Remember Me?

 Straight Dope Message Board Remember Me?

#1
08-05-2004, 05:46 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
Where does the energy of magnets come from?

I was playing around with two magnets when I realized that I don't have a clue where the repelling and attracting energy comes from. When you place two magnets close to each other they'll repel (if they're placed south-to-south or north-to-north) or attract (if they're placed north-to-south) each other. What energy causes the magnets to move as they slide across the table towards each other? What energy causes the magnets to resist my attempts to push them together?

More importantly, where is this energy stored? It cannot come from nowhere and there cannot be an infinite amount of it, so sooner or later every magnet must run dry, right? Why haven't I ever heard of or encountered an "empty" magnet?

I feel silly even having to ask this, but I honestly don't know.
#2
08-05-2004, 06:21 AM
 Garfield226 Guest Join Date: Jun 1999 Posts: 4,605
IANAQP, but...

Basically, it seems as if the atomic structure of magnetic element emulates an electromagnet. In other words, each iron atom is a tiny little electromagnet. When all the little tiny electromagnets' poles line up, the whole chunk of iron turns into a magnet. Electromagnetism is one of the four basic forces.

See this Wikipedia article as well:
Quote:
 Originally Posted by Wikipedia The physical cause of the magnetism of objects, distinct from electrical currents, is the atomic magnetic dipole. Magnetic dipoles, or magnetic moments, result on the atomic scale due to the two kinds of movement of electrons. The first is the orbital motion of the electron around the nucleus; this can be considered as a current loop, resulting in a magnetic moment along the axis of the nucleus. The second, much stronger, source of electronic magnetic moment is due to a quantum mechanical property called spin (although current quantum mechanical theory states that electrons do not actually physically spin, or orbit the nucleus for that matter). The overall magnetic moment of the atom is the net sum of all of the magnetic moments of the individual electrons. Because of the tendency of magnetic dipoles to oppose each other to reduce the net energy, in an atom the opposing magnetic moments of some pairs of electrons will cancel each other, both in orbital motion and in spin magnetic moments. Thus, in the case of an atom with a completely filled electron shell or subshell, the magnetic moments normally completely cancel each other out and only atoms with partially filled electron shells will have a magnetic moment, whose strength depends on the number of unpaired electrons. The differences in configuration of the electrons in various elements thus determine the nature and magnitude of the atomic magnetic moments, which in turn determine the differing magnetic properties of various materials.
So, to answer your questions, the energy is "stored" in the movement of the electrons in the individual atoms, and thus, as long as the electrons keep doing whatever it is they do, the magnet won't "run dry."

(That last part referring to the magnetism of an individual atom. The magnetism of a chunk of a particular metal can be made to go away by making some of the groups of atoms "point" one way, and some the other, thus "cancelling out" the magnetism of the entire chunk. You can do this by heating the metal, pounding it with a hammer, or stroking it randomly with another magnet.)

To understand it deeper, I think you'd have to get into Quantum Mechanics and Maxwell's equations.
__________________
"·BraheS· I object to your classifying tuna as fake meat. They're the cows of the sea, except for "sea cows," but manatees are illegal here. | ·GMRyujin· Sorry, fish isn't real meat. Real meat has legs. | ·BraheS· Hell, *I* have legs. Why isn't anyone eating me? | ·GMRyujin· Cause you eat tuna, dude"
#3
08-05-2004, 06:25 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
I know how magnets work. I just don't get where they get their power from.
Quote:
 Originally Posted by Garfield226 So, to answer your questions, the energy is "stored" in the movement of the electrons in the individual atoms, and thus, as long as the electrons keep doing whatever it is they do, the magnet won't "run dry."
But the electrons can't keep doing whatever it is they do forever. If there's no energy input, they must be doing it slower and slower all the time until they don't do it at all.
#4
08-05-2004, 06:36 AM
 Garfield226 Guest Join Date: Jun 1999 Posts: 4,605
Quote:
 Originally Posted by Priceguy I know how magnets work. I just don't get where they get their power from. But the electrons can't keep doing whatever it is they do forever. If there's no energy input, they must be doing it slower and slower all the time until they don't do it at all.
They get their power from the electrons whizzing around. Where the electrons get their power from, I don't know. That, I'd imagine, is where the quantum mechanics comes in.

