help understanding magnetic induction

[copper123]

I am having some problems getting my mind wrapped around magnetic induction: I am pretty sure I am making way to much out of it, but I sure would like somebody to set me strait. This what I think I have learned so far:

#1 whenever current flows through a conductor/coil, a magnetic field is created in the conductor.
The magnetic field strength can be changed in many ways. The amount of current flowing in the conductor will change the field strength... Or you can simply add more loops in the conductor to get more strength. Also, the polarity of the magnetic field is determined by the direction of the current flow.

#2 whenever a conductor passes through a magnetic field, a voltage is induced into the conductor. I guess this is what is called magnetic induction. The amount of voltage is determined by the strength of the field, speed of cutting action, or number of turns of wire.

Ok, so far I think I got it.
But now, I get messed up with the whole induced voltage/counter EMF or CEMF.

This is what I have got myself thinking and can?t shake. Break out the crayons!
Lets say you have a coil of wire.
You pass a magnet through the coil. As you pass the magnet through, you induce a voltage onto the conductors. This is one of the first basic laws. (Whenever a conductor cuts through magnetic lines of flux, a voltage is induced into the conductor) Ok, so now we have a voltage induced into the conductor. This is what I call the induced voltage. Correct? So now we have voltage or potential in the conductor. If we have a complete circuit, current will begin to flow. Ok, so now we have current flow, this current flow will create its very own magnetic field. This magnetic field will have an opposite polarity to the magnetic field that created the voltage that caused the current to flow. So, the two magnetic fields are opposite of each other, and the magnetic field caused by the induced current will want to pull the magnetic back into the coil. If we reverse the direction of the magnet, the polarity of the induced current will change and we will start all over again. I guess this is what is called Lenz?s law. This is how it is described in one of my books: The polarity of an induced emf will be such that any resulting current will have a magnetic field which opposes the original action that produced the induced current. Soooo, basically are they not just saying, that the two magnetic fields that have been created will oppose each other? The mag. Field creates a voltage that creates a current that creates its own mag. field. Right? What is getting me confused is what is the counter EMF?
This is what it says in Mike Holts exam prep book about induction.
When a magnetic field moves through a conductor, it causes the electrons in the conductor to move. The movement of electrons caused by electromagnetism is called induction. I guess he takes out the whole part of the induced voltage and goes right to just current flow. In an ac circuit, the current movement of the electrons increases and decreases with the rotation of the armature through the magnetic field. This is describing the induced voltage right? Conductors passing through a magnetic field create a voltage. The voltage is going to 0 to 180 on the positive sine wave and then going 180 to 360 on the negative. The degrees in which the conductors cut the flux lines make the voltage. This 0 voltage to 90 positive to 180 to 360 negative is what changes the current flow. He then says: As the current flowing through the conductor increases or decreases, it causes an expanding and collapsing electromagnetic field within the conductor. Yep, I get that, simple stuff. But this is where I get confused. He says: This varying electromagnetic field within the conductor causes the electrons in the conductor to move at 90 degrees to the flowing electrons within the conductor. The movement of electrons because of the conductor?s electromagnetic field is called self-induction. Self induction (induced voltage) is commonly called counter-electromotive force.
I just don?t understand. I understand everything right to there. Are we going all they way back through the entire process again?. If so, it makes me think of the nursery rhyme about the lady that swallowed the fly to catch the spider, and so on, and so on?It would seem that the varying electromagnetic field that is cutting the conductor would induce another voltage into it. .
In another book, it says, the induced voltage always opposes the original change in current. That is why the induced voltage is known as the counter-emf. Are we talking about two different voltages that get induced into the coil? The first that gets induced from the original magnetic that started the entire thing, and then the magnetic field from the induced current makes another induced voltage?? Man, I have really gotten myself confused. In Delmar?s book of electricity it shows a example of a coil and a magnet. It shows the coil with no induced voltage, the magnet is away from it. It says, if the magnet is moved toward the coil, magnetic lines of flux will begin to cut the conductors of the coil, and a current will be induced in the coil. Don?t I need the voltage first?? The induced current causes magnetic lines of flux to expand outward around the coil. As this expanding magnetic field cuts through the conductors of the coil a voltage is induced in the coil. Should there not be a voltage already in the coil?? Are they just simplifying the text or am I really missing something here? Anyhow, could you guys get me back on track? I am sure this is really simple but I have the rare ability to take something simple and turn it into something extremely difficult!
thanks

 

[physis]

Re: help understanding magnetic induction

Well, I read your book and think I can help with the voltage versus current issue.

