The energy in magnetic fields. - Page 1
Forum section: electronics / general

author: corrado33
author_rank: Frequent Contributor
timestamp: February 23, 2016, 04:19:42 pm
content: I know perpetual motion machines don't work. I'm not an idiot. My question is this.If magnets don't contain energy, the magnetic field contains energy, where does the energy come from to sustain that field?For example if you set two round magnets on top of each other (on a pole), opposite polarity, the magnets will stay separated, indefinitely.Where does the energy for that come from? The top magnet is opposing the force of gravity, there has to be a force associated with that. Forces require energy, right? I really should know this, but I can't find good information on it (heck the first site google comes up with is a perpetual motion site where someone says "Well GOD shows us of frictionless motion every day when we look at atoms...")Any insight?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 23, 2016, 04:28:44 pm
content: Quote from: corrado33 on February 23, 2016, 04:19:42 pmNo, force does not require energy. You know this of course, if you were to take some duct tape and attach that top magnet onto the pole and remove all of the other magnets, it would still stay elevated, but you don't think that duct tape is a free energy contraption do you?Work is force*distance. THAT is what requires energy, not the force itself. Like gravity, the force is just a means of energy storage/extraction, not the energy source itself.

author: corrado33
author_rank: Frequent Contributor
timestamp: February 23, 2016, 04:30:36 pm
content: Found a site that answers the majority of the follow up questions to this question.https://van.physics.illinois.edu/qa/listing.php?id=432

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: February 23, 2016, 05:02:13 pm
content: The energy stored in the magnetic field is put there when it if first magnetised. An external strong magnetic field does work when it establishes the permanent magnet's field.Like how the spring in a mousetrap contains stored energy from when you set it.

author: zapta
author_rank: Super Contributor
timestamp: February 23, 2016, 05:08:08 pm
content: Quote from: DenzilPenberthy on February 23, 2016, 05:02:13 pmWhat happens when you dissolve a compressed spring in acid? Will it get hotter? Will the end solution be different?Edit: or getting rust over time.

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2016, 05:29:22 pm
content: Quote from: corrado33 on February 23, 2016, 04:19:42 pmI think you may have the answer to this now, but a variation on this question is "If the magnet is placed on top of a table, it will stay there forever. The table is opposing the force of gravity and preventing the magnet falling to the ground, so where is the energy for that coming from?"The actual difference between the two cases is only the type of field involved. The forces in the table come from electric fields and the forces between the magnets come from magnetic fields. The only reason the magnet case seems mysterious is that the distances involved are much larger.

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2016, 05:31:33 pm
content: Quote from: zapta on February 23, 2016, 05:08:08 pmYes it will, but the difference in temperature will be too small to measure. (You can work it out from the mechanical energy stored in the compressed spring and the heat capacity of the acid solution.)

author: BravoV
author_rank: Super Contributor
timestamp: February 23, 2016, 05:45:36 pm
content: Quote from: IanB on February 23, 2016, 05:31:33 pmI guess in reality, once a single point of the spring is too corroded, that weak point will break the spring into two pieces and releasing the mechanical energy.

author: firewalker
author_rank: Super Contributor
timestamp: February 23, 2016, 05:53:40 pm
content: The nice thing with a compressed spring, is that it weights more than the same spring uncompressed!Alexander.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: February 24, 2016, 12:40:10 am
content: that is an interesting question, the effect of strain on corrosion.it seems the answer is very not simplehttps://www.google.com/search?q=effects+of+strain+on+corrosionsearch relativistic mass also

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: February 24, 2016, 04:55:16 pm
content: Quote from: zapta on February 23, 2016, 05:08:08 pmOh that's a really interesting idea for an experiment!I work in a university physics department so I will be asking around some colleagues to find out if this has been done or would even be measurable. It must end up as heat though, surely.Likewise, I wonder about dissolving a magnetised permanent magnet vs an unmagnetised one.

author: zapta
author_rank: Super Contributor
timestamp: February 24, 2016, 06:34:38 pm
content: Quote from: DenzilPenberthy on February 24, 2016, 04:55:16 pmIf you dissolve two opposing magnets that held together, there is a potential energy that's needs to go away. May be same mechanism as the strained metal mentioned above.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2016, 07:43:53 pm
content: Yes, the magnets store energy. And yes, they each store enough to support their weight. Clearly!Force alone is separate from energy. Fortunately, you have displacement evident in the picture: the magnets bearing more weight are compressed further, indicating their fields are acting as springs, and thus \$ \int \mathbf{F} \cdot d\mathbf{x} \$ is a nonzero amount, and a greater magnitude for each magnet going down. (It's not simply \$\mathbf{F} \cdot \mathbf{x}\$, because the force is not linear with distance.)Indeed, this energy is directly present in each magnet. The magnetic field stores energy as \$ e = \frac{\mathbf{B \cdot B}}{2 \mu} \$, where \$\mu\$ is the permeability of space and \$e\$ is the energy density at the same point as the other quantities (to find the total energy, integrate over all of space; this includes if \$\mu\$ varies because of surrounding materials like pole pieces, and the \$\mathbf{B}\$ field inside those materials; fortunately the magnet material itself usually has a low relative permeability, so this isn't automatically made harder).The magnetic energy is stored during manufacture, using a strong pulsed field.The magnetic field is conservative, meaning it acts like a spring. You perform work against the field stacking the magnets, but that work is recovered when you unstack them. This is how PM motors work (or any motor if you include electromagnets, and transformers as well when Faraday's law is considered). You never discharge the magnetic energy, and you wouldn't get much if you did, anyway (if you like, you can heat the magnet to its Curie temperature and perhaps measure a very small spike in temperature, or collect the energy electrically with a coil).Tim

author: zapta
author_rank: Super Contributor
timestamp: February 24, 2016, 08:03:20 pm
content: If I climb a pole and accumulate potential energy, were is it 'stored'?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2016, 08:44:57 pm
content: Quote from: zapta on February 24, 2016, 08:03:20 pmIn you? In the gravitational field (I assume you mean to climb against your weight on Earth)?Energy is more of a state, than a "thing" that gets "stored" somewhere. Where do these letters get stored in your computer? Well in memory okay, but what makes these letters different from any other information that might be in that memory? It's just a state.Tim

author: TimFox
author_rank: Super Contributor
timestamp: February 24, 2016, 10:01:37 pm
content: Quote from: zapta on February 24, 2016, 08:03:20 pmThe energy, wherever it was "stored", will manifest itself when as kinetic energy when you fall from the pole.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 24, 2016, 10:39:51 pm
content: Quote from: zapta on February 24, 2016, 08:03:20 pmIt's stored in the separation between the center of mass in your body and the center of mass of the Earth.As said before, potential energy is really just a state. It describes how much energy must be expended or will be recovered when that state changes.