Wikipedia:Reference desk/Archives/Science/2013 August 10

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August 10

Magnetic lines of force

Why do iron fillings align themselves along the magnetic lines of force of a bar magnet, even if lines of force are imaginary? Publisher54321 (talk) 08:16, 10 August 2013 (UTC)

I believe the filings conduct the magnetic field better than the air, so this tends to cause them to clump together, aligned in the direction of the magnetic field, since the magnetic field is stronger by other iron filings than away from them. StuRat (talk) 08:37, 10 August 2013 (UTC)

Magnetic field lines talks some about it. DMacks (talk) 10:06, 10 August 2013 (UTC)

The linked section seems to do little more than mention the phenomenon. Iron filings themselves become magnetically polarized in a magnetic field – either temporarily or permanently. If an individual filing is considered, this magnetic polarization is dominant along its long axis (if it has one, i.e. is not spherical). The polarisation generates its own magnetic field, which interacts with the overall magnetic field in such a way as to orient the long axis of the filing with the field if it can rotate freely. There is no preferential positioning of a single filing though: it will simply orient itself by the field, and not move, unless there is a magnetic field gradient, in which case it will experience a small force in the direction of the increasing gradient (assuming that its polarization is allowed to align freely). The gradient is generally strongest nearest a dipole source (like a normal bar magnet). This is why the force between two magnets increases so markedly when they get very close together: this is where the gradient of the field of a magnet is highest, and the force increases with the inverse cube of the distance (if I have not misremembered it).

Each filing, being magnetically polarized, generates its own tiny dipole field, which is superimposed on the surrounding magnetic field. This creates local gradients in the magnetic field such that there are effective forces between proximate filings. A simple way to think of it is that the filings all become aligned magnets due to the external field, an then they act as you'd expect many little magnets to act: they form chains of north-to-south magnets sticking to each other. The striations formed by clustering of adjacent chains is a little trickier, but is probably attributable to irregularities in the chains that causes them to stick together, leaving gaps between clusters, thus forming the striations. — Quondum 12:30, 10 August 2013 (UTC)

To put it more completely yet more concisely: The reason the filings form spaced lines is a) as Quondom said, as each filing become magnetised by the field, it increases the local flux density attracting filings ahead and behind it in a (North-South)-(North-South)-(North-South) chain as opposite poles attract, and b) the field is thus weaker between such chains, this establishing a gradient that makes locations between chains unstable, and because like poles repel. 121.215.72.7 (talk) 13:25, 10 August 2013 (UTC)

Not entirely: the spacing instability depends on non-uniform chains. Several closely (but not necessarily exactly evenly) spaced steel wires, all in and parallel to a uniform magnetic field, will experience no forces between them. In this, there is no field gradient between the wires (although the field strength between wires is reduced compared to no wires). A filing floating between them will also experience almost no force (other than orientation). A very small residual attraction will exist for the filing to the nearest wires though: its polarization will alter the polarization in the nearest parts of the wires, which is to say, with the uniform magnetic field removed the filing will experience the same attractive force to the adjacent wires by interaction with the polarization that it induces in them as a tiny permanent magnet of the same polarization. This depends on the filing not being part of a uniform chain (like another wire). Because the chains are not uniform this is not an accurate description, but it gives a feel for the mechanisms at work. — Quondum 14:10, 10 August 2013 (UTC)

Please update Magnetic field lines to clarify/expand. DMacks (talk) 08:03, 11 August 2013 (UTC)

We know that a tangent drawn from any magnetic line of force gives the direction of magnetic field. Suppose I have a figure showing a bar magnet and direction of magnetic field around that magnet. Can I call this arrangement a vector field? I asked this question because I am not able to understand what really a vector field is. Publisher54321 (talk) 14:38, 10 August 2013 (UTC)

In physics, a vector field is a mathematical tool that lets us define a vector at every point. So, it is a function whose inputs are coordinates (x,y,z); and whose output is a vector, also expressible as coordinates (x,y,z). Usually, we put extra constraints on that function to make sure the function is continuous. And if there are any mathematicians around, we have to refine our terminology in order to be much more precise: using stricter mathematical terminology, there's a distinction between the field itself, and the convenience function that defines it. In very elementary physics, this type of abstraction is not very useful. But as you study more advanced physics, it quickly becomes important, even in the study of magnetic fields; particularly when you study transforms, geometries, and gauge abstraction.

It is worth emphasizing that solid understanding of mathematical concepts and terminology are absolutely prerequisite for studying physics. If you want to know and understand magnetic fields, you need a good understanding of the ideas of vectors, multivariable functions, and the calculus as it applies to them. If you are truly not able to understand those concepts - if you don't enjoy those concepts, and you're not the kind of person who wakes up in the middle of the night from dreaming about differentiating multivariable fields - then maybe physics is not for you.

