7 ms·

Even on the macro level there are lots of tricky questions. A bar magnet is surrounded by magnetic flux lines. Apparently it takes no energy to maintain that m

Even on the macro level there are lots of tricky questions. A bar magnet is surrounded by magnetic flux lines. Apparently it takes no energy to maintain that magnetic field. If I heat the bar in boiling water for long enough, it becomes demagnetized. On the macro level there's no visible/measureable change in the bar. We need a lot of magnification to see that the 'particles are no longer aligned'. On the other hand, I move the bar magnet over a pile of iron filings, some of the filings will fly up against gravity to the bar, against the force of gravity. It takes energy to lift these filings. Where does that energy come from? (It never seems to be exhausted.)

pshc 5y ago
The energy comes from the incredible magnetic force spent to align the atoms when the magnet was created, and it will be slowly exhausted over time.
- esteth 5y ago
  Why would it be exhausted over time? When things come close to the magnet it uses its potential energy to pull them. When some force acts to remove those objects from the magnetic field, that energy is converted into potential in the magnet again. Are you claiming that the alignment of the magnet will fade slowly over time when the field is interacted with, more quickly than it would if it were left on a proverbial shelf for millions of years?
  - pshc 5y ago
    Yeah, that was my claim, that the atoms will slowly return to a chaotic high-entropy organization over time, and more rapidly as forces are exerted against the magnet. I guess I was wrong.
    - hither_shores 5y ago
      > that the atoms will slowly return to a chaotic high-entropy organization over time, and more rapidly as forces are exerted against the magnet This is true, it's just not why magnets can lift things.
- drdeca 5y ago
  Force isn't a resource which is spent.
BobbyJo 5y ago
> It takes energy to lift these filings. Where does that energy come from? (It never seems to be exhausted.) That's potential energy. When you made the magnet, you created a potential between everything else and the magnet. When you unmake the magnet, you change the potential again in the other direction. Changing said potential consumes energy. The potential can be exhausted as only so much mass/magnetic-potential can fit within the magnets field. To use the field with more stuff, you have to pull some stuff out of the field, which takes energy.
aaaaaaaaaaab 5y ago
Huh? Iron filings fly up because of the principle of minimum potential energy. They can lose potential energy by moving closer to the magnet, so they do. No energy is gained - potential energy is converted to kinetic energy, then heat when they hit the magnet.
- xyzzyz 5y ago
  Your answer is basically a tautology: if we observe work being performed, there must be some potential energy being converted, that’s what the potential energy is; the ability to perform work. The real question rather is: why doesn’t the same thing happen when I get a stick close to wood shavings? How is energy stored in a magnet in a way that is not stored in a stick?
  - kurthr 5y ago
    Because stick field is not a thing? Well, I guess there's gravitational fields, but those are not dipoles and they're really weak. Magnetic fields of large aggregates aren't simple to model, and there are some very complex systems, but individually they are very simple and well modeled.
  - AnimalMuppet 5y ago
    Because sticks don't have magnetic fields, and there isn't a "stick field" that has potential energy. And why is there a magnetic field? Basically it's a consequence of the electric field plus relativity.
tkluck 5y ago
> some of the filings will fly up against gravity to the bar, against the force of gravity. It takes energy to lift these filings. Where does that energy come from? In this example the energy comes from whatever is holding the bar magnet itself up against gravity. If it's not doing any work (ie. it's not adding energy; say it's a rope or a spring) then it's not just the filings that move: the bar magnet and the filings move towards each other (with the filings moving the bulk of the distance) and the end result is that some of the gravitational energy in the bar magnet is transferred to the filings. I think this only seems like a tricky question with an informal idea of what "energy" is. It doesn't have the same interpretation problem as what the linked article is about.
spiantino 5y ago
This is a good question to bring to /r/askphysics . It's pretty simple, and isn't the kind of problem with quantum mechanics that the article is referring to.
XorNot 5y ago
Nobody ever asks this question about a spring supporting a mass against gravity. There's nothing actually holding atoms together, it's all electric fields. So why is a magnet different? (it's not). People also don't ever ask this question about gravity itself.
akomtu 5y ago
I've been bothering myself with the same question. This rabbit hole in the wikipedia has led me to a dead-end explanation: those iron filings are pulled by virtual photons created by the magnet, those virtual photons are completely virtual and cannot be observed, except for the fact that they somehow pull the filings despite being virtual (this must be a small incursion of the purely mathematical world into reality). I've concluded that magnetism is an unexplained phenomenon, just like gravity: we have accurate formulas, but nobody knows what those formulas describe. For intuitive understanding, I see a magnet as a well, except that this well can be moved around and has two poles. Just like a regular well, things fall into it at no expense to the well, because the well isn't really a "thing" that pulls other things by force.
- hither_shores 5y ago
  > This rabbit hole in the wikipedia has led me to a dead-end explanation Trying to teach yourself physics by reading wikipedia is like trying to learn how to program by reading the C++ standard. It's a fine resource, but not for that. You need an introductory physics textbook, not an encyclopedia. Griffith's Introduction to Electrodynamics is a classic choice. > those iron filings are pulled by virtual photons created by the magnet No. Virtual particles are part of a notational scheme for carrying out certain computations in quantum field theory. They're not particles, and they're definitely not involved in classical electromagnetism. > I've concluded that magnetism is an unexplained phenomenon, just like gravity: we have accurate formulas, but nobody knows what those formulas describe. The formulas, ultimately, describe the relationship between our observations. This is the only sense in which we ever explain anything, and a sense in which magnetism and gravity are both very well-explained. > For intuitive understanding, I see a magnet as a well, except that this well can be moved around and has two poles. You should not try to understand electromagnetism intuitively yet. You haven't built the right intuitions, and to do that you first need to tear down your wrong ones.
- l33tman 5y ago
  By that logic you can't observe anything or explain anything, though. All the force carrying bosons (like the photon) have to interact with fermions (the electrons and quarks that make up your equipment and your body). Some of the interactions happen due to on-shell photons ("light") but most happen due to completely off-shell photons (EM force). They are really the same interaction. Don't get hung up on the "virtual" in the lingo about particles. All particles are more or less "virtual" depending on how you analyze the problem. I'm sorry there is not a more intuitive explanation of this.. The underlying QFT building blocks are so far from classical physics. Even concepts like energy and momentum are in some sense emergent properties that lose most of their classical intuition "at the bottom", this is reflected by the Heisenberg uncertainty relationship for example.

