Wikipedia:Reference desk/Archives/Science/2013 November 17

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November 17

Biting your tongue

If you bite your tongue or cheek with force, you get a painful area that feels "hard", as if the area swells or something like that. What generally causes this phenomenon (i.e. is it swelling, or something else?), and (if such a term exists) what's the official anatomical/physiological term for an area that's experiencing this phenomenon? Nyttend (talk) 03:23, 17 November 2013 (UTC)

When most parts of the body are subject to trauma (e.g. biting of the tongue, banging the head, trapping a finger) it is normal for two things to happen. Firstly the body has a reaction which involves extra blood being channeled to the injured area, this assists in bringing materials required to repair the area and the removal of any unwanted products of trauma. The other thing that occurs is bleeding within the soft tissue that is damaged. This causes discoloration, swelling and pain in variable amounts dependent on what and how badly the tissue is damaged. These two processes will cause the swelling you feel, the medical term for hardening of the tissue is induration. Richard Avery (talk) 07:49, 17 November 2013 (UTC)

Novel electric unicycle design

Does there exist a design for an electric unicycle, where the rider is positioned at the centre of a wheel, rather than atop it; the engine contains no moving parts; and the unicycle is steered by shifting the centre of mass along the axis of rotation? Plasmic Physics (talk) 03:59, 17 November 2013 (UTC)

I believe you have a monowheel in mind. However, note that there must be moving parts: in your proposed design, even though the center of mass is kept steadily ahead to move it forward, it has to be moving in a circle relative to the outer wheel (as is the rider). (speculation:) Using a weight in this way depends on it being massive in relation to the rider; I suspect it works out to be inefficient to push a weight to where it will move the wheel by force of gravity rather than simply pushing the wheel with the same motor mechanism. Wnt (talk) 04:22, 17 November 2013 (UTC)

Yes, it is a monowheel I'm thinking of. Plasmic Physics (talk) 05:39, 17 November 2013 (UTC)

I suppose you could set up a mag-lev connection between the wheel and the compartment where the rider sits. They would still move relative to each other, but there would be no friction (other than air resistance), since the moving parts are not in contact with each other. The rider could then lean forward to move forward or lean backward to move backward. Leaning to the sides should help to steer. StuRat (talk) 05:15, 17 November 2013 (UTC)

Yes, it is a monowheel that functions on the principle of the linear motor. To ensure that the wheel spins instead of the rider, it could be solved by increasing the difference in the moment of inertia between the wheel and "hub". Plasmic Physics (talk) 05:39, 17 November 2013 (UTC)

I meant that last sentence as a question, not as statement. Plasmic Physics (talk) 07:52, 17 November 2013 (UTC)

Yes, the moment of inertia should effect which rotates faster. I also imagine that the battery, motor, etc., would be under the driver's seat, to bring the center of gravity down. This would help prevent him from turning upside down. At high acceleration, though, it might still be a problem. Driver rotation might also occur when braking or if the vehicle hits something. StuRat (talk) 08:41, 18 November 2013 (UTC)

What if the entire vehicle was structured as a gyroscope? Plasmic Physics (talk) 12:08, 18 November 2013 (UTC)

Dynamo lights become ever brighter with increasing speed?

I made this circuit to light a couple of LEDs (in series) on my bike. The power comes from a bottle dynamo which I think is rated at 6 V and 3 W. My question is: does the light get ever brighter the faster I cycle or does something limit the output of the dynamo? The LEDs can actually handle at least an amp but I'm a pretty lazy cyclist so I'll never even approach that; I just wondered. If the rating is 3W, that must mean the limit for the dynamo is 500 mA; is that a point at which something inside the dynamo could start to melt? --78.148.107.181 (talk) 16:03, 17 November 2013 (UTC)

A bicycle dynamo is not, in fact, a dynamo, but a magneto (generator). However, although we have plenty of articles on magnetos, we don't have one which explains how they work. This paper describes the operation of an ignition magneto - a bicycle dynamo doesn't have the contact breaker or secondary winding, but it's more or less the same otherwise, and the Continental paper does imply that the output increases with speed. That being said, I assume that there will be a point where the rotor is continuously saturated and increasing the speed only increases the frequency. The best thing to do is probably just try it, and put in a voltage regulator if the output looks like it's getting too high. Tevildo (talk) 19:50, 17 November 2013 (UTC)

