Carlos

Science Brain Bogglers (Round 1?)

77 posts in this topic

They have double insulated fur coats. No border collie has ever been fired.

They have tenure?

They are all ages. They usually live longer than ten yures.

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This is very close, so here is the answer:

...

There is a much simpler explanation that everyone should routinely know and which does not required knowledge of thermodynamics: border collies herd the phlogisten to keep us warm and safe.

The principle of hot air rising is very different and is related to politicians.

This explains why politicians' heads sit on top of their shoulders rather than between their knees.

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This explains why politicians' heads sit on top of their shoulders rather than between their knees.

The hot air increases as they rise to higher political offices. What politicians do you have in mind who don't have their heads between their knees? This is a serious, scientific thread; please stick to facts, not fantasy.

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The changing magnetic field, from your movement, creates and electric field in the metallic coin. The electric field has a corresponding magnetic field which responds to the magnet in your hand.

Interesting example, but I have never tried to do that experiment.

Eh, I have to be picky and not be satisfied with this answer. I changing magnetic field does create an electric field, but that electric field doesn't have to necessarily create a lasting magnetic field. And even if the coin was somehow generating its own magnetic field, that magnetic field would be acting on the magnet and putting a force on a magnet, which doesn't explain the magnet's force on the coin.

It is a very fun experiment to try, but you need a smooth, low-friction surface and a pretty good magnet (the magnet I was using can lift my toaster).

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The changing magnetic field, from your movement, creates and electric field in the metallic coin. The electric field has a corresponding magnetic field which responds to the magnet in your hand.

Interesting example, but I have never tried to do that experiment.

Eh, I have to be picky and not be satisfied with this answer. I changing magnetic field does create an electric field, but that electric field doesn't have to necessarily create a lasting magnetic field. And even if the coin was somehow generating its own magnetic field, that magnetic field would be acting on the magnet and putting a force on a magnet, which doesn't explain the magnet's force on the coin.

It is a very fun experiment to try, but you need a smooth, low-friction surface and a pretty good magnet (the magnet I was using can lift my toaster).

What kind of coin was it? Does it have small amounts of iron in it? Any nickel? There are lots of materials that are paramagnetic.

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What kind of coin was it? Does it have small amounts of iron in it? Any nickel? There are lots of materials that are paramagnetic.
The coin was absolutely not magnetic. Even if it had a weak paramagnetic response, how would that paramagnetic response be significantly stronger in the presence of a moving magnetic field? It should be the same regardless.

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What kind of coin was it? Does it have small amounts of iron in it? Any nickel? There are lots of materials that are paramagnetic.
The coin was absolutely not magnetic. Even if it had a weak paramagnetic response, how would that paramagnetic response be significantly stronger in the presence of a moving magnetic field? It should be the same regardless.

Are you denying that the material is either ferromagnetic or paramagnetic? If so, then there would be no response at all. As far as the movement is concerned, I think that is a red herring. A paramagnetic material will show some response to a very strong magnetic field, moving or not. I didn't mean to imply that the movement was responsible for the magnetic field in the coin. I meant that the magnet's movement was responsible for the coin's movement because of the magnetic field generated in the coin.

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Are you denying that the material is either ferromagnetic or paramagnetic? If so, then there would be no response at all. As far as the movement is concerned, I think that is a red herring. A paramagnetic material will show some response to a very strong magnetic field, moving or not. I didn't mean to imply that the movement was responsible for the magnetic field in the coin. I meant that the magnet's movement was responsible for the coin's movement because of the magnetic field generated in the coin.

The coin is made of metal that is not magnetic at all. The coin shows no magnetic response when placed in the magnetic field of my magnet. However, when the magnet is moved relative to the coin swiftly, the coin tries to change its state of motion and move with the magnet. This is the paradox that I am asking an answer for.

The answer you are giving still isn't precise enough; magnets don't exert forces on magnetic fields. There is a different answer which is correct.

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Are you denying that the material is either ferromagnetic or paramagnetic? If so, then there would be no response at all. As far as the movement is concerned, I think that is a red herring. A paramagnetic material will show some response to a very strong magnetic field, moving or not. I didn't mean to imply that the movement was responsible for the magnetic field in the coin. I meant that the magnet's movement was responsible for the coin's movement because of the magnetic field generated in the coin.

The coin is made of metal that is not magnetic at all. The coin shows no magnetic response when placed in the magnetic field of my magnet. However, when the magnet is moved relative to the coin swiftly, the coin tries to change its state of motion and move with the magnet. This is the paradox that I am asking an answer for.

The answer you are giving still isn't precise enough; magnets don't exert forces on magnetic fields. There is a different answer which is correct.