And I guess maybe they can't do it FOREVER, but they've been doing it since the beginning of the Universe and aren't likely to stop anytime soon. I suppose they might be slowing down. I wonder how we'd detect that.
#5
08-05-2004, 06:38 AM
 alaricthegoth Guest Join Date: Jul 2002 Location: The Barbary Coast Posts: 2,125
Quote:
 Originally Posted by Priceguy I know how magnets work. I just don't get where they get their power from. But the electrons can't keep doing whatever it is they do forever. If there's no energy input, they must be doing it slower and slower all the time until they don't do it at all.
which would be operation of the second law of thermodynamics (I think it's the second) acting out, but it would take longer than you cloud notice with the magnets.

Meanwhile, consider the embarrassing accelerating expansion of the universe.

If you want to worry about kookie perpetual motion schemes that can't possibly work, I urge you to solve that one
#6
08-05-2004, 06:40 AM
 alaricthegoth Guest Join Date: Jul 2002 Location: The Barbary Coast Posts: 2,125
"It's the ripples of a disturbance in The Force, Luke,...."
#7
08-05-2004, 06:44 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
Quote:
 Originally Posted by alaricthegoth which would be operation of the second law of thermodynamics (I think it's the second) acting out, but it would take longer than you cloud notice with the magnets.
So magnets will become progressively weaker and eventually, one day, run dry? It's just that it's going to take a really, really, really, REALLY long time? And also, once that happens, magnets not working will be the least of our problems.
Quote:
 Meanwhile, consider the embarrassing accelerating expansion of the universe.
Which will eventually stop accelerating, right? Same way a bullet still accelerates after leaving the gun barrel but stops doing so after a while.
#8
08-05-2004, 06:47 AM
 alaricthegoth Guest Join Date: Jul 2002 Location: The Barbary Coast Posts: 2,125
Quote:
 Originally Posted by Priceguy So magnets will become progressively weaker and eventually, one day, run dry? It's just that it's going to take a really, really, really, REALLY long time? And also, once that happens, magnets not working will be the least of our problems. Which will eventually stop accelerating, right? Same way a bullet still accelerates after leaving the gun barrel but stops doing so after a while.
uh-uh

that's the embarrassing part..
#9
08-05-2004, 06:50 AM
 alaricthegoth Guest Join Date: Jul 2002 Location: The Barbary Coast Posts: 2,125
Quote:
 Originally Posted by alaricthegoth uh-uh that's the embarrassing part..
the expanision is not only not slowing down, its expanding faster and faster.

also there's the little thing about the speed of light, which we used to think of as rock solid

There were two things true everywhere in the universe:

Money now is better than money later

and

The speed of light is 186,000 miles per second, and there's no fuckin' give or takes.

Well the part about money is still true....
#10
08-05-2004, 06:52 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
Please dump a big cite in my lap.
#11
08-05-2004, 07:04 AM
 Dog80 Guest Join Date: Dec 2003 Location: Athens, Greece Posts: 3,354
Quote:
 Originally Posted by Priceguy So magnets will become progressively weaker and eventually, one day, run dry? It's just that it's going to take a really, really, really, REALLY long time? And also, once that happens, magnets not working will be the least of our problems.
It all has to do with atomic alignment. Each atom is essentially a tiny magnet with a North and a South pole. When all the atoms are aligned, all those South poles face one way and the North poles face the other way. The magnetic force of each atom is combined and the material has magnetic properties.

If the atoms point at random directions, then the magnetic forces cancel each other out and the material has no magnetic properties.

If you touch a magnet on a piece of iron and remove the magnet, the piece of iron will behave as a magnet for a little while. That is because the atoms in that piece of iron were aligned with those of the magnet. As time passes, atoms lose their alignment and assume random positions again. Hardened iron (steel) doesn't lose alignment that easily. I don't know about natural magnets (FeO4 if I'm not mistaken), I think they never lose their alignment.
#12
08-05-2004, 07:04 AM
 alaricthegoth Guest Join Date: Jul 2002 Location: The Barbary Coast Posts: 2,125
expanding accelerating. (0.96 seconds)

Universe Expansion is Accelerating, UK and Australian Researchers ...
Say By Royal Astronomical Society posted: 11:17 am ET 20 March 2002. ...
www.space.com/scienceastronomy/ astronomy/universe_expansion_020320.html - 37k - Aug 4, 2004 - Cached - Similar pages

