Explanation of inductance needed

[carbonzit]

I need a simple, yet comprehensive explanation of how basic inductance works.

I'm familiar with the concepts, and I've read a number of explanations, and I sort of understand how it all works. But I still can't comfortably wrap my head around the entire thing, including counter-EMF, expanding and collapsing fields and all that.

So I'm soliciting explanations from y'all, either your own finely-crafted short essays, or pointers to existing pages out there in Web-land.

I'd like to be able to understand the very basics of how coils work, starting with the right-hand rule and all that. Good illustrations would help, so I can finally grasp the relationships between the direction of current and the direction of the magnetic fields generated (and vice versa).

I have one request: Please do not try to explain it in terms like "the inductor tries to keep its current constant ..." I hate that! Not only does it not help to understand the concept, but it tries to anthropomorphize (big fancy word for "make it seem like it's human") a physical process, which has nothing to do with anything "wanting" to do anything. Please just explain it in terms of fields, forces, etc. See how good a teacher you can be!

I know it's basically a simple concept, but sometimes it's the simplest things that make our heads hurt the most ...

 

[carbonzit]

To get the ball rolling, check this page on inductance from one of my favored site for explaining technical stuff, HowStuffWorks. (I just now found this page.) In it, they say

Think About Water...

One way to visualize the action of an inductor is to imagine a narrow channel with water flowing through it, and a heavy water wheel that has its paddles dipping into the channel. Imagine that the water in the channel is not flowing initially.

Now you try to start the water flowing. The paddle wheel will tend to prevent the water from flowing until it has come up to speed with the water. If you then try to stop the flow of water in the channel, the spinning water wheel will try to keep the water moving until its speed of rotation slows back down to the speed of the water. An inductor is doing the same thing with the flow of electrons in a wire -- an inductor resists a change in the flow of electrons.

which actually helps.

So basically what they're explaining is that an inductor has inertia, electrically speaking (a resistance to change in imposed current). And to be more precise, when they say "an inductor resists a change in the flow of electrons", one should add "due to the creation of an opposing magnetic field". Correct?

Anyhow, I could really use some illustrations to help me grasp this simple concept.

 

[crutschow]

Let's start with the energy characteristics of the magnetic field.
The magnetic field stores energy equal to 1/2 LI².
Thus the greater the current the more energy stored in the magnetic field.
The energy is added or subtracted from the magnetic field by a voltage appearing across the inductor which changes the current where V = L di/dt.
If the inductor voltage is in the same direction as current flow, then energy is added to the field.
If the voltage is in the opposite direction to the current flow, energy is subtracted from the field
The magnetic energy does not change when the current is constant, thus there is no voltage drop across an inductor for that condition (neglecting inductor resistance).
Because of the voltage caused by the energy change due to an increase or decrease in the current, the inductor appears to resist the change in current, thus acting similar to an inertial mass in the physical world where it takes energy to increase or decrease it's speed, and the mass thus has a resistance (inertia) to this change.

Does that make it any clearer?

 

[OlPhart]

It seems we all have our favorite & despised analogies. I don't particularly like the water one, but if it helps the visualization... all good.

My take: electrons flowing through a wire cause a magnetic field (see RightHand rule) that surrounds the wire. Coiling the wire causes this field to reinforce itself, setting up its' "inductive characteristic". More coil causes denser/stronger magnetic field which increasingly "stifles" (can't use "resists" or "impedes" without ambiguity) changes to its' field "amount" (gauss).

A given amperage ((6.23x10e23 electrons/sec)/amp) through a given coil sets up a resulting gauss. The higher the gauss, the greater the inductive effect. Inductance (in Henrys) is the metric of a physical build: wire gauge, wire material, # turns, core diameter, core material, layout (toroidal/bar, etc), winding type (spiral/crosshatch, etc), (probably left some factors out).

Just my verbiage on a topic difficult to visualize, hope it (and those from others) helps... <<<)))

Crutschows' description is an Excellent Other view of the effect... Well Done.

