Science desk
< December 21	<< Nov \| December \| Jan >>	December 23 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 22[edit]

Electric Kettles[edit]

Just to clarify, by an electric kettle, I mean a device into which you pour water, turn it on, and then it boils the water and turns off automatically, not a device which maintains water near boiling point.

One day I wondered how a home-variety electric kettle knows when the water is boiling, so as to stop operating.

A few tests showed that (at least the four or five different kettles I tested, all of different brand and make) there is a small hole inside the kettle, near the top (above the maximum amount of water line), which is evidently intended for the steam produced by boiling the water.

I verified that assumption with the following three experiments.

Firstly, I removed the lid of the kettle, and started the boiling process, and waited. The kettle didn't stop heating the water even as it was boiling, and then almost immediately after closing the lid, the kettle shut down. I repeated this process several times to make sure it wasn't just chance that immediately after I replace the lid the kettle shut down.

The next experiment was to remove the lid of the kettle, and cover it, so that the small hole is outside the closed area. Again, several times, the kettle never stopped, until I removed the covering, and then closed the lid.

The final experiment was to cover the kettle in the same way as the previous experiment, except to include the hole in the closed space. This time, the kettle stopped by itself.

My conclusion is that the kettle's stopping mechanism relies on the small hole near the top of the device.

I raised three hypotheses as to what mechanism is actually used:

Testing conductivity - Air is very resistive, perhaps air with steam is a better conductor
Pressure - As the water is heated, steam is generated, increasing air-pressure inside the kettle
Temperature

The question is, what IS the mechanism? I haven't gone as far as taking apart a kettle, but I did try to find the answer on the web, and neither wikipedia nor HowStuffWorks had an answer, and web-searches gave only one answer, but the site in which it was seemed dubious.

--89.0.134.155 12:26, 22 December 2006 (UTC)[reply]

Just too say I too can confirm this experiment and believe the small hole to contain the 'mechanism'.87.102.11.80 12:46, 22 December 2006 (UTC)[reply]

I'm pretty sure mine works with a simple bleed-off pressure switch, the pressure build-up is similar to a whistling kettle. But I've looked everywhere and can't find anything definitive. If you go to Google Patents, you find every sort of mechanism that is possible. --Zeizmic 14:24, 22 December 2006 (UTC)[reply]

A suggestion for another experiment: put the nozzle of your hoover near the spout, and turn it on at the same time as the kettle. It should lower the pressure enough for a pressure switch not to work. What happened? yandman 15:29, 22 December 2006 (UTC)[reply]

THe most common method IMO, is a pressure trip mechanism attached to the on/off switch. You could test that simply by actrivating the on switch and blowing down the hole( keetle empty and disconnected). Souldnt take miuch presssure to trip it off.--Light current 15:44, 22 December 2006 (UTC)[reply]

OTOH, thinking about it more, after the steam has dipersed anand the kettle emptied, I cant get my kettle to switch on again. So in mine, it cant be pressure. It could be a bimetallic strip attached to the on/off switch--Light current 15:49, 22 December 2006 (UTC)[reply]

Coffee makers commonly and teakettles sometimes use a "Klixon" thermostat [1]. This is a small metal button with two Faston tabs that connect to the electrical circuits. In "drip style" coffee pots, the thermoswitch is set just above boiling point so when the main heating element runs dry, the switch heats above boiling and opens the electrical circuit to the main heater. A small heater in parallel with the thermostat now can operate, keeping the coffee in the caraffe warm. Hysteresis keeps the theromstat from re-closing until it has become quite cool (but it does cycle occasionally while the coffee is being kept warm). Percolators (and presumably, Light current's teakettle) use a slightly different scheme so the termostat can open before the water has boiled away but only after boiling has occurred for a given period of time, short for the tea kettle and longer for the percolator.

Atlant 16:05, 22 December 2006 (UTC)[reply]

Actually, I forgot to mention that I already did test blowing into the hole to see if it would trigger the switch, and it wasn't triggered, which points against the whole pressure idea.

But, thermostats seem unreliable for too - water's boiling point varies with air-pressure, and I haven't heard of special makes for people living in high altitudes (in which water boils at a measurably lower temperature, which may cause such thermostats never to trigger).