Bigger Faster: Universe Expansion May Be Accelerating
... The notion of the expanding universe is related to a mathematical quantity Einstein
devised called the ... If the universe is accelerating, however, it means ...
www.space.com/scienceastronomy/ solarsystem/accelerating.html - 30k - Aug 3, 2004 - Cached - Similar pages
[ More results from www.space.com ]CNN.com - Chandra unlocking mystery of 'dark energy' - May 18 ...
The Chandra Space Telescope has gathered further evidence the universe is expanding
at an accelerating rate, scientists at NASA and Britain's Institute of ...
www.cnn.com/2004/TECH/space/05/18/dark.energy/ - 43k - Aug 3, 2004 - Cached - Similar pages

en.wikipedia.org/wiki/Accelerating_universe
Similar pages

Nifty50:ACCELERATING, EXPANDING UNIVERSE
ACCELERATING, EXPANDING UNIVERSE. Two independent teams of astronomers, both with
NSF support, have concluded the expansion of the universe is accelerating. ...
www.nsf.gov/od/lpa/nsf50/nsfoutreach/ htm/n50_z2/pages_z3/01_pg.htm - 12k - Cached - Similar pages

q=universe+expanding+accelerating&sourceid=opera&num=0&ie=utf-8&oe=utf-8

note:I typed universe expanding accelerating in goo gle box which took me two seconds.

It's been four minutes posting this.

you're welcome, and i look forward to my reward.

1. you plant a tree in bedford-stuyvesant, the promised land of my people
#13
08-05-2004, 07:06 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
Kindly skip the attitude, I was talking about the speed of light. If you don't want to answer, just don't. It's that simple.
#14
08-05-2004, 07:13 AM
 alaricthegoth Guest Join Date: Jul 2002 Location: The Barbary Coast Posts: 2,125
#15
08-05-2004, 07:16 AM
 Alessan Guest Join Date: Jul 2000 Location: Tel Aviv Posts: 22,612
Quote:
 Originally Posted by Priceguy So magnets will become progressively weaker and eventually, one day, run dry? It's just that it's going to take a really, really, really, REALLY long time? And also, once that happens, magnets not working will be the least of our problems.
Sure. I've seen "dead" magnets lots of times. Haven't you ever had one that keeps on falling of your fridge for no reason? You can't get around the Law of Conservation of Energy - each time a magnet exerts force, it loses a bit of its charge. Home magnets just don't do much work so the charge can last for a very long time.
#16
08-05-2004, 07:20 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
So now we have two competing viewpoints.
1. Magnets do lose their power, but it takes a LONG time.
2. Magnets do lose their power, and it happens every day.
#17
08-05-2004, 07:33 AM
 Garfield226 Guest Join Date: Jun 1999 Posts: 4,605
Quote:
 Originally Posted by Alessan Sure. I've seen "dead" magnets lots of times. Haven't you ever had one that keeps on falling of your fridge for no reason? You can't get around the Law of Conservation of Energy - each time a magnet exerts force, it loses a bit of its charge. Home magnets just don't do much work so the charge can last for a very long time.
I imagine those magnets had lost thier charge NOT because the individual atoms had lost their charge, but because the atoms, for one of the reasons I listed above, weren't lined up properly. It probably could have been fixed by stroking another magnet across it several times in the same direction.
#18
08-05-2004, 07:40 AM
 Ponster Guest Join Date: Nov 1999 Location: Irish in Paris Posts: 933
Magnets do lose their power, and it happens every day.

Dog80 has already expleined it in easy terms.

Quote:
 It all has to do with atomic alignment. Each atom is essentially a tiny magnet with a North and a South pole. When all the atoms are aligned, all those South poles face one way and the North poles face the other way. The magnetic force of each atom is combined and the material has magnetic properties.
But if I find a way to destablise the magnet, it'll lose it's power.

When you heat a magnet up it loses some of it's power. as it cools down it regains it. but if I heat it up beyond a temperature known as the Curie temperature (Curie Temp of iron = 1043K), then the magent will lose all of it's power, even when it cools down to room temp.

Another way is to apply an opposite magnetic field.

If I have 2 magnets, one much more powerful than the other, and I push them together so that they repel each other, the weaker magnet will become demagnetized much faster than it normally would.
#19
08-05-2004, 07:42 AM
 scm1001 Guest Join Date: Jun 2001 Location: cambridge Posts: 2,340
I think we are getting confused with force and energy. A compressed spring can exert a force forever until the universe ends. No energy is expended or taken in until the spring expands or contracts. Where did the energy stored in the spring comes from? From the person who pushed it in. Where is it stored? Well ultimately if one goes deep enough, in the strained electric fields of the atoms making up the spring.

Two magnets can in theory exert a force on each other forever. Where did the stored energy come from? From the guy who pushed them together (NN alignment) or pulled them apart (NS alignment). Where is the energy stored? In the electron spin repulsion of the atoms.