 

[KeepItSimpleStupid]

My quick take:

inductors: Will not let current change instantaneously

Capcitors: Will not let voltage change instantaneously

Resistors: Let voltage change instantaneously

 

[Ratchit]

CarbonZ

Explanation of inductance needed
I need a simple, yet comprehensive explanation of how basic inductance works.

You are asking for two different things. First the explanation. As any physics book will tell you, inductance is the flux linkage per unit current of a circuit loop. In other words, it is the amount of linkage of magnetic flux between one circuit and another (mutual-inductance), or within the same circuit (self-inductance). So inductance does not "work". It is a quantity of magnetic flux transference. Magnetic flux works by storing and releasing energy. Remember, energy has an equivalency to work. All those other things you have read explain the ins and outs of how magnetic flux does what it does. No one can do much better than that.

Ratch

 

[trash]

It seems we all have our favorite & despised analogies. I don't particularly like the water one, but if it helps the visualization... all good.

I'm going to drift from carbonzit's request slightly too.
Yes. I rely on friends for their mechanical abilities. They rely on me for electronic knowledge.
They quickly visualise electronic components if I can explain them in mechanical terms.

An inductor is analogous to a flywheel
A capacitor is analogous to a spring

They then quickly consider the many ways in which these two types of devices can be varied or interconnected.

Newton's first law:
A body's motion (inertia) remains constant unless acted upon.

The rate at which that change takes place is Newton's third law:
For every action their is an equal and opposite reaction.

---

If one wishes to think of it in terms of say calculus;
Then the instantaneous voltage (thus resistance/impedance) is a function of the differential of the waveform with respect to the inductor's value.
V = L di/dt

I do believe the easiest way to understand it is to put it into a context which the person learning can understand.
The irresistible force vs the immovable object and the compromise that is the outcome.

 

[MrAl]

Hi Carbon,

If you are looking for very basic rules then you might start with a single straight piece of copper wire stretched between two nails or something. That conductor has inductance just like a larger 'inductor' and behaves just the same.

Note that it doesnt actually do anything by itself, just sitting there for a time. We have to connect a power source to it to get anything to happen, and that's when things start to change dramatically.
Connecting a battery to the inductor through some short leads causes a voltage to appear across the wire, and as the voltage appears a current also begins to appear through the wire. Along with that a magnetic field appears surrounding the wire with a certain orientation. The voltage forces current through the wire, but the current cant change instantaneously because the field is changing and that induces an emf that is opposite to that of the battery, and thus acts like a small battery that is in anti series with the external battery we connected. That action is what we commonly call inductance, well at least that's half the story.

The other half is when we go to remove the battery from the circuit. If we quickly replace the battery with a resistance then the field starts to collapse, and that generates a current through the wire with a voltage across the ends. Since the field is decreasing now the polarity of the voltage is opposite to what the battery was, but the current is in the same direction. An interesting observation is that at a very very short time after the battery is removed, even though there is a resistor there now the current appears to stay at the same level for that short time. What actually happens though is that the current starts to decrease, but that decrease over shorter time intervals is very small so we say that the inductor keeps the current constant. Obviously if it is decreasing it cant be constant, but for those very short times we say that it stays constant because after all it's hard to measure the difference between 1.000001 amps and 1.000000 amps. With greater time comes greater change, and the current decreases to zero.
If there could be some way to keep the internal and external resistance to zero however, that current would stay the same indefinitely. If we had 1 amp flowing at time t=0 then we would have that same current flowing an hour later, but with real life circuits this usually isn't possible nor desirable in say power converters.

So the main reason why we have inductance is because of the building up or collapsing of the magnetic field. It's the changing magnetic field that causes the inductance. And since when the field is increasing it in turn generates a counter voltage, we dont get a short circuit but rather what we commonly call an 'inductor'.

