Magnet Myths and Misconceptions

[MileHigh]

Tinman:

From the discussions that have taken place in this thread, is it safe to assume that you are getting the concept that the magnetic pull force attraction on a small iron test piece and the strength of the magnetic field itself are two separate things?  I am assuming that you either get that or are coming around to start to understand that.

So, this is going to be my "Revenge of the Nerds - Bench Version" posting discussing more interesting stuff that you can do on your bench.

First of all, a quick recap of measuring the magnetic field strength of the coils using a hypothetical example to illustrate the process.  We are assuming that you have two identical coils, one made with copper wire and the other one made with iron wire.  The assumption is that the copper wire coil produces a stronger external magnetic field for the same current flow because there is no field trapped inside the wire like you have for the iron wire case.

Very importantly, I am going to assume that you have a digital voltmeter with at least three digits of precision.  If you don't have one with three digits of precision then you can't do this test.

So you have your coil under test in the magnetic field strength measurement setup using the compass as was previously described.  This is all set up at the center of a wooden table.   Between your power supply and the coil you have about two feet of interconnect wire.  The wire is a tight twisted pair and like Mark says, you have at least three twists per inch.   You have a current sensing resistor next to the bench power supply.

So for the copper coil, let's say you measure 0.873 volts across the current sensing resistor to make the compass needle deflect exactly 45 degrees.

Now you swap out the copper coil for the iron coil.  You dial up your power supply voltage to see 0.873 volts across the current sensing resistor.  You look at the compass needle and this time it is deflecting about 42 degrees.

So that is telling you that the copper coil is producing a stronger external magnetic field than the iron coil for the same current flow.  This makes sense because the assumption is that the iron wire itself is "stealing" some of the magnetic flux that would normally be in the 3D space around the coil.

Next post will be the "Nerds Part 2."

MileHigh

[verpies]

When we supply a DC current to a solenoid,is any of that input power used/consumed to create the magnetic field around that solenoid?

Yes.  In an ideal solenoid all of the input energy is used to create the magnetic field around that solenoid, and in a resistive solenoid some of the input energy is used to create the magnetic field.  See here.

This means that it takes no power to create and maintain the actual magnetic field,as all power in is dissipated as heat.

No, it takes energy to create a magnetic field even in an ideal coil.
Also, not all of the energy delivered by the power supply is dissipated as heat in the resistance.  See here.

2- Once the magnetic field around that inductor is established, does it require any of the input power to that inductor to maintain that field, or is it all dissipated as heat?

If the inductor is ideal, then no energy input to that inductor is required, in order to maintain its field.
If the inductor is resistive then yes, energy input is required to make up for the energy dissipated in the resistance as heat.
Resistance constitutes an energy leak in an inductor's circuit.

[MileHigh]

Revenge of the Nerds - Bench Edition, Part 2:

I think you mentioned in your younger days you had a muscle car.  There is a good chance you added an instrument cluster that included real analog gages: water temperature, oil temperature, oil pressure, battery voltage, and battery charge/discharge amps.

In North America we call the simple indicator lights that do these functions "idiot lights" and I assume that it's the same in Australia.

So, many experimenters will buy a digital inductance meter.  How did people survive before digital inductance meters?  We are going to call the digital inductance meter an "idiot box."   So for this test there will be no inductance meter idiot box.

The assumption is that the iron wire coil has a greater inductance than the copper coil.  It stands to reason because the iron wire in many ways is simply acting like a ferrite core, where the ferrite core and the wire are one in the same thing.   And it's also interesting because at first glance it seems counter-intuitive - the coil with the larger inductance has the smaller strength of external magnetic field.  Normally it's not the case - a larger inductor will have a stronger external magnetic field.

So here is the nerd revenge challenge:  Without using the idiot box, measure the inductance for the copper coil and the iron coil and see it it is indeed true that the iron coil has the larger inductance.

If you or anybody takes up the challenge, you are on your own.  I am not helping at all.  Hey, it's up to other people if they want to help anybody that takes up the challenge.  Or perhaps a few experimenters will work together and brainstorm and try to come up with a procedure to make the measurement all by themselves.

One thing I can tell you is it is certainly doable, and there are probably(?) multiple ways to do it.

You like to bash the book nerds and say "It's all real on the bench and screw your egghead books?"  (I am saying this in jest.)  Okay then go for it and show us your stuff.  On your bench with your standard bench instruments and with your wits and possibly with help from your peers or even a nice nerd, measure the inductance of your two coils.  Put your money where your mouth is and make it real on the bench.  The bench is where it counts...

You are not going to get any hints from me!  Not even bread crumbs!

MileHigh

P.S.:  On a more serious note if you actually took up the challenge and got your results you will learn 10 times more than you would learn on yet another thread about building yet another pulse motor configuration.

[MarkE]

1-OK,so you agree that it takes energy to create a magnetic field?

Yes

2-And do you agree that when the power to the inductor is abruptly disconected,the magnetic field gives back the power it took to create it?-the rest of the power of course was disipated as heat.

"Gives back" is a bit ambiguous.  The energy in the magnetic field is not destroyed.  If the circuit is not arranged to reclaim most of the energy, then most will be dissipated as heat when trying to establish the voltage that will ultimately be across the disconnect, and some will radiate away.

3- do you believe this to be true in all situation's-Quote: In physics, the law of conservation of energy states that the total energy of an isolated system remains constant—it is said to be conserved over time. Energy can be neither created nor be destroyed, but it can change form,

Conservation of mass/energy is very fundamental.

[MileHigh]

2-And do you agree that when the power to the inductor is abruptly disconected,the magnetic field gives back the power it took to create it?-the rest of the power of course was disipated as heat.

My take on it is this:  First of all it's "give back the energy" not "give back the power."  It's very important here to draw this distinction.

You are basically talking about the so-called "back-EMF spike" in a Bedini motor here.  I will call it a pulse of current associated with the inductor discharging its stored energy.

To say that the back-EMF spike is "pure voltage with almost no current" like you know who, so-called "radiant energy," is basically an insane statement.  By definition it's a pulse of current.