Note that permanent magnets can lose strength just like a spring can, in the former case from magnetic domains losing alignment. This is a separate effect from above and varies from material to material.
#20
08-05-2004, 07:53 AM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
Thanks ponster and scm1001; between the two of you I think you answered all my questions.
#21
08-05-2004, 07:54 AM
 Garfield226 Guest Join Date: Jun 1999 Posts: 4,605
Quote:
 Originally Posted by Ponster Magnets do lose their power, and it happens every day. But if I find a way to destablise the magnet, it'll lose it's power. When you heat a magnet up it loses some of it's power. as it cools down it regains it. but if I heat it up beyond a temperature known as the Curie temperature (Curie Temp of iron = 1043K), then the magent will lose all of it's power, even when it cools down to room temp. Another way is to apply an opposite magnetic field. If I have 2 magnets, one much more powerful than the other, and I push them together so that they repel each other, the weaker magnet will become demagnetized much faster than it normally would.
Yes, but you can fix all those things. You can "remagnetize" the previously-magnetic chunk of metal.

Look at it this way:

This is a magnetic chunk of iron --
Code:
```|||||
|||||
|||||
|||||
|||||```
Each "|" is a "magnetic domain." A magnetic domain is basically a bunch of atoms all oriented the same way. Each of those atoms is magnetic, and when they're all oriented in the same way, their fields combine with each other into one big magnetic field.

This is a demagnetized chunk of iron --
Code:
```|-\/|
-/|\|
\|//\
\--|/
/-|\|```
Each symbol is a "magnetic domain." Each of those atoms is STILL magnetic, but since they're all mixed up, their fields can't combine with each other to make one big one. If you rub another magnet over this one several times in the same direction, the domains will realign, as in the first example.

If the first chunk becomes like the second chunk, did it "lose its magnetism"? Well, the chunk as a whole did, but the magnetic force is still there...as a whole, it's just cancelled out due to the opposing orientations of the magnetic domains (and individual atoms).
#22
08-05-2004, 08:05 AM
 Ponster Guest Join Date: Nov 1999 Location: Irish in Paris Posts: 933
Quote:
 Originally Posted by Garfield226 If the first chunk becomes like the second chunk, did it "lose its magnetism"? Well, the chunk as a whole did, but the magnetic force is still there...as a whole, it's just cancelled out due to the opposing orientations of the magnetic domains (and individual atoms).
The question is "Does the damn thing stick to the fridge anymore?" and if it doesn't, then to the average Joe Soap or John Smith, it isn't a magnet
#23
08-05-2004, 08:31 AM
 erislover Guest Join Date: Nov 2000 Posts: 13,476
I believe hitting a magnet hard will also ruin its magnetic field. Taking a hammer to it, for example. The explanation I got for that was that it jostled the atoms.
#24
08-05-2004, 09:40 AM
 Jurph Guest Join Date: Oct 2003 Location: Charm City, Hon Posts: 3,380
Hang on, I've got a hijack.

Quote:
 Originally Posted by Priceguy Same way a bullet still accelerates after leaving the gun barrel but stops doing so after a while.
That can't be right. I am not a gun owner, and it's been six years since I've fired one, but I can't figure out where you think the force is coming from. It's possible that, in the first centimeter or two after leaving the barrel, the expanding gases continue to provide some (small) force. But "accelerate" in any meaningful way? I just can't buy that.

Until today, I was blissfully certain that bullets were ballistic projectiles, accelerating down the barrel to their top speed, then slowing down for the rest of their flight due to air resistance. Far be it from me, however, to come down like a hammer and just say, "No, you're wrong." I concede that I may be missing something, and implore a more level-headed person to clear this up.
#25
08-05-2004, 10:17 AM
 Crafter_Man Guest Join Date: Apr 1999 Location: Ohio Posts: 10,527
Quote:
 Originally Posted by Jurph Hang on, I've got a hijack. That can't be right. I am not a gun owner, and it's been six years since I've fired one, but I can't figure out where you think the force is coming from. It's possible that, in the first centimeter or two after leaving the barrel, the expanding gases continue to provide some (small) force. But "accelerate" in any meaningful way? I just can't buy that. Until today, I was blissfully certain that bullets were ballistic projectiles, accelerating down the barrel to their top speed, then slowing down for the rest of their flight due to air resistance. Far be it from me, however, to come down like a hammer and just say, "No, you're wrong." I concede that I may be missing something, and implore a more level-headed person to clear this up.
Your hunch is essentially correct… as soon as the bullet leaves the barrel, air resistance slows it down.

But when it comes to ballistics, things aren’t always as simple as they seem.