I'll have to leave it to others to figure it out, or when you are ready to provide the answer. If a magnet does not exert a force on a ferromagnetic or paramagnetic material because of the induced magnetic field created by the magnet, that's as far as my understanding goes. When I put two negative ends of two magnets near each other and they repel each other, other than the interacting magnetic fields, I don't observe another physical interaction.

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I would like to comment on methodology first. I took physics in college many decades ago and still remember much of it. I remember issues like the reciprocal effects of magnetic fields on charged particles and vice versa. I remember something about a "right-hand rule" that describes the magnetic field that a charged particle creates along its path as it moves. I remember how the trajectory of a charged particle can be changed by a magnetic field, as in the cathode-ray picture tubes of TV sets (non-flat-panel types). And I remember something about what happens to metals (iron-containing or not) in the presence of a varying magnetic field -- varying either because of relative motion between the field and the metal object, or because the field itself is time varying.

But I couldn't quite remember or apply any of that to a net force on a charge-neutral metal object due to a magnetic field. Rather than speculate from incomplete knowledge or incomplete memory, I looked up one concept in particular that I remembered. I found a wealth of excellent insights when I did that. Here are some excerpts. I have blanked out the key concept, so as to leave it as a hint for others. Think of it as a "fill in the blanks" quiz.

[Excerpts from Wikipedia, entry on ______]

They [______] can thus be used to induce a magnetic field in aluminum cans, which allows them to be separated easily from other recyclables. With a very strong handheld magnet, such as those made from neodymium, one can easily observe a very similar effect by rapidly sweeping the magnet over a coin with only a small separation. Depending on the strength of the magnet, identity of the coin, and separation between the magnet and coin, one may induce the coin to be pushed slightly ahead of the magnet - even if the coin contains no magnetic elements, such as the US penny.

Superconductors allow perfect, lossless conduction, which creates ______ that are equal and opposite to the external magnetic field, thus allowing magnetic levitation. For the same reason, the magnetic field inside a superconducting medium will be exactly zero, regardless of the external applied field.

Identification of metals

In coin operated vending machines, ______ are used to detect counterfeit coins, or slugs. The coin rolls past a stationary magnet, and ______ slow its speed. The strength of the ______, and thus the amount of slowing, depends on the conductivity of the coin's metal. Slugs are slowed to a different degree than genuine coins, and this is used to send them into the rejection slot.

------ [end of excerpts] ------

I will be happy to fill in the blanks if Carlos' eventual answer differs from what I'm expecting. I must say, though, that I have been completely unable to reproduce any coin movement myself due to passing a magnet over the coin. Maybe I need a stronger magnet, but the ones I have are quite strong. I also wonder if it needs to be a simple bar magnet rather than a horseshoe shape (horseshoe magnets have the N and S poles relatively close together).

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Aluminum and copper are paramagnetic, so that explains its behavior in a magnetic field.

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....

What's going on here? Remember, the coin is not magnetic at all.

OK, I think I've got it figured out. Without looking at the internet or consulting notes and books about electricity and magnetism, here's what I remember:

If a conductor is moved through a magnetic field, there will be a voltage induced in the conductor. That's what we have here. The voltage in the conductor (coin) will cause a current to flow inside the coin.

The direction of the induced current will be such that it causes a force that opposes the motion. (I think this is called Lenz's law, but I'm not allowed to look -_- .) Here, the "motion" is the relative motion of the magnet and the coin, so the induced current will oppose this relative motion - that is, it will try to prevent the magnet and the coin from moving relative to each other. That means it will try to keep the coin moving along with the magnet - in other words, oppose the relative motion.

I'm thinking that the induced current in the coin is just circling around.

And this is the same phenomenon that, in a generator, gives rise to the force that opposes the turning of the generator - the current flows in the wires in a direction that causes a force that tries to stop the generator from turning, hence the need to supply energy to keep it going.

The whole thing depends on the motion. Just holding the magnet there wouldn't do anything.

Paramagnetism would not explain it: the force would be way too small, and it wouldn't depend on the motion.

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....

What's going on here? Remember, the coin is not magnetic at all.

OK, I think I've got it figured out. Without looking at the internet or consulting notes and books about electricity and magnetism, here's what I remember:

If a conductor is moved through a magnetic field, there will be a voltage induced in the conductor. That's what we have here. The voltage in the conductor (coin) will cause a current to flow inside the coin.

The direction of the induced current will be such that it causes a force that opposes the motion. (I think this is called Lenz's law, but I'm not allowed to look -_- .) Here, the "motion" is the relative motion of the magnet and the coin, so the induced current will oppose this relative motion - that is, it will try to prevent the magnet and the coin from moving relative to each other. That means it will try to keep the coin moving along with the magnet - in other words, oppose the relative motion.