Finally, I don't know who told you that lines of magnetic force are imaginary. They are invisible, insofar as the human eye cannot see them. But magnetic fields really are there; they do exist, they do have a force and have energy associated with them; they can do work (exerting a true force over a distance, causing physical objects to move). In the case of iron filings aligning with a magnetic field: the iron filings are arranging themselves according to the lowest energy they can get. The iron is ferromagnetic; it has a magnetic moment; so each granule interacts with the field; this can cause a torque, and can move the particle. The work to perform this motion comes from potential energy: somebody first did work against the magnetic field in order to place an iron filing in a geometric arrangement with a higher energy state. When you scatter or sprinkle iron filings, you might not be paying careful attention to how much work you're imparting to each grain: but just as you did work against gravity to lift the filings, providing energy for their motion as they fell back toward the table, you also did work against the magnet, providing the energy for them to rotate. It's a tiny amount of energy, but it's not imaginary at all. Add in some complex interactions to account for air resistance and collisions between grains, and there is so much perceivable randomness that you don't notice the work you're expending. Nimur (talk) 12:13, 11 August 2013 (UTC)

And that, friends, is what is great about this reference desk. -- Scray (talk) 13:53, 11 August 2013 (UTC)

Just a small but very important correction to the last part of Nimur's post that directly impinges on the original question by Publisher54321: While magnetic fields are most certainly real physical phenomena, lines of magnetic force are an imaginary conceptual tool that is useful in modelling or the design of magnetic devices, and are reflected in obsolete (non-metric) units. There is no texture or segregation onto lines in magnetic fields in free space or in homogenous magnetic materials - lines have no physical reality. The total magnitude of a magnetic field penetrating a given area is expressed in non-metric systems in units of lines, cf weber in SI. When showing the distribution of magnetic fields in magnetic circuits of some complexity, it is useful to draw magnetic field lines, much as it is useful to draw stream lines in a diagram of air passing over a wing for example. Doing so is not taken to imply the the air is confined to the lines. 121.221.72.86 (talk) 15:32, 11 August 2013 (UTC)

Another minor clarification: the link target in "using stricter mathematical terminology" as provided is not appropriate: the term field (as defined in mathematics) and the term field (as defined in physics) are unrelated. It is the second meaning that we are dealing with. — Quondum 16:14, 11 August 2013 (UTC)

Well, both definitions of "field" are related; it's just difficult to explain how - unless you're intimately familiar with the special mathematical language of set theory. I'm sure one of our better mathematicians could express why a scalar field (as we use in physics) is a special case of some abstract commutative ring over the set mapping R3 to R - but more to the point, they will probably have a difficult time explaining why that relationship is helpful for analysis of worldly problems. (I jab at the mathematicians in jest, but at the end of the day, I readily acknowledge that they are often more correct than the physicists). Nimur (talk) 17:45, 11 August 2013 (UTC)

Hmm. Your obfuscatory response makes me wonder about what you are trying to achieve. — Quondum 18:26, 11 August 2013 (UTC)

I guess that my point was: anybody who is qualified to understand the relationship between fields and fields is already a mathematician and already knows where to look for answers. Everybody else probably doesn't care about such details - even though the details are more correct. As physicists and engineers, we just need an occasional reminder that we're glossing over things for the sake of simplicity and succinctness. I guess that my post was a little obfuscatory for a reference desk answer. May I recommend A Transition to Advanced Mathematics - a very tiny book with very few pages-per-dollar, but still well worth the cost, for anyone who wants better answers about mathematical terminology than I could ever provide? And for everyone else, a scalar field is just a function with three inputs. Nimur (talk) 19:27, 11 August 2013 (UTC)

Unless my memory completely fails me, that they are both called "fields" is accidental and that they do not go by the same terms in every language. Would you kindly elaborate on what the connection between them is? Even if advanced/incomplete, please do. More importantly, is it that there is simply some connection between the algebraic and the geometric objects or is it a foundational connection? There's weird connections between just about every object these days, what makes this one special besides the identical names?Phoenixia1177 (talk) 08:15, 13 August 2013 (UTC)

To address the OP's second question succinctly, "Can I call this arrangement a vector field?": Yes, you can think of it as a vector field, which is how it is often treated (even mathematically, as in the vector treatment of Maxwell's equations). — Quondum 16:27, 11 August 2013 (UTC)

How many kinds of blood cells are there?