For some rifle bullets (e.g. the .50 BMG), the bullet is somewhat unstable during the first few hundred yards. After a few hundred yards, the bullet “goes to sleep” and becomes very stable. (An analogy is a spinning top; it is a little unstable when it starts to spin, and then becomes very stable after a few seconds.) This also has a direct affect on precision; the .50 BMG, for example, actually exhibits more precision at 600 yards vs. 100 yards, all else being equal. This is due to the bullet “going to sleep” a few hundred yards after leaving the barrel.

So here’s my question: does the .50 BMG bullet pick up a little velocity once it “goes to sleep”? I don’t know. I can only guess that there would be a slight increase in velocity once the bullet “goes to sleep,” but that this velocity (in absolute terms) is still less than the muzzle velocity due to the air resistance that is constantly acting on the bullet. I’m guessing that, if we graphed the velocity vs. distance from the time the bullet leaves the barrel, the overall slope would be negative, with a slight hump where the bullet “goes to sleep.”
#26
08-05-2004, 11:29 AM
 hajario Charter Member Join Date: Apr 2001 Location: Santa Barbara, California Posts: 15,221
Quote:
 Originally Posted by Alessan Sure. I've seen "dead" magnets lots of times. Haven't you ever had one that keeps on falling of your fridge for no reason? You can't get around the Law of Conservation of Energy - each time a magnet exerts force, it loses a bit of its charge. Home magnets just don't do much work so the charge can last for a very long time.
You may want to try cleaning the back of the magnets.

Haj
#27
08-05-2004, 11:53 AM
 Mort Furd Guest Join Date: Apr 2001 Location: An American in Germany Posts: 1,432
I'm probabla way the f... off base, but I've always considered it to be similar to the way a capacitor works.

A capacitor stores energy by distorting the electrical fields in the conducting plates. From what I've read/seen as diagrams, this equates to distorting the orbits of the electrons. Charge the plate, push the electron orbits out of whack. When you discharge the plates, the orbits snap back to normal by shedding the energy pushed into them by the charging process.

For magnets, I imagine something similar. A magnet by itself has no force or charge to exert. Then I coma along with a second magnet and push it against the field of the first magnet. This distorts the magnetic fields of both magnets, in effect charging them with potential energy. Now, I release the second magnet, and the distorted magnetic fields attempt to return to normal by shedding the energy that I put into the system by forcing them together.

Think of it maybe this way:

I've got a ton of lead sitting on the Earth's surface. Compared to the Earth's surface, that lead has a potential energy of zero. This is like the two magnets being separated by a large distance.

Now I use a crane and lift that block of lead fifty feet up from the ground. The lead now has potential energy that can be used to do work (smash somebody flat if he's dumb enough to get under it.) This is like forcing the two magnets together. Forcing them together takes work, which puts energy into the system.

Now I cut the cable holding the lead. The potential energy (that I've stored in the system) becomes kinetic energy and the lead moves, accelerates, smashes into the ground and generates heat. Similarly, I can release the magnets and allow the potential energy to become kinetic energy and the magnets move away from one another.

The only energy involved is that which is used to force the two magnets together. A magent loses its effectiveness when the particles (the smaller mganets it is composed of) lose their alignment. Forcing two magnets to stay together may cause the alignment to be lost sooner.

Magnetism is an inherent property of the material, like gravitational attraction, and can't really go away. Since magnetism is bipolar (unlike gravity, which is monopolar,) alignment is critical and can reduce the apparent strength even though the total magnetism of an object stays the same.

So. That's my understanding of it.

Take it with a big grain of salt, since I'm not a physicist and my last physics class was like 16 years ago.
#28
08-05-2004, 11:54 AM
 Algernon Charter Member Join Date: Nov 2001 Location: Milwaukee Posts: 2,721
Quote:
 Originally Posted by Crafter_Man I’m guessing that, if we graphed the velocity vs. distance from the time the bullet leaves the barrel, the overall slope would be negative, with a slight hump where the bullet “goes to sleep.”
I'll see you and raise you one. I'm guessing that the overall slope would be negative, with a plateau (no rise at all) where the bullet "goes to sleep".

I'm basing this guess on the many, many discussions I've had with people on both the belief that a bullet rises after it leaves the barrel, and the belief that a pitched fast ball can rise.
#29
08-05-2004, 11:55 AM
 Algernon Charter Member Join Date: Nov 2001 Location: Milwaukee Posts: 2,721
Oops. To clarify. I don't believe a pitched fastball will rise, but I know of other people who believe this.
#30
08-05-2004, 12:01 PM
 tracer Charter Member Join Date: May 1999 Location: Silicon Valley, Cal., USA Posts: 15,800
Quote:
 Originally Posted by scm1001 I think we are getting confused with force and energy.
God, yes. You wouldn't believe how many crackpot "alternative physics" theories are based on a simple confusion of these two concepts.