I'm thinking that the induced current in the coin is just circling around.

And this is the same phenomenon that, in a generator, gives rise to the force that opposes the turning of the generator - the current flows in the wires in a direction that causes a force that tries to stop the generator from turning, hence the need to supply energy to keep it going.

The whole thing depends on the motion. Just holding the magnet there wouldn't do anything.

Paramagnetism would not explain it: the force would be way too small, and it wouldn't depend on the motion.

One thing which I have not yet seen discussed is the relative strength of the mover (hands and fingers) of the magnet , as well as the speed of the motion. For example, the grip on the magnet must be at least strong enough to oppose gravity, or else the magnet would drop and that would be the only motion. And obviously, there would be no motion of the magnet if the hand was not itself moving. Does electricity in the hand (or the human body) have any effect?

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Hmmmm... Jay P is very close here, but to be picky it is not that the conductor is moving in a magnetic field. The conductor is stationary, and the magnetic field is being swept across it.

I'm still not completely satisfied though... will wait a bit longer before I give answers out.

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Hmmmm... Jay P is very close here, but to be picky it is not that the conductor is moving in a magnetic field. The conductor is stationary, and the magnetic field is being swept across it.

I'm still not completely satisfied though... will wait a bit longer before I give answers out.

Ok, here's the answer.

The conductor (metal coin) was stationary, so there can be no Lorentz Force acting in the usual way. What is happening instead is that in each instance the magnetic field is varying with respect to time, with respect to the location of the metal coin. Faraday's Law tells us that when the magnetic field is changing with time at a location, then an electric field will be created at that location. (More specifically, the law says that the negative of the partial derivative of the magnetic field with respect to time is equal to the curl of the electric field)

So the moving magnet causes there to be a changing magnetic field at the site of the metal coin. This changing magnetic field creates an electric field, and the electric field drives electric currents within the metal coin. Now that electric currents have been established within the metal coin, the magnetic field of the magnet can act on those currents through the Lorentz Force, and this force can push the metal coin. The way it works out is that this force always pushes in a direction so as to oppose any relative motion between the moving magnet and the coin!

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For those who may be interested in the "fill in the blanks" exercise that I posted earlier in this thread (#60), the Wikipedia topic that I was referring to is "Eddy current." The Wikipedia article begins as follows:

"An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body of the conductor. These circulating eddies of current create induced magnetic fields that oppose the change of the original magnetic field due to Lenz's law, causing repulsive or drag forces between the conductor and the magnet. The stronger the applied magnetic field, or the greater the electrical conductivity of the conductor, or the faster the field that the conductor is exposed to changes, then the greater the currents that are developed and the greater the opposing field."

I believe this is essentially the same as Carlos' explanation. In conductors, the outermost electron in each metallic atom is relatively free to move about from one atom to another, as long as the hopping electron remains relatively close to at least one of the atoms in the metal. The coin in Carlos' experiment moves because (ultimately) the magnet is attempting to push the electrons too far away from the "sea of atoms" to which the electrons are electrically compelled to remain close.

I trust that Carlos will elaborate further if needed. Note also the way in which this scheme conforms to other basic laws of mechanics (Newton's Laws), such as: "For every action [force] there is an equal and opposite reaction [opposing force]." The moving magnet "pushes" on the coin, and the coin pushes back (via the magnetic field produced by the eddy currents in the coin). If the coin is free to move (with only a little friction), the magnet wins the force-counter-force contest and the coin moves. If the coin is fixed in its position, it exerts more force on the magnet, which, in this example, might not be enough to be felt by a human holding and moving the magnet.

I would also advise caution in using terms like "stationary" vs. "moving" too loosely. One cannot define "stationary" except by reference to something else, and what matters in electromagnetism is relative motion between two or more bodies or force fields. It doesn't matter whether the magnet is stationary (relative to the Earth or other large object) and the coin is moving past it (as in a vending machine or magnetic brakes used on some railroad cars), or whether the coin is stationary (relative to the Earth again) and the magnet is moving past it. The forces involved will be the same. What moves or slows down as a result of the forces depends on what each entity is connected to (if anything), how firmly it is connected, how heavy (massive) it is, and how it may already be moving (if it is) relative to the whole arrangement.

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Sorry for my delinquency towards this thread; I only get random bursts of appreciable spare time, so sometimes this thread could be idle and sometimes it can be active...

NEW QUESTION

For the same reason that water spirals down a drain counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, hurricanes always rotate c-clockwise in the N. Hemisphere and clockwise in the S. Hemisphere.

Is this statement right or wrong, or a mix of both? Dissect it and find what the truth really is.

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Sorry for my delinquency towards this thread; I only get random bursts of appreciable spare time, so sometimes this thread could be idle and sometimes it can be active...