89.0.134.155 23:01, 22 December 2006 (UTC)[reply]

my friend took his broken kettle apart one day and said (hes an enigneer) theres a bimetallic strip in the switch that clicks it off —The preceding unsigned comment was added by 86.29.50.118 (talk) 01:00, 23 December 2006 (UTC).[reply]

All the recent (past 15 years) automatic kettles I've seen had this steam hole. The switch in a kettle which I converted from automatic to non-automatic was a bimetallic strip - which I assume they still are. The strip was specifically shaped to be bistable, i.e. tended to stay shut until it has absorbed enough heat to very rapidly flip over to the open state. This is where your hysteresis comes in, it is built in to stop a buzzing switch, and sparks flying, and the smell of burning plastic... The metal strip flips when it has absorbed a certain amount of heat, and this rate of heat absorption has to be greater than that which it is losing by conduction and radiation. Hot air (low thermal capacity) in relatively small volumes would not heat the strip up quickly enough to overcome heat loss , but steam would, since it gives off heat of condensation to the metal strip (but see "Even later, more breaking news" below). The pressure in the kettle at boil is not much above atmospheric, so that a kettle set for a certain pressure alone would not work at higher altitudes (Which is patently not the case, these switches do work at higher altitude. For living altitudes of 0-2000m, we cannot buy high altitude and low altitude kettles, the way we can low and high pressure tennis balls). Nevertheless, while the switch itself is not pressure activated, there does need to be enough pressure inside the kettle to cause an adequate flow of steam through the orifice to the switch, else it would simply not cycle - as when the lid is not closed properly. Late breaking news: My 20 year younger kettle flips the switch when mains power is cut, which implies that the strip is held by an electromagnet - its shape is probably not that important. This is likely to be a more recent safety feature, in that a stable closed contact (ON) can be maintained only while current flows. The over heat cut out switch (dry run protection) is mounted in the base, next to where the mains power enters. Even later, more breaking news: A point which my wife now made to me (While pointing out that my imminent application of a screw driver to the new kettle would be seen as a death wish, and that "as she loves" me she would fulfill my wish:-) is that one cannot get a steady flow of hot air from a dry kettle, so that my theory about thermal capacity is probably over complicating the issue. I will test that with a hair dryer rigged up to ventilate the switch, but Occam is often right. --Seejyb 05:50, 23 December 2006 (UTC)[reply]

Is it possible that a bimetallic strip could be used to control a much greater current by means of an electrical relay - thus avoiding 'sparks flying' when the bimetallic strip switches..87.102.4.34 10:38, 23 December 2006 (UTC)[reply]

Yes, quite likely, what with steam being involved. The bistable nature would explain the hysteresis, the sparking being less relevant. --Seejyb 14:52, 23 December 2006 (UTC)[reply]

Why are skis no longer made from wood?[edit]

I was wondering what are all the adventages that modern carbon fibre skis hold over their wooden counterparts? (Also it would be greatly appreciated if you could explain the physics behind an answer) Thanks —The preceding unsigned comment was added by 81.145.240.41 (talk) 13:00, 22 December 2006 (UTC).[reply]

A higher modulus, more strength, better formability, ease of integrating steel "edges", better durability with little care, and no rot.

Atlant 14:28, 22 December 2006 (UTC)[reply]

Consistency. Using synthesised materials means that every ski will perform exactly the same, and cannot fail due to some chance element in the grain of the wood.--Fangz 15:18, 22 December 2006 (UTC)[reply]

Hmm I believe making skis from wood can be a knotty problem but the Heroes of Telemark did it!--Light current 01:08, 23 December 2006 (UTC)[reply]

Be quiet, LC, ... or should I say schuss ? :-) StuRat 15:34, 23 December 2006 (UTC)[reply]

Textual vs Pictorial Memory[edit]

How do textual and pictorial memory compare? In the short term, and in the long term?--Fangz 15:16, 22 December 2006 (UTC)[reply]

In the brain or in computers?