As speed is increased from zero, the generator voltage will increase proportionately, up to a certain point where the load (the lamps or LEDs) draws a current near the rated output. Beyond that point, two effects come into play that limit the current: a) magnetic saturation of the stator core (Tevildo was on the right track here, but it is the magnets that rotate on the shaft, the windings are on a stator - this avoids needing slip-rings) and, b) pole reluctance. Pole reluctance arises because at high currents the magnetic filed in the stator poles created by the load current opposes the magnet field as the rotor magnet poles rotate into mesh with the stator poles. This effectively lengthens the air gap between the rotor and stator, weakening the generated voltage. The limiting is not a hard or staright line limit - further increase in load or speed will increase the current to a certrain extent. 120.145.133.195 (talk) 00:45, 18 November 2013 (UTC)

So would it then be approaching an asymptotic limit? 24.23.196.85 (talk) 02:00, 18 November 2013 (UTC)

The electricity-producing thing on my old bike was called an 'alternator' when I bought it, so presumably it produces alternating current. To drive LEDs, I would consider adding add a rectifier chip and a capacitor to smooth the ripple, and would tip the bike and spin it to make sure it could not overdrive the LEDs and burn them out. Edison (talk) 01:28, 18 November 2013 (UTC)

Blood glucose result discrepancy

All my fasting glucose tests taken at a medical lab (vein blood) show above 100 mg/dL. However home test (finger prick) results are always ~80 mg/dL, no fasting, before and 1.5 hours after a big meal. How can this be explained? Gil_mo (talk) 16:16, 17 November 2013 (UTC)

I'm afraid that because this bears on your personal medical issues, an answer would constitute medical advice, which we are not permitted to give. Looie496 (talk) 16:53, 17 November 2013 (UTC)

I'm asking this on a general note. Let me rephrase: what could be the explanation for such a discrepancy should it occur for anyone doing the test?Gil_mo (talk) 07:10, 18 November 2013 (UTC)

See Glucose meter, home tests measure whole blood glucose level, lab tests measure plasma glucose level which is generally 10% to 15% higher than the equivalent whole blood value. All the glucose is in the plasma, if you remove the blood cells, the volume decreases but the amount of glucose stays the same, so the amount of glucose per unit of volume rises. Ssscienccce (talk) 11:28, 18 November 2013 (UTC)

Obvious question, are the home testing strips expired? Have they been stored in an airtight container? Have they been stored in excess heat (like a hot car)? I know these can affect the accuracy. Also, many home testing units contain a little bottle of control solution (with a known glucose concentration) that you can test your strips against. --209.203.125.162 (talk) 18:55, 19 November 2013 (UTC)

science (magnification in parallel and series circuits)

magnification is independent in series or parallel circuit.true or false? — Preceding unsigned comment added by Mintoooo (talk • contribs) 17:51, 17 November 2013 (UTC)

The word "magnification" has no meaning for an electrical circuit. If you are going to ask "true or false", you need to use correct terminology. In this case, I can't even figure out what you are trying to ask. Looie496 (talk) 17:57, 17 November 2013 (UTC)

To paraphrase Chico Marx, then, the correct answer to the question is, "No." ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:52, 17 November 2013 (UTC)

