0

A simple experiment. Make an electric magnet as strong as a regular refrigerator magnet such that either can affix to the side of a refrigerator and both hold an equal mass with any required dead-weight.

  • Measure the voltage needed to keep the electric magnet attached.
  • Measure the ? needed to keep the permanent magnet attached.

Where and what is that question mark?

This is clearly not a "stable" system since if you turn off the electric magnet, stuff is going to fall.

IMarvinTPA
  • 1
  • 3
    The issue has to do with the fact it doesn't actually take any power to constantly hold a mass up - it only takes work to move it. We could add a third means of holding up a mass to your experiment - add a small table that the mass sits on top of, and this also won't need any power source. – jacob1729 Feb 27 '19 at 23:09
  • Related (possible duplicate?): https://physics.stackexchange.com/questions/1984/why-does-holding-something-up-cost-energy-while-no-work-is-being-done. One of the answers actually considers your question as an analogy. – jacob1729 Feb 27 '19 at 23:12
  • @jacob1729 I was contemplating the table idea as well (but as a nail in the refrigerator holding a weight up), what powers the bond that keeps solids solid? My example is designed to highlight the fact that it is taking energy to hold an electromagnet up while a permanent magnet seems to be free. Both magnets are resisting the force of gravity. – IMarvinTPA Feb 28 '19 at 03:31

2 Answers2

4

The magnetism of permanent magnets comes primarily from the fact that electrons have a quantum-mechanical property called spin. (Despite what this word suggests, they are not actually rotating on an axis like the Earth does.) Any charged particle with spin acts like a tiny magnet. (It is said to have a “magnetic moment”, which measures its amount of magnetism.) In a ferromagnetic material, the spins align and create a significant macroscopic magnetic field.

Electrons cannot stop “spinning”, and it takes no energy to keep them “spinning”. Their spin is just a kind of intrinsic angular momentum that they possess. So permanent magnets don’t need any power source. A ferromagnetic material can lose its macroscopic magnetism if its spins become non-aligned, but an electron can never lose its tiny magnetic moment.

G. Smith
  • 51,534
  • So an electron is a perpetual motion particle? – IMarvinTPA Feb 28 '19 at 04:59
  • I suppose you could put it that way, but physicists wouldn’t use that language. For one thing, quantum spin is not motion. For another, any particle can be a “perpetual motion particle” if you don’t exert a force on it. Newton’s First Law says that a body in uniform motion remains in uniform motion unless a force acts on it. – G. Smith Feb 28 '19 at 05:16
  • 1
    When people talk about “perpetual motion”, they mean systems that you can extract energy out of indefinitely. You can’t extract energy out of an electron’s spin. – G. Smith Feb 28 '19 at 05:26
0

The actual mechanism for how the permanent magnet works is not actually that important here (we'll come back to it though). What is important is the definition of work:

$dW = \vec{F} \cdot d\vec{r}$

If you look at it, you see that whilst to hold up an object does require a force to balance gravity, it does not take any work because the object isn't moving. Ultimately this is the answer to your question: it's just not required that energy be expended to supply a constant force and its not a paradox if this happens.

OP raises a good point that if we used an electromagnet to supply the force we would need to continually provide energy to it. A permanent magnet on the other hand would not. A similar situation is that if a human held the object they would get tired, but if we simply hung the object from a string or placed it on a table no more energy needs to be supplied. This simply reflects that some ways of generating constant forces are not energy efficient. An electromagnet only functions whilst there is a current flowing and you will get continuous Joule heating because of this. A human's muscles cannot stay contracted for long periods of time and lose energy in cycles of contraction/relaxation. Conversely, the permanent magnet has no such mechanism to lose energy since its magnetism comes from the alignment of fundamental magnetic dipoles that cannot be demagnetised, only realigned and so can produce a continuous force without needing an energy supply.

jacob1729
  • 4,556