How do magnets work?
- Marsha Borski (age 9)
That’s an excellent question.
Try the following experiment with a magnet, some needles (the plain kind used to hold stuff together as you sew it), and a screwdriver. It will help you understand the explanation.
Touch a needle with a screwdriver and nothing unusual should happen (unless the screwdriver is already magnetized). Now, take some of the needles and hold them against the end of a magnet for a while (the stronger the better). After doing this, you will have turned each needle into a little magnet, and when you touch them with a screwdriver they will stick to it.
Some kinds of metals (like steel that the needles are made of) are made up of billions and billions of individual atoms that each have the properties of a microscopic magnet. The atoms in steel naturally tend to get together in tiny little groups called domains, and within each domain the atoms tend to point in the same direction, which makes the domains behave like a tiny little bar magnets just like the kind you have probably played with at school. The needle of a compass is also a bar magnet, and we know what this does: it points north because it likes to line itself up with the magnetic field of the earth.
When you make a piece of steel (like a needle), all of the tiny domain-magnets inside tend to get stuck pointing in different directions, which means that they more or less cancel each-other out, so to begin with the needle does not behave much like a magnet at all.
However, if you bring the needle close to another magnet of some kind, (like the ones on your fridge, or better yet some stronger ones you can find in your classroom) something interesting can happen: Because of the other magnet you are holding it near, the little domain magnets in your needle will tend to line up so that they are pointing along the same direction. (You can visualize this by holding two bar magnets near each other and noticing how they like to line up a certain way). The domains in the needle will do the same thing, and after you take the needle away from the other magnet, the domains in the needle will tend to stay pointing this way (they sort of get stuck pointing in the same direction). Now, since you have lots of these little domain magnets in the needle all pointing in the same direction, the needle will itself have become a small magnet. (This is exactly how bar magnets are made).
Magnets attract other magnets (as you can see), and also attract some kinds of metal like many kinds of steel. If you touch your magnetic needle with a steel screwdriver it will stick. However, stainless steel is not a very good magnetic material, so if you touch your magnetic needle with something made of stainless steel it will probably not stick (try it).
(published on 10/22/2007)
Follow-Up #1: why not more magnets?
I need a more in-depth answer to Marsha’s question. I know that natural magnets have magnetic fields becuase of the electrons moving aroung inside thier atoms(while electric fields are present so are magnetic fields), but why then doenst matter like water and tree bark have natural magnetism if they also have electrons inside thier atons?
In most materials almost all the electrons form pairs, with the magnetism from the two paired electrons exactly canceling. The result ultimately is due to something called the Pauli exclusion principle, which says that no more than one electron can exist in any particular quantum state. So if there's some nice low-energy state waveform for an electron to sit in in some molecule, it tends to get two electrons for the two possible quantum states with that form: one spin up, the other spin down.
This very fact is the origin of most of the properties of chemical bonding. Remember how a typical covalent bond consists of two shared electrons? The reason is the exclusion principle plus spin. In ionic bonds, typically one electron will mostly move from one atom with an odd number of electrons over to another atom with an odd number, leaving each with an even number.
So the tendency of most materials not to be magnetic is closely tied to the same quantum mechanical facts which account for the prevalence of pair-bonds and ionic bonds in chemistry.
(published on 03/12/07)
Follow-Up #2: sad truth
why are most materials non-magnetic? (no quantum physics please!)
- ivan brown (age 13)
Newport, Wales, Britain
Ivan- I sympathize with your question, but the sad fact is that there’s absolutely nothing honest one can say about the origin of magnetism in materials without using a quantum mechanical basis. Sorry.
(published on 05/31/07)
Follow-Up #3: pauli exclusion principle
what is pauli exclusion principle?
- starlai (age 18)
It states that for particles called ’fermions’, including electrons, protons, neutrons, etc., no two particles can be in exactly the same quantum state. There must be something different about them, either that they’re in different places or have different spins or something. There’s another class of particles, called ’bosons’, including photons, for which this principle does not apply.