Atlant 15:55, 22 December 2006 (UTC)[reply]

I suspect he means the brain; it would likely have been phrased quite differently if it was in respects to computers. --24.147.86.187 16:54, 22 December 2006 (UTC)[reply]

It varies from person to person. What would one expect of an analphabet or a blind person - I don't know. You could get some ideas at How do Presentation Modality and Strategy Use Influence Memory for Paired Concepts?, and take it from there. --Seejyb 18:22, 22 December 2006 (UTC)[reply]

I am told that people remember things better in pictures and sounds, rather than in words (spoken or written). I think intuitively most people would agree - but I have only anecdotal evidence. 220.253.91.28 01:07, 23 December 2006 (UTC)[reply]

Water displacement[edit]

Does an object in water displace the same weight of water as the object?129.112.109.250 16:35, 22 December 2006 (UTC)[reply]

If you put a bowling ball in a tub full of water, and then put a balloon (the same size as the bowling ball) in the water, they both displace the same amount of water, but they weigh a lot different. So, the two objects don't weigh the same, but what do they have in common? — BRIAN0918 • 2006-12-22 16:46Z

Archimedes Principle may be what you're looking for. WilyD 17:39, 22 December 2006 (UTC)[reply]

Short answer: yes. In Brian0918's example, the situation is different because you would have to force the balloon down. If no force is applied, it would displace its itsy bitsy teeny weeny weight. Clarityfiend 17:56, 22 December 2006 (UTC)[reply]

But only if the object does not sink. If it is made of iridium, it weighs more than 20 times the water it displaces. --Lambiam ^Talk 18:11, 22 December 2006 (UTC)[reply]

Ok, if I am understanding this, it sounds like if 2 objects have the same volume the amount of water they displace is equal regardless of their weight.129.112.109.252 18:29, 22 December 2006 (UTC)[reply]

If they float, they displace a volume of water equal to their weight. Clarityfiend 18:38, 22 December 2006 (UTC)[reply]

If they sink they displace a weight of water equal to their volume. Jokem 19:03, 22 December 2006 (UTC)[reply]

As phrased these two together have a curious chiastic structure. I'd formulate it as follows:

If they float, they displace an amount of water of the same weight.
If they sink, they displace an amount of water of the same volume.

--Lambiam ^Talk 21:16, 22 December 2006 (UTC)[reply]

If you want to know the weight and the volume of an object fill a container that will hold it to the very top with some arrangement to direct the overflow into a container with volume marked off in increments. Place the object in the water and catch the overflow. If the object floats the weight of the overflow is the weight of the object. This can be found approximately from volume: 1 kg of water is approximately one liter of water (approximately). Next then submerge the object without allowing your hand to go below the surface of the water. The total volume of overflow will then equal the volume of the object. If the object sinks then its weight can be measured by hooking the end of a rope thru a ceiling mounted pulley and the other to a big bucket. Fill the container with water until the object lifts off the ground. The weight of the water will equal the weight of the object. (Correct me if i'm wrong.) 71.100.6.152 19:45, 22 December 2006 (UTC)[reply]

I see Harvard has a cannon ball boat puzzle illustrating the above on one of their sites. Which reminded me of a story about the person who had to check whether the king's (or emperor's) gold cannons were really gold, and use immersed displacement of water for volume, and Archimedes' principle - using barges - for weight, thus calculated density. Was it in a novel? Eastern, possibly Siam?? I forget... It would make a nice reference for this question, which surely comes up every now and then. --02:14, 23 December 2006 (UTC)

Are you thinking of Hiero's crown (Vitruvius, On Architecture 9.9[2])? EricR 02:32, 23 December 2006 (UTC)[reply]

No, this was definitely cannon, but I shall research Hiero's crown and keep it as a reference for the purpose of questions. It is seems to be a similar story, and the smaller object may make a demonstration possible. The name "Shogun" popped up in my head, but I have no copy of the book. That would make the reference recent fiction, rather than old legend, and I would go for the legend angle. --Seejyb

Slight nitpick: the weight of an object which is floating actually is equal to the sum of the weight of the water displaced and the air displaced. While the weight of the air displaced is normally trivial, this isn't true for light objects, where there may be little, or no, water displaced. It's wrong to conclude from this that a balloon or other lighter-than-air object is weightless, however. StuRat 15:23, 23 December 2006 (UTC)[reply]

Something else interesting, is that the water is sometimes displaced without having anything but air in that location. Insects, lily pads, etc., which use surface tension to float "above the water", actually deform the surface of the water somewhat:

      bug
--+   ***   +---- waterline
   \  /|\  /
    +-----+

StuRat 15:29, 23 December 2006 (UTC)[reply]

Gauss (unit)[edit]