I have red in the entrey of blood cells that there are three general catgories: Red blood cells (leucocytes), White blood cells (thrombocytes) and platlets (trombocytes). I can understand from the word GENERAL that there are more kinds of blood cells, it says that there are sub-categoies of blood cells. So, how many kinds of blood cells are there in all? (In the Hebrew Wiki there are 10 kinds of blood cells: Two of them are belong to the Red blood cells, and seven of them belong to White blood cells, and one of them is a platelet. these are the kinds: 1.Erythrocytes. 2. Reticulocyte. 3. Neutrophils. 4. Eosinophils. 5. Basophils. 6. Macrophages . 7.lymphocytes . 8. Monocytes. 9. Phagocytes . 10. platlets ). So, is it correct to say that there are 10 kinds of blood cells or there are more or you have another something to say about... Thank you מוטיבציה (talk) 10:22, 10 August 2013 (UTC)

You've got things a bit confused. The usual classification is:

Red blood cells, or erythrocytes. Reticuloctes are young erythrocytes.

White blood cells, or leucocytes. There are three main classes:

Granulocytes, with three subclasses:

Neutophils

Eosinophils

Basophils, which are also called mast cells

when they leave the circulation and enter

the tissues;

Lymphocytes, consisting of three main groups:

B cells

T cells, including

Cytotoxic T cells

Helper T cells

Memory T cells

Regulatory (or suppressor) T cells

Natural killer cells

Monocytes, which are which are called

macrophages when they leave the circulation

and enter the tissues. Together, monocytes

and macrophages are refered to as phagocytes.

Platelets or thrombocytes. Not really cells strictly speaking, but rather cell fragments. We have articles on all the terms used above. Look at them for further information. Dominus Vobisdu (talk) 10:54, 10 August 2013 (UTC)

-

-

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Well, if I understand you properly, there are 10 kinds of blood cells, here is the acount:

Red blood cells (reticulocytes are young cells)- It's one or two in the acount.

White blood cells incloud three main classes:

granulocytes with three subclasses:

1. :Neutophils. 2.Eosinophils. 3. Basophils (mast cells).It's plus three to acount.

Lymphocytes, consisting of three main groups:

B cells

T cells, including 1.Cytotoxic T cells. 2.Helper T cells. 3.Memory T cells. 4. Regulatory (or suppressor) T cells. 5. Natural killer cells.

Monocytes (macrophages/phagocytes). It's plus six to the acount.

Platlets - It's plus one to the acount.

In all, there are 10 kinds of blood cells. (I did not understand why you don't acount platlets as blood cells). Thank you for the help :) מוטיבציה (talk) 14:25, 10 August 2013 (UTC)

No. Monocytes are not under T lymphocytes. And the answer from your question ranges from three to a big bunch, depending what you mean by "kinds" of blood cells. I already explained why platelets are not cells. They are cell fragments. Dominus Vobisdu (talk) 14:34, 10 August 2013 (UTC)

I made order again in my things, and monocytes no more under lymphcytes. In regard to the platlets, I understnad your things, but I ment that I don't understand why some books and wiki itself call it "cell" (in the hebrew wiki it's called expressly: blood cell), so I wondered if what that you say is it not known to all above mentiond? מוטיבציה (talk) 15:27, 10 August 2013 (UTC)

It is a common misconception for platelets to be called cells; they are cell fragments that lack nuclei, as reflected in our article on the topic. It is appropriate to ask whether the same applies to red blood cells, but the latter are often nucleated (e.g. in infants, people with functional or anatomic asplenia, and other vertebrates) whereas platelets are not nucleated and are produced by pinching off cytoplasm from a megakaryocyte (rather than cell division). -- Scray (talk) 15:51, 10 August 2013 (UTC)

Further, it might be tempting to call anything membrane-bound in the blood a cell, but then what to call exosomes, apoptotic bodies, and enveloped viruses that might be found in the blood? Platelets are better grouped with "cells" than with "plasma", but they are even better described as cell fragments. -- Scray (talk) 15:57, 10 August 2013 (UTC)

Should we assume that the original question refers to human blood cells, or does it embrace the blood cells of all creatures having blood?

—Wavelength (talk) 16:45, 10 August 2013 (UTC)

It's pretty much the same for all vertebrates, at least as far as the main groups and subgroups of blood cells go. The last evolutionary event was the divergence of lymphocytes from monocytes, which might have occured soon after fish evolved, or perhaps soon before, so it is possible that some primitive fish like sharks and lampreys may not have lymphocytes, though I'm not sure. Basophils are perhaps an even younger derivitave of the granulocyte prototype, and whether fish have them or not is not settled. Originally, all the WBC types evolved from a phagocytic ancestor in invertebrates. Dominus Vobisdu (talk) 17:37, 10 August 2013 (UTC)

It splits further: Th0, Th1, Th2, various combinations of cytokines... no two cells are really -precisely- alike, and sooner or later, someone finds a way to distinguish them. And all the ways matter, once you understand them. Wnt (talk) 04:04, 12 August 2013 (UTC)

Main Battle Tank

My question is about Leopard 2 Tank when looking at the image in the picture of the link downward you find that part number 2 is (Main Hydraulic Pump), what is the purpose of this pump and why this pump does not exist in other tanks like M1 Abrams or T-90 -as I expect- http://imageshack.us/a/img811/4895/leopard2wieakoncpecja.png Tank Designer (talk) 11:10, 10 August 2013 (UTC)

Well, the M1 Abrams does have a hydraulic pump, but it's located in the hull rather than the turret. The main purpose of the hydraulic pump, as I understand it, is to move the turret and gun. Looie496 (talk) 14:49, 10 August 2013 (UTC)

Thank you very much but where does the pump exist in T-90 there is no where to place it-as expect only - as yo know it is very cramped from inside 86.108.126.173 (talk) 20:49, 10 August 2013 (UTC)

Actually I know hardly anything about tanks. My skills lie in the direction of knowing how to find information on the internet, not in the direction of knowing about tanks. Regards, Looie496 (talk) 15:35, 11 August 2013 (UTC)

If it isnt in the article its probably secret. --Kharon (talk) 02:07, 13 August 2013 (UTC)

on difference between serum and plasma?

I have red that the main difference between serum and plasma of the blood is coagulation factors (proteins of cougulation). Serum is without these couglations factors, and plasma is with it. Why is it needed to take out the couglation factors? what is the profit/ adventage/ use in the serume that we can not do with plasma? Thank you. מוטיבציה (talk) 14:58, 10 August 2013 (UTC)

There are many differences between serum and plasma, some of which affect laboratory assays; this is sometimes called matrix effect (a consistent effect on an assay of the matrix from which the sample is taken). Because matrix effects are hard to predict, and lab assays are developed empirically, an important principle is to stick with the matrix (specimen type, e.g. serum or plasma) for which the assay has been validated. Examples of dominant influences on assays include: (i) fibrinogen, present in high concentration in plasma but nearly absent from serum, can interfere with interactions of other proteins; and (ii) anticoagulants such as EDTA used in plasma collection but absent from serum, can interfere with assays that depend on cations like Ca²⁺. If you search for some of these things, you may find some more examples and specific references (I may do the same if others don't). -- Scray (talk) 15:24, 10 August 2013 (UTC)

• some references: review articles PMID 21400551 PMID 17377775 PMID 15915347; example 1 - differential stability in stored serum vs plasma PMID 23570966; example 2 - differential effects of heparin, citrate, and EDTA anticoagulants in plasma PMID 23473258. -- Scray (talk) 18:46, 10 August 2013 (UTC)

String bass v fretless bass guitar

What is the principal reason for the marked difference in tone between a double bass (or an electric double bass) and a fretless bass guitar? The only differences I can think of are the string thickness and length. So how do these factors affect the tone so much and cause the double bass to give a much deeper sounding richer tone?--86.177.63.179 (talk) 18:51, 10 August 2013 (UTC)

If you're comparing an acoustic double bass with an acoustic bass guitar it's because the double bass is a much bigger instrument with a larger sounding box and so it produces a deeper sound in the same way that a longer string produces a deeper note than a short one. An electric bass and an acoustic double bass can't really be compared as the sound from the electric bass is produced in an entirely different way and then manipulated electronically to give the desired sound. Richerman (talk) 22:59, 10 August 2013 (UTC)

The strings on electric basses and double basses are tuned to exactly the same pitch. So that cannot be the reason for the difference in sound. Doesnt anyone have any suggestions?--86.177.63.179 (talk) 14:28, 11 August 2013 (UTC)

Tuned to the same pitch does not mean things sound exactly the same. Richerman is right above: the size of the sound box will change the timbre. For instance, see the selection of pictures at mandolin. The round-back mandolins will sound much fuller than the flat backed ones, and so will basses with different body types. I also agree with Richerman that it makes more sense to compare within electric of acoustic groups, not between, because the method of sound production is so different. But maybe you mean to compare two basses that are identical in all aspects, except for fret/no fret? In that case, have a look at Bass_guitar#Fretted_and_fretless_basses, which describes some of the differences that come from lack of frets specifically. SemanticMantis (talk) 14:41, 11 August 2013 (UTC)