banned editor and off-topic discussion
The following discussion has been closed. Please do not modify it.
It would be nice if people would answer questions only if the question is within their competence. Magnification certainly does have meaning in electical circuits containing inductance (L) and capacitance (C), and is of fundamental importance to radio. It is the basis of tuning. Magnification occurs in parallel L and C circuits - at or near the resonance fequency, the current circulating in the L-C circit may be many times greater that the externally supplied current. This phenomena is referred to as magnification. In a series L-C circuit, the voltage across either L or C at and near resonance may be many times greater than the externally applied voltage - this is also referred to as magnification. The degree of magnification depends on the Quality of the inductor and capacitor. Quality here essentially is degree to which resistance is kept out of the inductor and capacitor. For this reason, various companies in Britain, before British industry lost its way, used to sell instruments known as Circuit Magnification Meters. Marconi being the most well known in the industry (Model TF1245A). In the USA, the various manufacturers (eg Hewlett Packard) called them Q-Meters - this name is easier to say and has become the dominant term world-wide. However the correct term for what happens in resonant circuits is still magnification. To answer the OP question, magnification is independent in magnitude regardless of the circuit being series or parallel - however what gets magnified, voltage or current, depends on whther the circuit is series or parallel. Magnification can also occur in circuits containing only resistors and capacitors, and in circuits containing only inductors and resistors. Magnification of either voltage OR current, but not both at the same time, can be arranged by combining current OR voltage in suitable phases. However such circuits are not simple series or parallel arrangements. It can be done becasue the arithmetic sum of voltages in a series RC circuit is greater than the applied voltage, due to differing phase. 120.145.133.195 (talk) 01:00, 18 November 2013 (UTC) Ah Wickwack, it's always such a pleasure to have you around. Looie496 (talk) 05:13, 18 November 2013 (UTC) Unfortunately, he shouldn't have answered, as this is an obvious homework question with the student making no attempt to solve it on their own. StuRat (talk) 05:19, 18 November 2013 (UTC) I am the previous poster 120.145.133.195. It had occurred to me that it might be homework, but it doesn't smell like it. The OP wrote it in quite poor grammar. A question written in a textbook or set by a teacher could well use the term "magnification", but they would not (or should not) word a question as "Magnification independent in series or parallel circuits..." Independent OF what? Independent IN one case and not IN the other?? Quite possibly the OP is a beginning electronics hobbyist or amateur. If so, it was very bad of Looie496 to say that the word magnification had no meaning in electric circuits - rather than helping such a person, it would set them back. That was my reason for posting - to prevent a beginning amatuer from being misled. In any case it's ok to just not answer a homework question, but it is never ok to mislead, intentionally or accidentally. And I am not Wickwack. 58.170.182.209 (talk) 13:38, 18 November 2013 (UTC) Ah, then it's an amazing coincidence that you geolocate to Perth and write in the same style as Wickwack, with the same mannerisms. (I do make mistakes sometimes and am willing to acknowledge them, but it is always hard when they are pointed out in such an unpleasant way -- the typical Wickwack style.) Looie496 (talk) 16:17, 18 November 2013 (UTC) Probably best to let this lie, but there may indeed be more than one cranky old electrical engineer in the Perth area. I'd imagine their styles and tones could come across rather similarly in this medium ;) SemanticMantis (talk) 18:10, 18 November 2013 (UTC) Wickwack and aliases are more known for criticizing other responders for being wrong, regardless of whether he, or the person he criticized, are correct or not. Looie, sorry your pride was hurt. I suggest using this method: "I've never heard of 'magnification' with regard to an electric circuit". This leaves you an out, in case it turns out to be real. And even an expert may not be aware of some obscure terminology used by a small group on the other side of the globe. StuRat (talk) 18:56, 18 November 2013 (UTC)

I added a useful title. StuRat (talk) 06:03, 18 November 2013 (UTC)

We have some scant coverage at LC_circuit#Applications, which uses the "magnification" terminology, and mentions the tuning discussed above. SemanticMantis (talk) 18:10, 18 November 2013 (UTC)

Most likely because the term is most likely and commonly called resonance and specifically electrical resonance. --DHeyward (talk) 05:30, 19 November 2013 (UTC)

banned editor and focused reply
The following discussion has been closed. Please do not modify it.
Actually, both terms are in use - there is an important difference in meaning. Resonance is what allows magnification to occur. Resonance also causes other phenomena to occur - for instance when a parallel L-C circuit is brought into resonance, a dip (reduction) in the current drawn from the external voltage source occurs, and the phase of the current drawn from the supply is brought into alignment with the supply voltage phase. An analogy is cooking food. Applying heat typically causes several changes - reduction in moisture content, browning, reduction in vitamin content, denaturing of protein, etc. Just as one does not use the term "heating" or "cooking" when one wants to discuss "browning", an electrical engineer does not use the term resonance when he/she means magnification, any more then he/she would say "brought into phase" if he/she meant magnification or resonance. Magnification can be quantified - one can say such-and-such a circuit has a magnification of (say) 96 times. Saying the resonance is 96 times is a nonsence - you can only say things like "off resonance", "near resonant", "resonant" and the like. Further, as I said before, magnification can occur in L-R or C-R circuits, but in these cases there is no resonance in the sense that electrical engineers use the term. 120.145.154.249 (talk) 06:45, 19 November 2013 (UTC) Never heard "magnification" used in engineering. "peaking", "resonance", "Q", "Q factor", "FWHM", "magnitude", etc. Never heard of "magnification." Some passive filters can peak, but I've never heard it called magnification. It would be an odd term as I believe the maximum is always referenced as 0db. --DHeyward (talk) 07:12, 19 November 2013 (UTC)