pshc 5y ago

The energy comes from the incredible magnetic force spent to align the atoms when the magnet was created, and it will be slowly exhausted over time.

esteth 5y ago
Why would it be exhausted over time? When things come close to the magnet it uses its potential energy to pull them. When some force acts to remove those objects from the magnetic field, that energy is converted into potential in the magnet again. Are you claiming that the alignment of the magnet will fade slowly over time when the field is interacted with, more quickly than it would if it were left on a proverbial shelf for millions of years?
- pshc 5y ago
  Yeah, that was my claim, that the atoms will slowly return to a chaotic high-entropy organization over time, and more rapidly as forces are exerted against the magnet. I guess I was wrong.
  - hither_shores 5y ago
    > that the atoms will slowly return to a chaotic high-entropy organization over time, and more rapidly as forces are exerted against the magnet This is true, it's just not why magnets can lift things.
drdeca 5y ago
Force isn't a resource which is spent.

BobbyJo 5y ago

> It takes energy to lift these filings. Where does that energy come from? (It never seems to be exhausted.) That's potential energy. When you made the magnet, you created a potential between everything else and the magnet. When you unmake the magnet, you change the potential again in the other direction. Changing said potential consumes energy. The potential can be exhausted as only so much mass/magnetic-potential can fit within the magnets field. To use the field with more stuff, you have to pull some stuff out of the field, which takes energy.

aaaaaaaaaaab 5y ago

Huh? Iron filings fly up because of the principle of minimum potential energy. They can lose potential energy by moving closer to the magnet, so they do. No energy is gained - potential energy is converted to kinetic energy, then heat when they hit the magnet.