All right, all you wackos out there who are thinking about creating your own perpetual motion machine, listen up and pay attention:

Exerting a force does not require any energy! It is only when a force is applied to a moving object over a distance that energy must be expended. Just because the muscles in your arms have to vibrate (thereby expending energy) in order to exert a static force against a motionless object does not mean that springs, magnets, gravity, etc. have to expend energy to exert a static force against a motionless object. If this were not the case, it would be impossible to set anything down on a table, because the table would have to expend energy to keep it from falling.
#31
08-05-2004, 12:14 PM
 Crafter_Man Guest Join Date: Apr 1999 Location: Ohio Posts: 10,527
Quote:
 Originally Posted by Algernon I'll see you and raise you one. I'm guessing that the overall slope would be negative, with a plateau (no rise at all) where the bullet "goes to sleep". I'm basing this guess on the many, many discussions I've had with people on both the belief that a bullet rises after it leaves the barrel, and the belief that a pitched fast ball can rise.
I agree it’s impossible for a bullet to rise above the barrel axis. In fact, this is true regardless of the barrel’s elevation angle. But we’re talking about velocity here. My point is that it’s possible for the incremental change in velocity to be positive during the time the bullet goes from “partially stable” to “stable.”
#32
08-05-2004, 12:17 PM
 Thudlow Boink Charter Member Join Date: May 2000 Location: Lincoln, IL Posts: 24,405
Quote:
 Originally Posted by Priceguy What energy causes the magnets to move as they slide across the table towards each other? What energy causes the magnets to resist my attempts to push them together? More importantly, where is this energy stored? It cannot come from nowhere and there cannot be an infinite amount of it, so sooner or later every magnet must run dry, right? Why haven't I ever heard of or encountered an "empty" magnet?
One could just as easily ask:

What energy causes a bowling ball to roll down a slanted ramp towards the floor? What energy causes the bowling ball to resist my attempts to push it back up the ramp? Where is this energy stored? Why haven't I ever encountered an "empty" bowling ball?

I am not a physicist, but IIRC, gravitation and electromagnetism are two of the Four Basic Forces of the Universe. The answers to my questions involve gravity and potential energy; the answers to yours are analogous but involve magnetism. Right?
#33
08-05-2004, 12:19 PM
 Algernon Charter Member Join Date: Nov 2001 Location: Milwaukee Posts: 2,721
Quote:
 Originally Posted by Crafter_Man But we’re talking about velocity here. My point is that it’s possible for the incremental change in velocity to be positive during the time the bullet goes from “partially stable” to “stable.”
I confess that my subsequent comments about a rising bullet/fastball were confusing and irrelevant. Sorry. Yet, I still would think that the velocity curve would at best flatten out. I can't quite grasp where the energy would come from to allow the bullet to speed up. Wouldn't it simply decelerate more slowly. (that make any sense?)
#34
08-05-2004, 12:26 PM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
Quote:
 Originally Posted by Thudlow Boink One could just as easily ask: What energy causes a bowling ball to roll down a slanted ramp towards the floor? What energy causes the bowling ball to resist my attempts to push it back up the ramp? Where is this energy stored? Why haven't I ever encountered an "empty" bowling ball?
Apart from the last question (the bowling ball isn't the thing doing the lion's share of the work, if you catch my meaning), you're right; there is a definite analogy. I hadn't thought about it that way.

Could everyone please stop implying that I'm trying to build a perpetual motion machine? I was just wondering whence the energy comes to perform the work needed to move two magnets towards each other.
#35
08-05-2004, 12:29 PM
 CandidGamera Guest Join Date: Feb 2004 Location: Morgantown, WV Posts: 10,857
Quote:
 Originally Posted by Crafter_Man I agree it’s impossible for a bullet to rise above the barrel axis. In fact, this is true regardless of the barrel’s elevation angle. But we’re talking about velocity here. My point is that it’s possible for the incremental change in velocity to be positive during the time the bullet goes from “partially stable” to “stable.”
Disclaimer : I am not a physicist or ballistics expert, nor do I play one on TV. I do, however, have fairly recent experience with college level physics.