We can take a macro view of the inductor for a very short time period by examining the voltage and current in a circuit with an inductor in series with a resistor and battery. With a current level of say 1 amp and the inductor still charging, we dont see the entire battery voltage across the resistor. What we see is some of the voltage across the inductor. That means that there is less voltage across the resistor and thus less current flowing than if the inductor was a short circuit. If we let time progress however, that inductance would charge up and start to look like a short circuit so all of the voltage would appear across the resistor.
Connecting the battery up to the resistor and inductor, first all of the voltage would appear across the inductor (inductance totally opposes all of the voltage) and then as the field becomes steady the current would start to flow and voltage would start to appear across the resistor. Eventually all the voltage appears across the resistor.

 

[mbarazeen]

if you understand how a capacitor works, then its bit similar how an inductor works. capacitors are charged by increasing the terminal voltage, but inductors are charged by increasing current passing through. try to understand comparing both..you will find it easy..

 

[carbonzit]

Back to basics

OK, I'm going to try to focus like a laser beam on the one aspect of inductance that I really want to understand. I think it's clearer to me now, since I sat down and drew some pictures (see below). For me, that always seems to help.

The one aspect I was having trouble with was this: the effect that a changing voltage has on a coil.

As we all know, when a changing voltage is impressed on an inductor, either increasing or decreasing, the inductor acts to oppose that change (hence the analogy to "inertia").

What I was having trouble with was why this is true. I think most people simply accept this as a given. Not me; I really need to know at least something about why this happens. I'm referring to the basic physics of electromagnetism and induced current.

Here's how I see it, illustrated. This seems correct to me. However, if it is not, I would very much appreciate being shown the error(s) in my explanation.

So we all know that when a current flows through a conductor, a magnetic field is produced, shown here as magnetic "lines of force":

**broken link removed**

(The direction of current is that of "conventional" current flow. For fans of "electron flow", just reverse it. The magnetic lines of force, however, rotate in the direction shown.)

OK, so what we have is a static magnetic field, for some constant flow of current through the conductor.

Now let's change the current (by changing the voltage). When we increase the current flow, the magnetic field expands (looking at the conductor end-on):

**broken link removed**

(and vice versa: when the current decreases, the magnetic field shrinks)

So here's what I think happens to cause the opposition to the change in current. As the magnetic field around a conductor expands, its lines of force cut across adjacent conductors:

**broken link removed**

This causes a current to be induced in the adjacent conductor, just as if the conductor were moved through a magnetic field, as in a generator; instead, here the conductor's position is fixed and the magnetic field itself is moving against the conductor, by virtue of its expansion.

So that's why we get "reverse EMF" from changing currents. Now, my explanation is missing a few details, which would explain why the induced current as shown above is in opposition to the current already flowing through the conductor. It must have something to do with the direction of current relative to the direction of the moving magnetic lines of force.

Can somebody complete this explanation? (or correct it if needed?)

(Wacky side thought: if the action were the opposite--if any change in current was aided by, rather than opposed by, the inductor, would we have some kind of free energy generator? Because the more you increase the current, the more the current would increase, ad infinitum, which of course is impossible ...)

 

[Ratchit]

CarbonZ,

OK, I'm going to try to focus like a laser beam on the one aspect of inductance that I really want to understand. I think it's clearer to me now, since I sat down and drew some pictures (see below). For me, that always seems to help.

The one aspect I was having trouble with was this: the effect that a changing voltage has on a coil.

As we all know, when a changing voltage is impressed on an inductor, either increasing or decreasing, the inductor acts to oppose that change (hence the analogy to "inertia").

...