xyzzyz 5y ago
Your answer is basically a tautology: if we observe work being performed, there must be some potential energy being converted, that’s what the potential energy is; the ability to perform work. The real question rather is: why doesn’t the same thing happen when I get a stick close to wood shavings? How is energy stored in a magnet in a way that is not stored in a stick?
- kurthr 5y ago
  Because stick field is not a thing? Well, I guess there's gravitational fields, but those are not dipoles and they're really weak. Magnetic fields of large aggregates aren't simple to model, and there are some very complex systems, but individually they are very simple and well modeled.
- AnimalMuppet 5y ago
  Because sticks don't have magnetic fields, and there isn't a "stick field" that has potential energy. And why is there a magnetic field? Basically it's a consequence of the electric field plus relativity.

tkluck 5y ago

> some of the filings will fly up against gravity to the bar, against the force of gravity. It takes energy to lift these filings. Where does that energy come from? In this example the energy comes from whatever is holding the bar magnet itself up against gravity. If it's not doing any work (ie. it's not adding energy; say it's a rope or a spring) then it's not just the filings that move: the bar magnet and the filings move towards each other (with the filings moving the bulk of the distance) and the end result is that some of the gravitational energy in the bar magnet is transferred to the filings. I think this only seems like a tricky question with an informal idea of what "energy" is. It doesn't have the same interpretation problem as what the linked article is about.

spiantino 5y ago

This is a good question to bring to /r/askphysics . It's pretty simple, and isn't the kind of problem with quantum mechanics that the article is referring to.

XorNot 5y ago

Nobody ever asks this question about a spring supporting a mass against gravity. There's nothing actually holding atoms together, it's all electric fields. So why is a magnet different? (it's not). People also don't ever ask this question about gravity itself.

akomtu 5y ago

I've been bothering myself with the same question. This rabbit hole in the wikipedia has led me to a dead-end explanation: those iron filings are pulled by virtual photons created by the magnet, those virtual photons are completely virtual and cannot be observed, except for the fact that they somehow pull the filings despite being virtual (this must be a small incursion of the purely mathematical world into reality). I've concluded that magnetism is an unexplained phenomenon, just like gravity: we have accurate formulas, but nobody knows what those formulas describe. For intuitive understanding, I see a magnet as a well, except that this well can be moved around and has two poles. Just like a regular well, things fall into it at no expense to the well, because the well isn't really a "thing" that pulls other things by force.

hither_shores 5y ago
> This rabbit hole in the wikipedia has led me to a dead-end explanation Trying to teach yourself physics by reading wikipedia is like trying to learn how to program by reading the C++ standard. It's a fine resource, but not for that. You need an introductory physics textbook, not an encyclopedia. Griffith's Introduction to Electrodynamics is a classic choice. > those iron filings are pulled by virtual photons created by the magnet No. Virtual particles are part of a notational scheme for carrying out certain computations in quantum field theory. They're not particles, and they're definitely not involved in classical electromagnetism. > I've concluded that magnetism is an unexplained phenomenon, just like gravity: we have accurate formulas, but nobody knows what those formulas describe. The formulas, ultimately, describe the relationship between our observations. This is the only sense in which we ever explain anything, and a sense in which magnetism and gravity are both very well-explained. > For intuitive understanding, I see a magnet as a well, except that this well can be moved around and has two poles. You should not try to understand electromagnetism intuitively yet. You haven't built the right intuitions, and to do that you first need to tear down your wrong ones.
l33tman 5y ago
By that logic you can't observe anything or explain anything, though. All the force carrying bosons (like the photon) have to interact with fermions (the electrons and quarks that make up your equipment and your body). Some of the interactions happen due to on-shell photons ("light") but most happen due to completely off-shell photons (EM force). They are really the same interaction. Don't get hung up on the "virtual" in the lingo about particles. All particles are more or less "virtual" depending on how you analyze the problem. I'm sorry there is not a more intuitive explanation of this.. The underlying QFT building blocks are so far from classical physics. Even concepts like energy and momentum are in some sense emergent properties that lose most of their classical intuition "at the bottom", this is reflected by the Heisenberg uncertainty relationship for example.