If nothing is connected to your conductor or coil that has an induced voltage on it then no current can flow. The electrons wanting to move is what the voltage is. If they're allowed to move they will become current. It's just Ohm's law stuff from there.

As for the induced field trouble I'll have to look at your post a bit closer. When you brought up Lenz law I was going to look it up but I couldn't find it, it's around here somewhere.

 

[bphgravity]

Re: help understanding magnetic induction

When current flows through a conductor, a magnetic field is created aronf the conductor. When the voltage source is disconnected to the conductor, the current rapidly drops to zero. In the process, the magnetic field collapses back into the conductor, inducing a voltage and opposing the change in current. This is counter-EMF.

In an inductive alternating current system, the voltage goes to zero 120 times per second or twice per cycle. This action creates a continuously expanding and collapsing field around the conductors. This self inductance opposes the change in current and thus "lags" the current behind the voltage.

 

[charlie b]

Re: help understanding magnetic induction

I don?t know if this will help, but here goes:

Originally posted by copper123: Whenever a conductor cuts through magnetic lines of flux, a voltage is induced into the conductor.

That voltage is the ?counter-emf.?

Lenz?s law. . . . The polarity of an induced emf will be such that any resulting current will have a magnetic field which opposes the original action that produced the induced current.

That is indeed Lenz?s Law, and it might explain the origin of the name ?counter-emf.?

But this is where I get confused. He says: This varying electromagnetic field within the conductor causes the electrons in the conductor to move at 90 degrees to the flowing electrons within the conductor. The movement of electrons because of the conductor?s electromagnetic field is called self-induction.

I don?t have Mike?s book, and I hesitate to contradict something I have not read. But the term ?self-induction? means something else to me. Take a 3 foot section of AWG #10 and launch it into space. Let it drift forever far away from any magnetic fields, any batteries or ac generators, and any flowing currents; it will still possess the property of ?self-induction.? That property is determined from the shape and size of its cross-section, and can be expressed on the basis of ?per linear foot.? It is a measure of how the wire would behave, if you were to connect it to a voltage source. But it has that property even before the voltage is connected.

There is a different property called ?mutual induction,? that comes into existence when two or more conductors are in close proximity with each other. It is determined by their relative geometry (i.e., Do they run in parallel with each other? Are they at a constant distance from each other?). This property is a measure of how the two wires would behave, if you were to connect one to a voltage source (i.e., thus creating a current in the first, and therefore a magnetic field around the first), and lets you know how much counter-emf would be generated in the second. But the set of two or more wires has the property of ?mutual inductance,? even before any voltage is connected to either wire.

Self induction (induced voltage) is commonly called counter-electromotive force.

I agree with this statement, as it is the same as I have made above.

In another book, it says, the induced voltage always opposes the original change in current. That is why the induced voltage is known as the counter-emf.

Ditto.

Are we talking about two different voltages that get induced into the coil? The first that gets induced from the original magnetic that started the entire thing, and then the magnetic field from the induced current makes another induced voltage??

No. There is just the one, the ?counter-emf.?

In Delmar?s book of electricity . . . , it says, if the magnet is moved toward the coil, magnetic lines of flux will begin to cut the conductors of the coil, and a current will be induced in the coil. Don?t I need the voltage first?? Should there not be a voltage already in the coil??

That was a simplification. They omitted stating that there will first be an induced voltage, and that that voltage causes current to flow.

 

[copper123]