While the explanations in your last post appear to be true, taken as a whole, they appear to be convoluted and unnecessarily complicated. I think you should look at how both coils and capacitors work from an energy point of view. You are wondering why the current does not build and decay in a coil as fast as it would in a resistor only circuit, right? Let's start with the basics. Voltage is the energy density of the charge (joules/coulomb), OK? The more voltage, the higher the energy density, and the more energy is available to send charge flowing in a circuit. This charge flow is called current. Current exists, but does not flow. "Current flow" is a redundancy, like "velocity flow". Now let's apply a voltage to a circuit containing only a coil and see what happens. At the very instant the voltage is applied, the current is limited by the resistance all practical coils have. Otherwise, the current pulse would be infinite. Now you wonder why the current does not build up as fast as it would if only the resistor were present? The reason is that energy is diverted to build the magnetic field around the coil. With less energy available, the energy density of the charge (voltage) becomes less. Therefore less current will exist through the coil. Now you can think of the less voltage available to be the result of a "back-voltage" opposing the forward voltage if you want, but doesn't it make more sense to think of the lower forward voltage a result of an energy drain? The opposite happens when the driving voltage is lowered. The magnetic field collapsing returns energy to the circuit. This keeps up the energy density of the charge (voltage) for a period of time causing the current to exist at a higher value than it would with only a resistor present. So by considering coils and capacitors as energy storage elements, much of the wonderment of how they work is eliminated. Think energy!

Ratch

 

[carbonzit]

While the explanations in your last post appear to be true, taken as a whole,

Phew! I was hoping I wasn't too far off base there.

they appear to be convoluted and unnecessarily complicated. I think you should look at how both coils and capacitors work from an energy point of view.

With all due respect, I feel that describes your explanation. I'm not saying it's wrong, just unnecessarily abstract.

I tried to describe the physical mechanism that inhibits the change of current in an inductor. This physical process, assuming I've described it correctly, is the underlying cause of everything you described (pertaining to inductors, anyhow; obviously, there's a different process with capacitance).

Of course, it is useful to look at inductance from an energy point of view; I'm focusing on a specific aspect of how these things work, to try to wrap my head around it.

 

[MrAl]

Hello again Carbon,

You dont need to consider the expansion of the field to understand these basics though, so you can reduce that complication too. Instead, you only need to consider the increase or decrease of the intensity of the field.
If you apply a voltage across a horizontal wire with plus on the right, you first get a field that is increasing, and that increasing field induces a voltage in the wire just like the battery, only in reverse. So it's like having two batteries in series but the inductive one is in reverse to the real battery. Because the voltages are the same, no current flows (yet). The field later starts to increase less quickly, so the opposing voltage decreases and so the battery starts to have to supply current as well as voltage.
Eventually the field no longer increases and the reverse voltage falls to zero so the battery supplies more current.
Something else has happened too though, and that is that the field intensity has built up to some max value held by the current flow, and if we remove the battery and replace it with a resistor the current now decreases a little (before it was always increasing). That decreasing field causes the voltage to reverse across the wire and that keeps the current flowing for a while. After a time the field falls to zero, and the voltage falls to zero, and no more current.

So to sum up, the changing field regulates the current through the inductor (wire). Once the field becomes static it only stores energy.

To look at it in a sort of pseudo sequential fashion, we would have:
Forward voltage creates field, opposing field only allows some current to flow, when field reaches max the most current can flow.
Removed battery means removed forward voltage, field decreases inducing a current that is decreasing, when field reaches zero current reaches zero.

Make sense?

 

[carbonzit]

Yes, it makes sense.

The crux of the matter, as you stated:

If you apply a voltage across a horizontal wire with plus on the right, you first get a field that is increasing, and that increasing field induces a voltage in the wire just like the battery, only in reverse.

But what you (and everyone else) seem to gloss over, perhaps because it seems trivial to you, is why the field induces a reverse voltage. Which is what I tried to explain: just that mechanism. So do you agree with my explanation, above? None of the texts I've found, either in my house or on the web, seem to explain that part of it.

The rest of it I feel I understand pretty well.

 

[MrAl]

The electrons interact with the field in a manner similar to a metal ball and a magnet. The magnetic field pushes or pulls the free electrons so they form a current which induces a voltage in the wire.
You can get a very rough picture of this (and with new theories this is indeed rough now) by placing a metal ball on a flat table and moving a magnet near it sweeping from say left to right.
Electrons exhibit electrostatic properties but also magnetic properties.