The article relates Tesla (unit) to Gauss (unit) but no where can I find pounds of force per square inch pull or repulsion related to Guass or Tesla. Can you provide a definition? 71.100.6.152 18:57, 22 December 2006 (UTC)[reply]

More info on the force exerted will be found at Electromagnet. Please ask for more info if needed beyond what is in the article. Edison 19:03, 22 December 2006 (UTC)[reply]

Perhaps it will help if I explain what I am doing... I'm drawing a diagram of a horseshoe permenant magnet on top and a horseshoe electromagnet on the bottom so that you have the shape of a racetrack. In the Java or Excel code I am letting the viewer/user enter by mean of control knobs the surface area of the permanent and electro core faces, the Gauss for the permanent magnet, the weight of the permanent magnet, the weight of anything sitting on top of it, the number of turns of wire around the electromagnet core, and the amperes and voltage being supplied to the electromagnet. The code then displays the lifting force and other parameters of the unit for the purpose of measuring the weight of the load per the amount of ampers required to keep the cores from touching. It seems like all of the necessary information is there but I keep getting hung up on what seem to be extraneous variables like air gap which might possibly be ignored. 71.100.6.152 21:17, 22 December 2006 (UTC)[reply]

Experience such as that of Michael Faraday showed that the smoothness of the contact between the surfaces had a huge effect on the lifting power due to small air gaps. The electromagnet article shows the relation of the different variables: the ampere turns, the ares, the length, etc. Not sure why you want to use a permanenet magnet and an electromagnet; it might be simpler to posit an electromagnet and a soft iron pole piece/ keeper. Faraday in the 1830s went from electromagnets lifting ounces to electromagnets lifting thousands of pounds, and did some nice parametric experiments, with parallel versus series windings, number of turns, etc. Edison 08:18, 23 December 2006 (UTC)[reply]

The reason is as a classroom, science fair or museum interactive educational software. The reason for the permanent magnet is that I'm in this demonstration levitating (repulsing) rather than attracting. 71.100.6.152 10:37, 23 December 2006 (UTC)[reply]

Use 2 horseshoe electromagnets, and allow the circuit for one 2 by switched via a 2 pole double throw switch to either attract or repel. Plus it can all be done with, say 1 D cell alkaline battery. You could also connect a separate D cell (or perhaps even AA cell) to each horseshoe magnet. This would tie in easier to electrical technology such as motors. If one horseshoe is fixed in place and the other can move toward and away (supported by a spring or a counterweight (it would move nicely as it would if it were at a fixed distance but free to rotate. If the electric supply is a D cell alkaline, it could supply 1/3 amp for hours, or 1/2 amp for a demo. This means keeping the coil resistance above 3 ohms, so a great many turns of say #28 magnet wire could go on the horseshoe. A 1 cell battery holder could be mounted on each horseshoe.Edison 19:17, 25 December 2006 (UTC)[reply]

Bromelain, the protease enzyme in pineapple[edit]

Why does pineapple, which is not made of protein, contain a protein digesting enzyme?

For my A-level biology coursework I am finding the optimum pH of Bromelain. This is an enzyme (possibly a group of enzymes?) present in pineapple fruits, (Papaya has a similar enzyme, papain). I assume that enzymes are energetically expensive for the plant to make, but I can't work out how this one benefits the plant. Some possible theories I have come up with:

-The earlier bromeliads which later evolved into the pineapple plant were insectiverous and used the protease to digest their food (if so, why is the enzyme present in the actual fruit of the plant?)

-Pineapples disperse by explosion. The protease digests part of the fruit which is made of protein to make the dispersion more successful.

I would also be grateful if anyone knows the optimum pH of bromelain. 81.154.254.217 19:07, 22 December 2006 (UTC)Siobhan[reply]

This scholarly review may give you some clues towards another theory of why plants evolved protein digesting enzymes. Rockpocket 20:05, 22 December 2006 (UTC)[reply]

Oh, and bromelain extracts typically have an effective pH from 4.0 - 8.0 and an optimal pH from 4.5 - 5.5. (see this pdf file). Rockpocket 20:16, 22 December 2006 (UTC)[reply]

This article: Multifunctional role of plant cysteine proteinases(pdf) gives an overview of the functions of all of these proteases. I gave it a quick look through, and it does suggest answers in a general way - they have many functions, but there seems to be much work still to be done. