density generates the back EMF. In other words, what causes this linkage between the expanding (or contracting) magnetic field and the resulting opposing current flow?

If you are asking why an increasing flux density applies a force to electrons and generates a back EMF? the answer is "because that is what a magnetic field is observed to do". The mechanism is unknown. The are hundreds of basically philosophical questions in physics such as why is the charge on an electron negative?, or why does a moving electron generate a magnetic field? Those are observations that have been documented and accepted but the underlying reason for such behavior is unknown.

 

[carbonzit]

If you are asking why an increasing flux density applies a force to electrons and generates a back EMF? the answer is "because that is what a magnetic field is observed to do". The mechanism is unknown. The are hundreds of basically philosophical questions in physics such as why is the charge on an electron negative?, or why does a moving electron generate a magnetic field? Those are observations that have been documented and accepted but the underlying reason for such behavior is unknown.

I realize that this discussion could easily veer far too far towards extremely abstract physics, perhaps even into quantum physics, which is certainly not my intention.

So accepting what you've said, I guess I'll just leave it at "my (incomplete) explanation works for me". Which allows me to move forward to the behavior of inductors stemming from this observed phenomenon. (Which after all is more important to an understanding of inductances in electronic circuits than this underlying physics; that's just for my own peculiar personal satisfaction. Gotta know why things work.)

 

[MrAl]

If you are asking why an increasing flux density applies a force to electrons and generates a back EMF? the answer is "because that is what a magnetic field is observed to do". The mechanism is unknown. The are hundreds of basically philosophical questions in physics such as why is the charge on an electron negative?, or why does a moving electron generate a magnetic field? Those are observations that have been documented and accepted but the underlying reason for such behavior is unknown.

Hi there Carl,

I think it's an interesting question though, because what it comes down to is a field changing magnitude only yet it somehow gets electrons to move perpendicular to the traditional 'direction' of the field. There appears to be nothing parallel to the wire to push the electrons as in the motor effect, yet the electrons still move. It just seems to be that the extra energy has to cause something, so it causes the electrons to move

 

[carbonzit]

**broken link removed**

IV. Self-Induction
When the current in a conductor varies, the resulting changing magnetic field cuts across the conductor itself and induces a voltage in it. This self-induced voltage is opposite to the applied voltage and tends to limit or reverse the original current. [...]

I somehow missed this in Eric's post yesterday. This appears to be almost exactly what I proposed in my explanation (although I posited that the voltage was induced in adjacent conductors), so I feel totally vindicated here.

Now, of course this doesn't explain how this phenomenon works at a deeper level, but it is satisfactory for my purposes (to comfortably wrap my brain around the concept of self-induction).

In fact, it seems self-evident to me now. How would this be any different from the well-known phenomenon of induction caused by moving a wire (physically) through a magnetic field, as in a generator? In both cases, the lines of magnetic force "cut" through the wire. It's just that in the case of an inductor that there's no physical movement; the expanding or collapsing magnetic field supplies the "motion".

What's that they say about Occam's Razor and all? that the simplest explanation is the best?

 

[MrAl]

Yeah but it's interesting that even if you look at it as "expanding and collapsing" the direction is still perpendicular to the wire, which still doesnt tell us how the electrons get forced to move. That's what i meant by 'interesting'.

OR's quote might say that the simplest is probably the best, but that's only if another better explanation doesnt turn up later even if it is more complicated, which always seems to happen these days.

 

[carbonzit]

Yeah but it's interesting that even if you look at it as "expanding and collapsing" the direction is still perpendicular to the wire, which still doesnt tell us how the electrons get forced to move. That's what i meant by 'interesting'.

IOW, works the same way as a magneto/generator, correct?

The point being that we don't really know what makes the electrons move in either case .

All of which can be put under the heading "The Limits of Human Knowledge", which appears to be boundless, regardless of all the things we think we know ...