Are you smart enough to you solve mysterys of the electromagnet?

[CasualKilla]

Anybody can tell you that if you wrap some wires around a something then pass a current though it, you will make an electromagnet, but did a single one of you stop to consider what was actually going on?

 

Faraday's Law tells us that is we apply a voltage to this coil, the flux within the coil will increase with time.

 

Consider a voltage being applied directly to a coil, well this should produce a ever increasing strength magnet right? But we know that DC electromagnets create a constant strength magnet. I sure as hell haven't heard of somebody creating some kind of magnetic singularity in their basement.

 

 

So who can tell me why a DC electromagnet creates a constant strength magnet field? I figured out the answer myself, so only reply if you wanna show of your mad intellect.

[John Cuthber]

Resistance is futile.

[swansont]

Anybody can tell you that if you wrap some wires around a something then pass a current though it, you will make an electromagnet, but did a single one of you stop to consider what was actually going on?

 

Faraday's Law tells us that is we apply a voltage to this coil, the flux within the coil will increase with time.

 

Faraday's law tells you that if you change the flux, you get a voltage.

[Enthalpy]

Hey CasualKilla, if you really want to test this forum's members, you need something more difficult. I mean, seriously more difficult.

 

Resistance is futile.

[CasualKilla]

 

Faraday's law tells you that if you change the flux, you get a voltage.

Yes, and also a voltage induces changing flux, so why does the magnet have constant strength, it should be increasing right?

[swansont]

Yes, and also a voltage induces changing flux, so why does the magnet have constant strength, it should be increasing right?

 

A constant voltage can induce zero flux, or a constant flux. It depends on whether the circuit is open or not. A constant current from a voltage gives you a constant flux.

[CasualKilla]

 

A constant voltage can induce zero flux, or a constant flux. It depends on whether the circuit is open or not. A constant current from a voltage gives you a constant flux.

YES and NO!!

 

Yes you will need a closed conductive circuit, but change in flux depends only on the voltage across the coil. Faraday's Law always holds.

 

Actually not sure if you even need a closed circuit, but good luck applying a voltage to the coils without one.

 

Edit: Wait I think you are wrong, I don't think you need current at all.

Edited March 22, 2015 by CasualKilla

[John Cuthber]

If you have a perfectly conducting coil and a perfect power supply with zero output impedance then you don't get a constant magnetic field- it keeps rising forever.

But what actually happens is this

http://en.wikipedia.org/wiki/RL_circuit#Current

And the field varies with the current.

 

So, the "problem" doesn't exist in the real word- even with superconductors the "magic" breaks down when the field strength get high enough.

As for "But we know that DC electromagnets create a constant strength magnet.".

Well we know that because all electromagnets have a finite resistance.

[CasualKilla]

If you have a perfectly conducting coil and a perfect power supply with zero output impedance then you don't get a constant magnetic field- it keeps rising forever.

But what actually happens is this

http://en.wikipedia.org/wiki/RL_circuit#Current

And the field varies with the current.

 

So, the "problem" doesn't exist in the real word- even with superconductors the "magic" breaks down when the field strength get high enough.

As for "But we know that DC electromagnets create a constant strength magnet.".

Well we know that because all electromagnets have a finite resistance.

Correct, well done sir.

[swansont]

 

Yes you will need a closed conductive circuit, but change in flux depends only on the voltage across the coil. Faraday's Law always holds.

 

 

Only when applied properly. A changing flux induces a voltage. But the reverse is not true. A constant voltage giving a constant current produces a constant magnetic field, which you can see from Ampere's law.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

[CasualKilla]

 

Only when applied properly. A changing flux induces a voltage. But the reverse is not true. A constant voltage giving a constant current produces a constant magnetic field, which you can see from Ampere's law.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

That is just not right. A constant current produces a constant magnetic field and a constant voltage produces a changing magnetic field. In the case of a super conductor, you can produce a constant magnetic field with zero voltage.

 

Poop, I think your right about the voltage though.. Since the closed loop integral of an inductor circuit is 0, I do not think it will produce changing flux. Conservative field and so on

One thing still confuses me though, why does a constant current around a solenoid produce a constant magnetic field inside it, apllying a constant magnetic field inside the solenoid does not produce a constant current.

 

Makes sense from a practical point of view since we could just get free energy from wrapping coils around magnets, but don;t know how to reconcile it with amperes law.

Edited March 22, 2015 by CasualKilla

[swansont]

One thing still confuses me though, why does a constant current around a solenoid produce a constant magnetic field inside it, apllying a constant magnetic field inside the solenoid does not produce a constant current.

Because you need a changing field to produce a current, which is what Faraday's law tells us. You have to remember that the V in that law is the induced voltage, not a voltage already present in the circuit.

[Enthalpy]

CasualKilla, the puzzles you submit here correspond to what every electronics student must know - not to what electromagnetics experts may ignore.

 

If you're willing to test the knowledge of forum members, add 40dB difficulty.

[CasualKilla]

 

Because you need a changing field to produce a current, which is what Faraday's law tells us. You have to remember that the V in that law is the induced voltage, not a voltage already present in the circuit.