(published on 07/29/07)
Follow-Up #4: non-magnetic trees
i cant exactly get the answer of why magnets has no effect in trees example where infact it has also an electron inside thier atoms... please expain it in a simle way for me thanks!
- lai (age 18)
The basic reason is that almost every electron is paired up with one other one that is in the same state except that it is spinning the opposite way. (see the Pauli exclusion answer above) That means that the magnetism of the two just cancels.
In leaves, however, as photosynthesis occurs some electrons get shuttled around individually as part of the light-driven chemical process. So their magnetism isn't automatically canceled, and magnetic resonance signals from them can be measured.
(published on 12/17/07)
Follow-Up #5: domain interactions
Do domains work agnaist each other
I think what you're asking is whether the forces between magnetic domains push them toward pointing opposite ways. That actually depends on whether the domains are side-by-side or end-to-end. If you picture the domains as little arrows pointing up or down, side-by-side ones tend to point opposite directions but end-to-end ones tend to line up the same way.
Think of it this way. If you have two bar magnets, each has a North pole and a South pole.
If the two magnets have their two N poles pointing up then when they are side by side they will repel each other but when they are head-to-tail they will attract. In the other case when one has a N pole pointing up and the other has a S pole pointing up then they will attract each other when they are side by side but repel when they are head-to-tail.
(published on 12/19/07)
Follow-Up #6: permanent magnets
So what makes a magnet magnetic? You told me how iron was magnetic and i understand that but what is the difference between a magnet and something that is magnetic?
- Anonymous (age 13)
Most strongly magnetic materials, like iron, have magnetic domains which act like small magnets themselves. However, the directions of those magnets are all scrambled up, so the total magnetism cancels out. If you put them in a magnetic field, the domains tend to line up with the field, so you get something which really has net magnetism and can feel a force from the field.
To make a permanent magnet, you need some way to keep those domains lined up even after the applied field is removed. In certain special materials, it's hard for the domains to change which way they point. Usually those materials have some mixture of non-magnetic atoms, which tend to catch and trap the domain walls, the regions where the magnetism changes from one direction to another. Trapping the walls keeps them from moving, and keeps the magnetic direction from changing.
If you heat up a permanent magnet, the walls tend to come loose and the net magnetism will go away.
(published on 02/21/08)
Follow-Up #7: magnetic vs. electric fields
So it sounds like you're saying that what makes something magnetic is the presence of these "single" particles or particles that are not in a pair (if so they would cancel each others magnetism)and there are regions of these single particles that are called domains?
I have two question.
How can one particle be magnetic?
Is the source of magnetism the attraction/repulsion relation between positive and negative charges? And if so, what makes magnetic fields different from electric fields? (detailed please)
Actually, for something to be very strongly magnetic it takes more than just some unpaired particles. There has to also be a tendency for the magnetism of different particles to line up together, and that tendency is present in only some materials.
One of the sources of magnetism is electrical current. That's why a current through a copper coil, for example, creates a magnetic field. Now (and this isn't quite being honest) imagine a charged particle as a little ball of charged stuff. If it's spinning, then that's like having currents go around in a loop, and you should get fields like those when current goes around in a coil. Elementary particles like electrons do indeed have "spin", measurable angular momentum. However, if you took this story too literally you'd calculate a magnetic moment of an electron which is only half the actual value. On a small scale, that spin is really something quantum mechanical, not something you can picture as charges running around in circles.
Magnetism is related to the electrical fields which pull on charged particles. However, it's connected not just with the positions of the particles but also their velocities. If you look at a charged particle (without spin) from a frame where you say it's standing still, it will only have electrical fields. If you look from another point of view and say it's moving, it will also have magnetic fields. Of course the net physical effects on its neighbors will be the same in either case. We obviously cannot fully introduce this area here, but if you want to see a beautiful book on the topic, I recommend E. Purcell's "Electricity and Magnetism"..