Ignoring the effect of gravity (which will begin to accelerate the bullet downward) there is nothing that would increase the velocity of a bullet after leaving the gun. Conservation of Momentum applies. As Algernon suggests, I suspect you'd see a decrease in the 'air drag' on the bullet.. thus a decrease in the deceleration.. but for the velocity to increase, the deceleration would have to drop all the way to zero, and become acceleration temporarily.
__________________
-Official Doper Brat #007- When life gives you harlequins, make a harlequinade.
I am the very model of the modern kaiju Gamera / I've a shell that's indestructible and endless turtle stamina. / I defend the little kids/ and I level downtown Tokyo/ in a giant free-for-all mega-kaiju rodeo.
#36
08-05-2004, 12:36 PM
 MikeS Charter Member Join Date: Oct 2001 Location: New London, CT Posts: 3,770
scm1001 and Mort Furd have got the answer to the OP right: when you push two magnets together, you're storing energy in the system, and once they're released that energy can be converted back into mechanical motion (the magnets sticking together or flying apart, depending on their alignment.)

I'll just add that the energy you put into the system when you push the two magnets together can be viewed as being stored in the combined magnetic fields of the two magnets. There's an energy density per volume of space associated with an magnetic field (given by E = B2/m0, where m0 is the "permeability of free space".) When you push two magnets together, you increase the total energy present in the combined magnetic field of the two magnets, and thus the mechanical energy used to push them together is converted into field energy. Similarly, when you let go, the energy stored in the fields is converted into mechanical energy as the two magnets fly apart.
#37
08-05-2004, 12:43 PM
 Mort Furd Guest Join Date: Apr 2001 Location: An American in Germany Posts: 1,432
The energy needed to move two magnets together is harder to imagine.

Start with two magnets stuck together, north pole to south pole. If you pull them apart, then you must do work to get them to separate. If you don't turn them, then the energy that you put into the system by separating the two magnets is the same energy that will be expended in pulling them back together.

But, you say, the two magnets were never together to begin with. This is the strange part. Regardless of whether the two have ever met, they have potential energy with respect to one another because of the magnetic fields and their physical relation with one another. In moving the one magnet around, you have done work in moving it such that its potential energy with respect to the other magnet has increased. It is this energy that then pulls the two together.

Will a real physicist please step in here and rescue us?
#38
08-05-2004, 01:19 PM
 Dunderman Guest Join Date: Dec 2002 Posts: 8,838
#39
08-05-2004, 01:54 PM
 theckhd Guest Join Date: Dec 2002 Location: Rochester, NY Posts: 265
GAH i had a great post whipped up and the hamsters ate it on preview.

Anyway, IAAP, so here's the straight dope on magnetism.

Magnetic Fields are created by moving charges. In the case of a bar magnet, the moving charges are the electrons orbiting the atoms of the material the bar magnet is made out of. The moving electrons create what we call Magnetic Dipoles. If the Magnetic Dipoles of all of the constituent atoms line up, you have a bar magnet.

Magnetic fields do not, in fact, repel or attract one another. Magnetic fields can only act on moving charges. There's a caveat though -- in a uniform magnetic field, a magnetic dipole will feel no net force (a torque, perhaps, but not a force)! The reason bar magnets exert a force on one another is that their field is non-uniform, and thus they can exert a force on each others' magnetic dipoles.

Mort Furd was correct about everything except how a capacitor works. A capacitor does not store energy by deforming fields within the plates, it stores energy by creating a buildup of opposite charges on either plate, which creates an electric field between the plates. The energy it stores is the energy stored in the electric field, with an energy density u=(1/2)*epsilon*E^2.

The same is true for magnets, in that there is an energy density associated with magnetic fields. MikeS's explanation about how magnets push each other apart is exactly correct.

I have to run now, i hope this helps. If there are still questions, i'll try to stop back later tonight.
#40
08-05-2004, 02:34 PM
 GargoyleWB Guest Join Date: Mar 2001 Location: Somewhere cold 'n squishy Posts: 5,199
Physics/engineering guy here...

Just to nitpick terminology....

Asking whether the velocity of a bullet decreases (or a baseball) is an improper way of asking the question...

It can be technically correct to say that the velocity of a bullet increases after being shot, because velocity is a vector. The downward velocity of the bullet steadily increases due to normal gravitational acceleration. The horizontal component of the bullet's velocity is what is decreased by air resistance.

So one needs to be very explicit when asking questions of this sort, or you will get some unintended replies from the physicist/scientist crowd. Unfortunately, the layperson often is unaware or is confused by the differences between velocity/speed/momentum and force/energy and mass/weight.

As for the magnets and the OP's question...