If you put the same strings on a Stradivarius and a cheap mass produced violin you will get a completely different sound - otherwise why would anyone pay millions for a Stradivarius? It's the whole instrument that produces the sound, not just the strings. Richerman (talk) 15:34, 11 August 2013 (UTC)

Yes, exactly. The frequency at which the strings vibrate will not change - but the overtones and undertones produced by all of the complicated vibrations of the rest of the instrument and the air contained in it's sounding box will be quite utterly different. Since those sounds dominate our perception of the frequency, we hear different notes. Particularly, if the string sounds (say) a middle C, the resonances through the instrument will produce a C note an entire octave down from there. On some instruments, the volume of that lower C will exceed the volume of the middle C produced by the string - and in other instruments, the middle C will be louder. Our ears would then perceive the first instrument as producing a deeper bass sound than the second. But that's a vast over-simplification - every part of the instrument vibrates - including the other strings and even the body of the musician that's playing it. The resulting vastly complex soundwave is what we hear and it's hardly related to what the string produced. Think of the string not as the object that's producing the sound - but as the stimulus that causes the remainder of the instrument to produce the sound...that's a more accurate physical description of what's happening. SteveBaker (talk) 16:53, 11 August 2013 (UTC)

Though, I would bet there are no double-blind studies comparing the tone of a Stradivarius and the tone of a reasonably-good (but not famous) instrument. Those violins command an immense price, but not only because the Stradivarius is of excellent quality. It is also a luxury good; an artificially scarce commodity whose exact scarcity is defined by brand identity (not by the actual rarity of the constituent pieces of the instrument); it is a premium price, and the higher the price, the more valuable and rare the violin becomes. Stradivarius instruments command a high price because they are expensive - a bit of ouroboros-esque logic that is also seen in the retail-pricing of diamonds and gemstones. The instrument only need be of sufficient quality to maintain its brand identity, but not to actually compete with other instruments in specific quality metrics like musical timbre and tone. With today's near-perfect machine-tools, and the cheap availability of raw materials of all sorts - in principle, we should be able to import any desired exotic wood and varnish and so forth - and we could manufacture a violin that matches Stradivarius' every detail, and sounds exactly the same in every respect; yet, we would not produce an instrument that the marketplace for antique instruments would perceive to be equally valuable.

The very same affliction strikes modern instruments: a Rickenbacker can sell for ten or a hundred times the retail price of my Peavey electric bass. The real reality of it is, nobody can tell (by listening) which brand of guitar Paul McCartney played; but because he played a Rickenbacker, it must have been the best... Nimur (talk) 18:15, 11 August 2013 (UTC)

Steve Baker makes an obvious point about overtones. Im not sure how an Undertone can be generated on a bass. In fact it cant. Obviously the body on a bass fiddle will make a difference to the sound, but what about an EUB. A 3/4 size EUB with 41 in long scale (and thicker strings) sounds completely different from a fretless bass with a 35in scale . Why should that be?--86.177.63.179 (talk) 18:40, 11 August 2013 (UTC)

Note, though, that the quality of a musical instrument, especially any sort of guitar, is not merely what sort of tone it has. Other qualities are how easy is it to play - some need tougher fingers than others; how well does it stay in tune; can it in fact be put properly in tune? Cheap electric guitars can have twisted or bowed necks and thus no amount of twidling of the tuning screws will put them right. Double truss and steel framed guitars, including the Rickenbacker, stayed in tune under 1960's and 1970's TV studio hot lights - that is why they were highly regarded. 121.215.13.245 (talk) 00:35, 12 August 2013 (UTC)

The studies Nimur describes have been done and are even mentioned in our article Stradivarius#Comparisons_in_sound_quality. Vespine (talk) 02:50, 12 August 2013 (UTC)

Yeah I think this thread is veering off the point. Im not talking about subtle differences between different instrument makes but the pretty damn obvious difference in sound between a real bull fiddle and a fretless bass when both are played pizzicato. Is it something to do with the strings buzzing against the fingerboard more on the bull fiddle because of the thicker strings or the greater tension?86.177.63.179 (talk) 15:18, 14 August 2013 (UTC)

Ok well to get back on point, the most obvious differences are that your finger acts like a dampener at the end of the string on a fretless instrument rather then a rigid anchor on one with frets. Also very small and subtle movements of the finger against the fret board will affect the strings vibration and tamber and the precise placement of your finger, even by fractions of a milimeter will cause the tone and pitch to be slightly different, whereas with a fretted instrument you have a fixed anchor point where it doesn't really matter precisely where you placed your finger. Vespine (talk) 00:32, 15 August 2013 (UTC)