To add one more twist to the tale, Maxwell's equations are consistent with terms involving magnetic monopoles. None have ever been seen. For more information check out:http://en.wikipedia.org/wiki/Magnetic_monopole
And for another twist, if those magnetic monopoles exist, then an observer who sees one moving will say it produces electric as well as magnetic fields. mike w
(published on 03/01/08)
Follow-Up #8: making magnets and demagnetizing them
So magnets are made by putting a magnetic object under a magnetic field, this makes all the domains spin in the same direction, which is what defines a magnet. Also electricity running through a wire creates a magnetic field because it has a flow of electrons in one direction. so does that mean that if you try creating a magnet off of an electrical current, then the domains of the magnet will spin in the direction of the current, and if so, does that mean that making a magnet using AC current would be very ineffective?
The direction the domains will point is set by the current in the electromagnet, although it is not the same as the current direction. If the magnet is in a coil, running the current one way around the coil will make the magnetism point one way; reversing the current makes it point the other.
So you're absolutely right that an ac current is no way to magnetize something. In fact, when you want to demagnetize something (e.g. erase a magnetic recording) the standard way is by applying a big ac field and gradually decreasing the magnitude. That pretty much scrambles up the domains. It's often called "de-Gaussing" since one standard unit of magnetic strength is a Gauss.
(published on 03/09/08)
Follow-Up #9: Magnets do not bend light, however...
I was wondering if Magnets have any effect on light. I know that light is really electromagnetic waves, but I was wondering if these waves could be attracted/repelled against a magnet. Thanks!
- Joe Shock-Ra
In a vacuum there is no effect of a magnet on light. It doesn't bend it or anything.
There is a subtle effect, however, when light passes through a dielectric medium like glass or sugar water solution. It turns out that in the presence of a magnetic field the light beam is not bent but the plane of polarization can be rotated a bit. This is called Faraday rotation. You can find out more about it at http://en.wikipedia.org/wiki/Faraday_rotation
(published on 04/28/08)
Follow-Up #10: first magnets
First, thank you for answering for our questions. My question is, you say that to obtain a magnet we need to have a magnet. Then how was the first magnet made, when there were no any other magnets? How to make a magnet if i have only a piece of iron and nothing else??? Thank you again.
Really nice questions!
Some stones are magnetic as found. They are magnetized because they cooled in the Earth's magnetic field. The Earth is magnetic for complicated reasons which I don't really understand, but have something to do with currents stirred up as the hot core gradually cools, creating convection patterns.
You don't need a permanent magnet to magnetize a piece of iron. It's usually done using an electromagnet, so what you really need is a current source to drive the electromagnet. Although many current sources are ultimately based on generators which themselves use magnets, there are other current sources which do not need magnets. For example batteries can be made using chemicals and no magnets.
(published on 01/26/09)
Follow-Up #11: magnetic water?
can a magnet work when there is ice between the two magnets and if there is water between them ( the icce and water will be between them at different times ) if so will the magnetic force be different
Water (whether liquid or frozen) has so little magnetic susceptibility that you will not notice its effects unless you\'re deliberately making a sensitive experiment.
(published on 02/18/09)
Follow-Up #12: magnetic forces
What causes magnetics to attract to each other?
And why do they stick for so long?
- Tiffany (age 13)
Richmond hill, GA, US
Those are great questions but I can't think of any way to answer the first one without giving a little course. As for the second one, it's normal for the forces which hold things together to last forever.
Think of the gravity that holds you and the Earth stuck together. As long as you and the Earth are around, that gravity will be there. The same rule applies to magnets. As long as they stay magnets, the force between them will be there. There is one difference, however. You can think of a magnet as made up of little magnets, called domains. Each one has a North and a South pole. Over a very long time, those can jiggle around so the little magnets don't line up with each other. Then they won't add up to make a big magnet. Gravity doesn't have poles, the gravity from every part just adds together. So even as parts of the Earth shift around, the gravity won't go away.