Re: help understanding magnetic induction

Thanks guys for reading my novel! I think I am heading in the right direction.
I think what is happening to me is the fact that I am taking the text two literal or something.. As Charlie said, they are simplifying things, and I don?t grasp that. This is what Bhgravity said:
When current flows through a conductor, a magnetic field is created around the conductor. When the voltage source is disconnected to the conductor, the current rapidly drops to zero. In the process, the magnetic field collapses back into the conductor, inducing a voltage and opposing the change in current. This is counter-EMF
So back to the coil/magnet analogy.
Here we have a coil of wire that we have just captured from outer space.(Thanks Charlie)!
At this time as it sits on the table, it has no voltage induced in it. It?s just a plain ol piece of copper with one valence electron in its outer shell. If we grab a magnet and start to bring it close to the coil things will start to happen. Number one, we must induce a voltage into it, right???. Its got to have voltage before anything can happen. The electrons are always there, but you need something to give them the kick in the pants. The electrons will start to flow. If it is in a rotating mag field, the voltage will go to 0, 120 times per second, or twice per cycle. This change in voltage will make the current continually expand and collapse around the magnetic field. This expanding and collapsing will induce a voltage onto the conductor. This is the CEMF.
But?.. here is the deal that has thrown me. Like I asked before. Don?t you really have two voltages that are getting induced into the coil???? In my way of thinking it would go like this in the sequence of operation. This is using the magnet analogy.
Have coil, bomb with mag flux, create voltage, voltage starts current to flow, current makes mag lines of flux, flux lines continually expanding and collapsing create a voltage. This is the counter emf. Am I wrong?
This is how one of my books explains a transformer. This stuff is so darn wordy and it just sends me reeling. #1 Voltage applied to one of the coils will magnetize the core. Yep, current will flow, create a magnetic field. Core material will really help out the process. Mag + core material = lot stronger mag field. #2 This magnetic field induces voltage in the second coil. Yep, piece of cake. Pass a wire through a magnetic field, get a voltage. #3The voltage in the second coil opposes the initial voltage. Not sure if they are talking about the first initial voltage that starts the chain of events. #4A counter-voltage is built up which will oppose the voltages being applied. This counter-voltage creates voltage in the second coil by induction. That is what they say in a nutshell about how a transformer works. Is this over simplification again? Say again, How does a transformer work?. If they are going to lay out text in a sequence of operations, should they not lay out all the parts? This is why I am so stuck on the counter EMF. Right now, with my tiny Christmas tree light burning as bright as it can in my head, I still don?t understand a basic operation of a transformer. And my textbook says. If you want to be an electrian, you better understand the principles on induction. Yikes!

 

[charlie b]

Re: help understanding magnetic induction

You keep trying to bring into existence one too many voltages.

Originally posted by copper123: #3 The voltage in the second coil opposes the initial voltage. Not sure if they are talking about the first initial voltage that starts the chain of events. #4A counter-voltage is built up which will oppose the voltages being applied.

Let?s eliminate the ?first voltage,? by using a bar magnet (not a coil) as the source of the magnetic field. Make the magnetic field oscillate by physically moving the bar magnet towards and then away from the section of wire.

Now let?s eliminate any possibility of current in the wire. Use that three foot section you retrieved from outer space. Don?t connect a ?load? to it, and don?t create a complete circuit in any other way. Just keep it as a straight section of wire.

As you move the bar magnet towards the wire, there will be a voltage created in the wire. That voltage is the ?counter-emf,? and there will be no other voltage in this discussion.

Some number of the free electrons in the wire will move toward one end of the wire. That is a current, but a brief one. The result will be a net positive charge at one end of the wire and a net positive charge at the other end.

Now move the bar magnet away from the wire. The electrons that had moved to the one end will move towards the other. The result will once again be a net positive charge at one end of the wire and a net positive charge at the other end, but the two ends will have switched roles.

You could attach a voltmeter to the ends of the wire, and watch the voltage in the wire, the ?counter-emf,? varying up and down, as the magnet is moved in and out.

This counter-voltage creates voltage in the second coil by induction.

Not so! Here may be the source of your confusion. The counter-voltage (counter-emf) is itself in the second coil. It cannot create an additional voltage within itself. And there is a simple reason: A current-carrying wire will create around itself a magnetic field, but it cannot ?cut the lines? of that magnetic field. You need relative motion between a conductor and a field, in order to get a counter-emf. There can be no relative motion between a field and the wire that created that field.

 

[physis]

Re: help understanding magnetic induction

Its got to have voltage before anything can happen. The electrons are always there, but you need something to give them the kick in the pants.

It might be a small point but it's important to understand that voltage and current are part of the same phenominon and happen simultaniously. The current begins as the voltage appears.

 

[ronaldrc]

Re: help understanding magnetic induction

While were on the subject of counter EMF I have a question and I think Ed would be the best candidate.

Ed as we no the reason a high quality transformer doesn't present a load on the line without a load on the secondary is because of a near perfect impedance or equal magnetic lines of force collapsing in the opposite direction of the ones being generated by the source takes place my question is?

From the start of the cycle where would these lines of force start bucking I think it would be after the second quarter of the first cycle would that be right?

Ronald

[ December 05, 2004, 07:57 PM: Message edited by: ronaldrc ]

 

[Ed MacLaren]

Re: help understanding magnetic induction

From the start of the cycle where would these lines of force start bucking I think it would be after the second quarter of the first cycle would that be right?

Well, now that you ask, I'm not sure I agree with everything that has been said on the subject thus far.
And if we all agreed on everthing, this wouldn't be nearly as much fun.

I'll briefly outline how I see it, in point form, for discussion.

- With no load connected to the secondary, no current flows and no flux is contributed, by the secondary.