What you are thinking of the magnets' "energy" is what we call the "potential energy" between the two respective magnetic fields of each magnet. When held static, there is a resultant force (that you are feeling) based on the attraction and repulsion of the accumated total of all of the individual magnetic dipoles of the atoms and molecules in the magnet (explained by another poster). You can think of it just like the force of a spring. When compressed, a spring does not get "energy" from anywhere in a literal sense, it is instead providing an increasing resistive force. Same with magnets. Things get quickly hairy to explain when we get into potential energy, stored field energy, and such however.

In a very small nutshell, there is a physical principle that charged particles possess an electric field, and charges in motion produce a magnetic field. Charges in motion within a magnetic field feel an induced force upon them. All of the wiggly electrons within the magnet are producing a magnetic field that all of the other wiggly electrons in the other magnet feel as a force.
#41
08-05-2004, 03:00 PM
 Jurph Guest Join Date: Oct 2003 Location: Charm City, Hon Posts: 3,380
Okay, so the bullet leaves the barrel with an initial velocity vector parallel to the ground, and, unconstrained by the barrel, is now free to be acted on by two forces:

1) air drag, which acts in the direction opposite the velocity vector, and
2) gravity, which acts perpendicular to the initial velocity.

In the first full second of flight, the bullet falls ~4.9m, so its velocity vector is always ever-so-slightly below the horizontal, and drifting farther and farther off horizontal. In that time, air drag (which grows with the square of the velocity) is drastically lowering the horizontal speed, and having a negligible effect on the speed at which the bullet falls.

There's a big however here though. If you fired the bullet off the top of Mt. Everest, the magnitude of its horiztonal velocity will eventually slow to zero and the magnitude of its vertical velocity will approach the terminal velocity (in the downward direction). Since the terminal velocity is going to be much lower than the muzzle velocity, the velocity curve will never have a positive slope.

As for "going to sleep", unless there's some weird gyroscopic effect that causes the bullet's angular momentum to be transferred into linear force, it's not going to do anything but increase the bullet's "beta" (ballistic coefficient) which reduces drag but definitely will not accelerate the bullet by any means.
#42
08-05-2004, 04:25 PM
 trupa Guest Join Date: Mar 2003 Location: Calgary Alberta (Canada) Posts: 992
Quote:
 Originally Posted by scm1001 I think we are getting confused with force and energy. A compressed spring can exert a force forever until the universe ends. No energy is expended or taken in until the spring expands or contracts. Where did the energy stored in the spring comes from? From the person who pushed it in. Where is it stored? Well ultimately if one goes deep enough, in the strained electric fields of the atoms making up the spring. Two magnets can in theory exert a force on each other forever. Where did the stored energy come from? From the guy who pushed them together (NN alignment) or pulled them apart (NS alignment). Where is the energy stored? In the electron spin repulsion of the atoms. Note that permanent magnets can lose strength just like a spring can, in the former case from magnetic domains losing alignment. This is a separate effect from above and varies from material to material.
Thank you, SCM, for posting this and scratching that itch of irritation in my brain, not to mention saving me the trouble. Good analogy with the spring.
#43
08-06-2004, 01:26 PM
 Spectre of Pithecanthropus Charter Member Charter Member Join Date: Aug 1999 Location: Not here. There. Posts: 18,781
Quote:
 Originally Posted by Garfield226 I imagine those magnets had lost thier charge NOT because the individual atoms had lost their charge, but because the atoms, for one of the reasons I listed above, weren't lined up properly. It probably could have been fixed by stroking another magnet across it several times in the same direction.
Another way is to hold it, if it is long enough, so that one end is on a hard surface, and the magnet is pointing towards the North Magnetic Pole. (From Sweden, I'm not sure what direction that is...west by northwest?. You give it a sharp rap with a hammer and you'll knock those atoms back into line. You can weakly magnetize any piece of iron or steel in this manner. I found this very helpful when I had to insert a very tiny Philips screw into a narrow, awkward place. I couldn't get the screw positioned without having it fall before I could drive it in. So I followed the above procedure with the screwdriver shaft, and, sure enough, it was now a weak magnet. The tiny screw clung to the end long enough for me to drive it in. Yay, me!

 Bookmarks

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is Off HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Main     About This Message Board     Comments on Cecil's Columns/Staff Reports     General Questions     Great Debates     Elections     Cafe Society     The Game Room     Thread Games     In My Humble Opinion (IMHO)     Mundane Pointless Stuff I Must Share (MPSIMS)     Marketplace     The BBQ Pit

All times are GMT -5. The time now is 08:25 AM.

 -- Straight Dope v3.7.3 -- Sultantheme's Responsive vB3-blue Contact Us - Straight Dope Homepage - Archive - Top

Send questions for Cecil Adams to: cecil@straightdope.com