(published on 03/31/09)
Follow-Up #13: compass physics
The South Pole is magnetized, correct? How come a compass never points south. If we switched the charge in our compasses would they point south?
Also, how come compasses become more inaccurate as you near the north pole? That seems counter-intuitive. I'd think they'd become more accurate since the force is stronger.
- John (age 33)
It's just a convention to label the north-pointing end of the magnet in a compass by sharpening it or painting it with an arrow head. You could do that to the other end if you wished.
The inaccuracy near the north pole arises because the magnetic north pole isn't actually at the same place as the rotational north pole. The difference becomes important in that vicinity. If you stood in between them, your compass would point south.
(published on 09/23/09)
Follow-Up #14: Do two bar magnets attract or repel each other?
If two magnets are put end to end with the current patterns lined up in the same direction, the two poles next to each other would do what?
- Drew (age 17)
| | | | Currents aligned
| N A S | | N B S | Attraction
| | | | Currents anti-aligned
| N C S | | S D N | Repulsion
Just to be clear, in the picture above A and B attract, C and D repel. You might also wonder about what happens between A and C and between B and D, in the positions shown. A and C repel. B and D attract. If you imagine each magnet as a tube of current going around the axis, the ones that attract are the ones where the nearby parts
of the current go the same way in each tube. Mike W.
(published on 03/25/10)
Follow-Up #15: magnets attract or repel
How do magnets stick together only sometimes ?
- Isabel (age 10)
Bar magnets only sometimes stick together and sometimes repel because each has a north pole and a south pole. Opposite poles attract, and like poles repel. If you were to put two north poles together, they would repel, but a north and a south would stick together. The picture above shows an example.
Chris F. and Mike W.
(published on 06/06/11)
Follow-Up #16: are magnets permanent?
In permanent magnets what sort of energy do they use to push or pull other magnets if any at all? I was also wondering does a permanent magnet ever lose its magnetism?
A permanent magnet will lose its magnetism fairly quickly when it's heated enough. A good permanent magnet stays that way for a very long time at room temperature. In the extremely long run, its little domains should lose their alignment, but that's not in our lifetime.
You also ask, if I understand right, what sort of energy is involved in the magnetic forces between magnets. There is energy stored in the magnetic field itself. The density of that energy is proportional to the square of the field strength. When magnets move near each other, that field energy generally changes.
(published on 07/06/11)
Follow-Up #17: shielding a magnet with steel
I know a magnet can attract steel paper clips through paper, cardboard, plastics, cloth, etc. But can a magnet attract steel paper clips through a sheet of sheet?
I have beeen confused as some sources say it will not.
I know you can make a iron nail a temporary magnet by attracting it to a magnet at one end and have paper clips attracted to the nail. Shouldn't it be the case for the sheet of steel?
- SARAH (age 26)
I assume that was a typo, and you're asking about a sheet of steel. it's an interesting question.
Here's a nice physical way to picture what happens. Picture the magnetic field in the usual way, as a set of field lines coming out of one pole of the magnet and returning to the other. There's a tendency for those field lines to get sucked into pieces of iron or steel or other easily magnetized material. That's exactly the reason that the paperclips stick to the magnet. Some of the field lines get stuck in them.
Now what happens if you put a big sheet of steel in between the magnet and the paper clips? The field lines mostly get drawn into the sheet, spread out in the sheet, and return without going past the sheet to the paperclip. So the clip doesn't stick much if it's just behind the sheet.
This effect depends a lot on the shape of the sheet. A small piece of steel can pull field lines into it leaving more field just behind it than were there before. That's just what happens when you chain paperclips together hanging from a magnet. Each clip tends to steer field lines on to the next one. So the steel redirects the field, with a sheet redistributing it away from the clip just on the other side. It would tend to grab clips right near the edge of the sheet, where some of the field lines exit.