I get that now since the induced EMF breaks Kirchhoff law, but what about Ampere's law, it seems to indicate that a constant current corresponds to a constant H field, or that also "induced H field" in this case?

[swansont]

I get that now since the induced EMF breaks Kirchhoff law, but what about Ampere's law, it seems to indicate that a constant current corresponds to a constant H field, or that also "induced H field" in this case?

 

Right. That's the H that's created by the current.

[CasualKilla]

 

Right. That's the H that's created by the current.

I think i finally get it now. Some doofus on youtube was going on about symmetry in Maxwell's equations, but I don't see it. Statements like "if a wire a constant current inside it, it has a constant magnetic field around it, similarly if a wire has a constant magnetic field around it, it must have a constant current inside it" are simply untrue, and which lead to my intuitive misunderstanding.

Edited March 23, 2015 by CasualKilla

[studiot]

Note the difference between the terms constant and steady.

 

 

Edit in the light of your previous thread about bar magnets you might find this extract from an old text interesting.

 

Note I have indicated statement (b) about the field lines.

 

Edited March 23, 2015 by studiot

[swansont]

I think i finally get it now. Some doofus on youtube was going on about symmetry in Maxwell's equations, but I don't see it. Statements like "if a wire a constant current inside it, it has a constant magnetic field around it, similarly if a wire has a constant magnetic field around it, it must have a constant current inside it" are simply untrue, and which lead to my intuitive misunderstanding.

 

What's untrue about that statement? If I know the source is a current and I measure a constant field, I can conclude a constant current. The problem is incorrectly inferring that the field is the source of the current, rather than the other way around.

[Enthalpy]

JC told "resistance", why look elsewhere?

[CasualKilla]

Note the difference between the terms constant and steady.

 

 

Edit in the light of your previous thread about bar magnets you might find this extract from an old text interesting.

 

Note I have indicated statement (b) about the field lines.

 

farady1.jpg

That is indeed very interesting, it helps explain the behaviors of those iron filings. I would still love to see the behavior of iron filing in a 0 g environment.

 

I still think your explanation of the like pole repulsion makes sense, the more Iook at iron filing structures the more it makes sense. it explains why some cluster are thinker than other like below.

 

 

 

 

What's untrue about that statement? If I know the source is a current and I measure a constant field, I can conclude a constant current. The problem is incorrectly inferring that the field is the source of the current, rather than the other way around.

 

But it is somewhat misleading and redundant, since if I measure a constant field around the conductor I cannot conclude anything other than no varying current inside the conductor. I still have no idea how much current, or if there is any at all..

Edited March 23, 2015 by CasualKilla

[swansont]

But it is somewhat misleading and redundant, since if I measure a constant field around the conductor I cannot conclude anything other than no varying current inside the conductor. I still have no idea how much current, or if there is any at all..

The field strength drops of as 1/r from a wire. The field at a known distance tells you the current in the wire.

[CasualKilla]

The field strength drops of as 1/r from a wire. The field at a known distance tells you the current in the wire.

No, the magnetic field (or a portion of it) could be applied by an external magnet, you do not know for a fact that there is current in the wire.

[swansont]

No, the magnetic field (or a portion of it) could be applied by an external magnet, you do not know for a fact that there is current in the wire.

Sure, if you change the conditions of the experiment, then things will be different.

 

If I know the source is a current and I measure a constant field, I can conclude a constant current. The problem is incorrectly inferring that the field is the source of the current, rather than the other way around.

 

But an external magnet will not give you a radially-varying field around the wire, and in the shape of a circle around the wire. Given enough information, you could still discern the current.

[CasualKilla]

Sure, if you change the conditions of the experiment, then things will be different.

 

 

But an external magnet will not give you a radially-varying field around the wire, and in the shape of a circle around the wire. Given enough information, you could still discern the current.

Ok, but it is still possible to apply a field that mimics that created by a current exactly, which would not induce a current, but I see your point now, it is still useful and possible to use the law in reverse, I was just a bit butthurt that it didn't work like I originally thought.

 

I have an intersting question, is I have an varying current that produces B(r,t) outside the wire, then I apply an external field around the wire that produces exactly -B(r,t), what will be the current in the wire ie. 0, constant, AC current etc.

 

Facepalm, I already said I have a current a producing B(r,t) so, but the -B(r,t) will produce an EMF right? how does that effect the system?

Edited March 24, 2015 by CasualKilla

[swansont]

Ok, but it is still possible to apply a field that mimics that created by a current exactly, which would not induce a current, but I see your point now, it is still useful and possible to use the law in reverse, I was just a bit butthurt that it didn't work like I originally thought.

 

I have an intersting question, is I have an varying current that produces B(r,t) outside the wire, then I apply an external field around the wire that produces exactly -B(r,t), what will be the current in the wire ie. 0, constant, AC current etc.

 

Facepalm, I already said I have a current a producing B(r,t) so, but the -B(r,t) will produce an EMF right? how does that effect the system?

 

It should cancel, but the only way to produce that field is to run a canceling current down the wire, so that's really the trivial solution.