NEW QUESTION

For the same reason that water spirals down a drain counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, hurricanes always rotate c-clockwise in the N. Hemisphere and clockwise in the S. Hemisphere.

Is this statement right or wrong, or a mix of both? Dissect it and find what the truth really is.

Hardly ever. The Coriolis effect is too weak to manifest itself in a small size sink or tub.

Bob Kolker

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Depends on how close to the equator you are. Coriolis effect increases with latitude.

I have never heard of reverse rotations in the higher latitudes - except for small slower masses such as the bathtub water.

Back when we were navigating by the stars (I have long forgotten my spherical trigonometry), the movement of the aircraft over the rotating earth, displaced the bubble in the sextant. We had to correct for this coriolis effect, which varied with ground-speed and latitude. Any body moving over the rotating earth is affected the same way (force acts to the right in the N.H.)

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The Coriolis effect is too weak to manifest itself in a small size sink or tub.

When Stephen was doing his "Mr. Science from Caltech" features on the radio, he did one on the Coriols effect and said essentially what Bob is saying. He then went on to talk about the other forces that make the water go round and round when it goes down the drain.

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The Coriolis effect is too weak to manifest itself in a small size sink or tub.

When Stephen was doing his "Mr. Science from Caltech" features on the radio, he did one on the Coriols effect and said essentially what Bob is saying. He then went on to talk about the other forces that make the water go round and round when it goes down the drain.

Good point. For any body of water there are going to be forces other than the Coriolis "force" (the Coriolis effect is not a true force). For example air moving over the surface of the water and convection currents in the water due to temperature gradients. Also inertial effects from the water being let into a vessel in the first place. To see the Coriolis effect one would have to prevent air drafts or currents, make sure the water is at a uniform temperature and wait until any inertial residues from filling the vessel have died down. Even then the drain has to be opened very carefully to make sure no superfluous radial forces are being introduced. In any small vessel the water would have to be drained rather slowly to permit the Coriolis effect to act on the water for a sufficiently long time to be visible to observation. In a larger vessel like a swimming pool it could probably be made to manifest itself in a more obvious way. One thing for the sure, the detective story scene where the water in a toilet drains one way in the Northern Hemisphere and the other way in the Southern Hemisphere is pure fiction or a canard.

In any case we do not need much proof for the existence of the Coriolis effect. The cyclonic flow of large air masses is quite sufficient to show that the Coriolis is real. Also the way an artillery shell is diverted from a true course is another manifestation. But to measure this a shall must be fired to go miles or at least thousands of feet.

If I am not mistaken the the strange movement of the Focault Pendulum is yet another proof. The Pendulum has to swing for a considerable time for the divergence of its plane of swing to be readily visible.

While I am on the subject, I note the irony that Galileo invented the very thing that would prove the earth rotates (contrary to the Aristotelian-Ptolemaic view). It is of course the pendulum itself. Unfortunately physics was not sufficiently developed for Galileo to see he had the proof that would have put the Church fathers in their place.

Bob Kolker

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Depends on how close to the equator you are. Coriolis effect increases with latitude.

I have never heard of reverse rotations in the higher latitudes - except for small slower masses such as the bathtub water.

Back when we were navigating by the stars (I have long forgotten my spherical trigonometry), the movement of the aircraft over the rotating earth, displaced the bubble in the sextant. We had to correct for this coriolis effect, which varied with ground-speed and latitude. Any body moving over the rotating earth is affected the same way (force acts to the right in the N.H.)

Back in the Good Old Days of battleships, corrections had to be made in the aiming of large caliber naval rifles, like the big 16 inch guns on a battle wagon. As you point out, in the N.H. a shell fired in a northerly direction would diverge rightward and a shell fired southward would diverge to the left. In those days the shells would fly 15 to 20 miles. I think that is rather impressive. Corrections for the Coriolis effect were one of the first applications of electronic or electro-mechanical computers. Back in the days of the Great War, the corrections had to be done by looking up the location of the ship, the direction of the target and the latitude in a specially prepared gunnery table.

Bob Kolker

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Sorry for my delinquency towards this thread; I only get random bursts of appreciable spare time, so sometimes this thread could be idle and sometimes it can be active...

NEW QUESTION

For the same reason that water spirals down a drain counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, hurricanes always rotate c-clockwise in the N. Hemisphere and clockwise in the S. Hemisphere.

Is this statement right or wrong, or a mix of both? Dissect it and find what the truth really is.

After this, how about starting a new thread for each question? It makes it easier to find the beginning question.

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After this, how about starting a new thread for each question?

Because I don't want to bloat the forum with tons of threads, especially if I pose a new question roughly every few weeks.

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