(published on 09/28/11)
Follow-Up #18: magnetized nail
We made electromagnets in class with wire, a large nail and a 6 volt battery. The nails were not magnetic, but after we disconnected the batteries from the nails, the nails would pick up staples. How did the battery or the electricity magnetize the nail?
- Karen (age 10)
The nail started off already magnetized in small regions, called domains. At room temperature the little magnets of the electrons in the iron tend to line up with other, making those magnetic domains. However, the magnetism of the different domains points all different directions, so overall it cancels out.
When you put the nail in the electromagnet made with the coiled wire powered by the battery, it lines up a lot of those domains to point the same way. After you take the nail away, some of the domains stay stuck, so you're still left with more of the magnetism pointing one way than any other. That means the nail is a bit magnetized.
This is temporarily posted without the usual check, until Lee gets back from Paris.
(published on 11/27/11)
Follow-Up #19: magnets and their fields
1!: Can you make a 'doughnut' magnet so as to have, say, all N charge on the outside and S on the inside, in the hole of the doughnut??
2!: what exactly is the field itself? the magnetic field, i mean. The magnetic fields' COMPOSITION.
The answer I usually get is something like:
" well, the magnet makes the field"
"...I am aware of that. But what exactly is the field MADE-OUT-OF?"
"It is the magnetic force in the magnet interacting with things around it"
But you seem thorough. so please answer in that manner,I'll understand everything right up to the point where there needs to be calculus to understand. I'm still in Algebra 2.
- Atticus (age 14)
Salt Lake City, Utah,
Your first question is easy.
Sure, you can make a donut-shaped magnet with the N poles on the outer part and the S poles near the donut hole. It's not a standard form, but it's certainly possible. (Those poles aren't really "charges" though.)
Your second question is extremely hard to answer. I think for now perhaps the best non-answer is this. Whatever the universe consists of, at some point our description has to get down to the most basic ingredients, or, if every level of ingredient should be thought of as made of something deeper, at some point we'll just get to the deepest level we know about. At that point, all you can do is describe the properties, not say what things are made of. Electromagnetic fields are pretty close to that point. We could give maybe a step or so deeper description, but it would be in terms of quantum fields. Those are even more abstract mathematical entities than magnetic fields.
(published on 01/05/12)
Follow-Up #20: magnetism from moving charges
Well, i know that moving or spinning charged particles can cause a magnetic field. But how does it do that? And why don't stationary charged particles create magnetic fields, even though they create electrostatic forces? And what causes those forces in the first place? For example, gravity is caused by mass curving or warping space time or something of the sort, what causes magnetic and electrostatic forces? I know very basic quantum mechanics. Thank you!
- Rahul (age 15)
Rahul- You're asking just the right questions to start in on Purcell's Electricity and Magnetism book, from the Berkeley series. The outline of Purcell's presentation looks something like this.
1. Start with the existence of simple electrostatic forces following Coulomb's law.
2. Assume that special relativity gives the right rules for how things look in different reference frames.
3. Look at a moving charge near a neutral, current-carrying wire. There's no electrical force.
4. But if you now look in the rest frame of the charge, the wire is no longer neutral, thanks to the different Lorentz contractions of the differently-moving plus and minus charges.
5. So there's an electrical force in this frame.
6. So there must have been a velocity-dependent force back in the frame where the wire was neutral.
7. We call that velocity-dependent force magnetism.
Ok, that handles the part about why magnetism, given electricity. The part about why electricity in the first place is unfortunately over my head. Here's a few words to get you started. Purcell derives the magnetism from electricity from assuming a symmetry, special relativity, a rule about how the world obeys the same laws of physics even as you represent its contents in different ways. There are some other subtle symmetries (gauge symmetries) in relativistic quantum field theory, and I've heard that they require the electrostatic force.
(published on 05/29/12)
Follow-up on this answer.