- The primary winding's impedance is mostly composed of inductive reactance. The effective resistance, including the effects of the core losses, is quite low in comparison.

- Therefore the small (excitation) current that does flow in the primary is lagging by almost 90 degrees, let's say for this example 87 degrees.

- Because the CEMF is generated by the changing (alternating) primary current, it will be maximum when the current is changing most rapidly, which is while it is going through 0.
And, the CEMF will be 0 when the current is changing least per unit of time, which is when it is at it's maximum value.

- This means that the CEMF is 90 degrees out-of-phase (lagging) with the current.

- As a result the CEMF is almost 180 (minus 3 degrees), out-of-phase with the supply voltage. It is almost "opposite and equal", cancelling most of the supply voltage, and that is why the primary current is so small at no load.

- And, the only true power taken by the transformer is the copper and core losses.

So in answer to your question -
"where would these lines of force start bucking",
my answer is "3 degrees after the start of the cycle".

Ed

[ December 05, 2004, 09:09 PM: Message edited by: Ed MacLaren ]

 

[ronaldrc]

Re: help understanding magnetic induction

Thanks Ed

Assuming we are starting at zero volts at zero degrees and a clean undistorted sine wave. I wouldn't expect it to start bucking until the peak of the first half cycles starts to collapse which would be 90 degrees.

I'm sure you are right about the 3 degrees I will wait for your graph.I am very interested to see how it starts at 3 degrees.

Ronald

 

[Ed MacLaren]

Re: help understanding magnetic induction

Ronald,
I just picked 3 degrees as an example. The actual phase angle could be more or less than 87 degrees, and would depend on the resistance of the primary winding.

The sketch above is intended to represent a "snapshot" of a typical cycle after the core is fully magnetized.

A transformer's "inrush current" can be quite high, many times higher than the exciting current, for a few cycles until the core is magnetized.

Ed

 

[ronaldrc]

Re: help understanding magnetic induction

Thanks for the graph Ed I was typing my answer while you were editing yours.

I still don't understand the 3 degrees I will do more studying and maybe it will come to me.

Whops done it again I understand about the 87 degrees thanks"

Ronal

[ December 05, 2004, 09:32 PM: Message edited by: ronaldrc ]

 

[physis]

Re: help understanding magnetic induction

If it's me Ed I assert that in a case where you have voltage and no current you still have a simultanious existance of zero current!

 

[copper123]

Re: help understanding magnetic induction

Ok, you guys are just showing off now! I really sincerely thank everyone for helping me out. Ok, back to grade school again.
Charlie, I want to make sure I understand what you are talking about.

You say, (The counter-voltage is itself in the second coil. It cannot create an additional voltage within itself. A current -carrying wire will create around itself a magnetic field, but it cannot , cut the lines of that magnetic field. You need relative motion between a conductor and a field, in order to get a counter-emf. There can be no relative motion between a field and the wire that created it.)
I think this is what has been confusing me so much. I have always thought as the magnetic lines of force in the current carrying conductor expand and collapse it creates a voltage in the conductor.
This is what BPHgravity wrote:
When a current flows through a conductor, a magnetic field is created around the conductor. When the voltage source is disconnected to the conductor, the current rapidily drops to zero. In the processs, the magnetic field collapses back into the conductor, inducing a voltage and opposing the change in current. This is the counter-EMF.

My problem may be in the current/voltage issue. Do you see where I might be having the confusion??
So to make sure, as you described Charlie, the induced voltage is in fact the CEMF? When I induce voltage into a coil from a magnetic field the induced voltage is the CEMF

 

[rattus]

Re: help understanding magnetic induction

Hey, I almost missed this one which is one of my favorite topics. Here goes:

Steady state voltage applied to the primary causes a current in said primary. This current creates a magnetic field in the iron, and since we are applying a sine wave, the current and field are constantly changing.

It is the changing magnetic field which induces the back emf in the primary. This back emf is the essence of an inductor.

This same changing magnetic field also induces an emf in the secondary. Secondary current will flow only if a load is applied.

It turns out that the magnetizing effects of the primary and secondary load currents cancel each other and may be ignored.

There is an equation which states that the induced emf is proportional to the rate of change in the magnetic field in the core.

Another neat thing to remember is the "Right hand rule". With a current in a wire, point your right thumb in the direction of (conventional) current and curl your fingers, then the direction of the magnetic field is indicated by your finger tips.

Voltage, potential, emf, whatever you call it, is a an indication of the potential energy difference of the electrons at two points. This energy difference is a requirement for current to flow.

[ December 31, 2004, 12:56 AM: Message edited by: rattus ]