References: New Approach to the Digital Measurement of Magnification Factor

Marconi TF 1245 circuit magnification Q meter, the instrument identified by the first correct responder

(image) 84.209.89.214 (talk) 22:37, 19 November 2013 (UTC)

Why “g = GM/d^2” prevail over the higher derivatives of position w.r.t time?

Although higher derivatives of the position are not discussed by the Newton while formulating “F = GMm/d^2” (where reduction in “d” is lopsided for the falling of small objects on earth) but verily the subtle nuances of such spawning can easily be perceived through arcane reasoning if imagined two identical planets of mammoth masses “M” separated by conspicuous on-center distance “d”. Since both masses "M" are under the influence of gravitational force "F" therefore reduction in “d” would be equal but abrupt on either side of point “c” lucidly if “c” is the mid point of “d" as both F = GM^2/d^2 and "g =GM/d^2" are time varying.

The simultaneous abruptness of such motion starts equally on either side of "c" right at the inception of "g" of each M which sent the aforementioned masses “M” swiftly into different higher types of motion such as gravitational jerk, jounce, crackle, pop, lock, drop etc. This means two objects that fall towards each other are under the influence of higher types of gravitational motion/ higher derivatives of position w.r.t time - Theoretically.162.157.235.1 (talk) 23:24, 17 November 2013 (UTC)EEK

Under Newtonian gravity, there are no higher derivatives of position due to gravity. The acceleration really is a=GM/d^2, and the third, fourth, fifth, etc. derivatives of position are all zero. --Bowlhover (talk) 02:16, 18 November 2013 (UTC)

Not correct in general. If d is not constant then a is not constant either, so higher derovatives of position such as da/dt are not zero. Only exception I can think of is a circular orbit when the magnitudes of d and a are constant (although even then they are not constant when considered as vectors). Gandalf61 (talk) 10:05, 18 November 2013 (UTC)

But this hold true only when the difference in two masses is extremely gigantic. For example it is said that both earth and the Newton's apple (smaller objects) accelerate toward each other gravitationally but apple looks a lot to the earth due to its greater acceleration as compared to the earth toward an apple which is so minuscule to be distinguished. Here in this case the on-center distance “d” reduces totally on one side of the mid point of “d” as earth remains at its position (almost) therefore a=GM/d^2 is noticeable but the picture is totally different according to the equation “F = GM^2/d^2” if both the masses are mammoth as illustrated where on-center distance “d” reduces equally on both sides of the mid point of “d” therefore aren’t the said two objects under the influence of type of gravitational motion which is greater than “a=g=GM/d^2”162.157.235.1 (talk) 04:45, 18 November 2013 (UTC)EEK

The force acting on each object is F=GM^2/d^2. Therefore, by F=Ma (the definition of force), a=GM^2/Md^2 = GM/d^2. Since the acceleration varies with position (and position with time), there will be higher derivatives w.r.t. time, but these just describe how the acceleration changes, it's not correct to say that jerk "prevails over" accelration, instead, each of jerk, acceleration, velocity etc are used to describe how the position of the masses changes. Acceleration just tends to be the most used representation, since higher derivatives tend to be less mathematically useful. Also, in the graviational case, aceleration can always be expressed purely as a function of position, with no initial condition needed for velocity, but higher deivatives will require at least such conditions to be calculated accurately. (da/dt = da/dx . dx/dt = v.da/dx = -2vGM/d^3 in the case where the velocity is parallel to the acceleration). 91.208.124.126 (talk) 09:38, 18 November 2013 (UTC)

Wouldn't the said both identical masses in scenario collided each other earlier than expected in acceleration?74.200.19.65 (talk) 11:20, 19 November 2013 (UTC)EEK