Another way of thinking of it is to imagine a field already formed, a static field, and an electron shot through that field. There is a force on the electron as it passed into the field which makes it turn because it interacts with the field. There's various ways to look at this more exactly, but the basic idea is that a magnetic field produces a force on an electron and so the electron, if it is free to move, actually does move, and we know that movement forms a current in the wire.

You're explanation, if i understand you right, should not really talk about the field expanding, but rather the field increasing or decreasing at any given point. In this kind of classical discussion we would assume that the field goes out to infinity even when there is little intensity near the source. To bring in the expansion of the field brings in other properties that we dont need to worry about for this discussion (ie wave propagation).

If you want to look at the field intensities at various points in space away from the 'wire', you can use Biot Savart. That makes it a little easier to visualize the field.

 

[john1]

Hi carbonzit,

I think you are correct.
The expanding shape of the lines of force around the conductors is responsible for the back EMF.

********

As to your next paragraph about missing a few details .....
I dont think you missed anything.

Yes, the induced effect is in opposition to the existing current.
Yes, it does have to do with the direction of the moving lines of force.

********

Sounds about right to me.
And your drawings are too good to have been drawn with a mouse.
Although they are clearly drawn by hand.
My guess is a touch screen.

Regards, John : )

 

[john1]

Hi carbonzit,

Just read your last post. #14.

Your question here is substantially clearer now.

********

But to illustrate a reply, i would like to describe a physical active situation.

********

You are pushing a cart along with your hands, along fairly even ground.
You can feel the pressure on your palms as you push it along.
Then it comes time to stop, so you slow down to stop.
You feel the cart pulls on your hands a bit, then you stop.

Very simple, whats to know here you ask.
Well, the cart always went forwards, but the pressure on your hands
went from pushing to pulling.

I hope this illustrates to you
why the induced current produces a reverse voltage.

********

You said earlier that you are not keen on these illustrative kinds of explanation,
but this one is fairly simple.
And its hard to explain in other more technical ways.

Regards, John : )

 

[MrAl]

Hi again,

Well you know the simplest way to explain it is probably with Faraday. E=-d(Phi)/dt where Phi is the flux. Otherwise we have to figure out how the field which is perpendicular to the flow of electrons can impede and quicken the flow by simply increasing and decreasing. The field would have a rotation effect on the electrons either toward the center of the wire or toward the surface. The expansion of the field would be outward away from the wire and not parallel to the wire, and would take place very quickly.

 

[carbonzit]

I think you are correct.
The expanding shape of the lines of force around the conductors is responsible for the back EMF.

********

As to your next paragraph about missing a few details .....
I dont think you missed anything.

Thanks for your affirmation. Good to know one isn't completely crazy.

And your drawings are too good to have been drawn with a mouse.
Although they are clearly drawn by hand.
My guess is a touch screen.

Nope. Just good old pencil and paper (ball-point pen, actually), digitized by a scanner.

Y'know, people still use that ancient stuff to make drawings ...

Regarding your cart analogy, I actually like those kinds of illustrations (like the water-wheel one I posted, it describes the "inertia" that inductance imposes on current flow). I just don't like explanations that impute some kind of conscious action to the coil: "The inductor wants to keep current flowing through it after the voltage is removed".

 

[ericgibbs]

hi cz,

The expanding shape of the lines of force around the conductors is responsible for the back EMF.

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

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

When the current thru the inductor is switched off, the magnetic field surrounding the inductor collapses and induces a current back into the inductor.
When the magnetic flux change becomes zero, the current stops flowing in the inductor.

If you wind a coil on a ferrite core, the increase in 'inductance' is because of the ability of the coil and ferrite core to create a higher magnetic flux density around the inductor. If the magnetic field density is increased to a point were the ferrite cannot support the increasing magnetic field flux, it will 'saturate' and again the inductor will become purely resistive.

So its the rate of change of magnetic flux around the conductor that creates the back EMF.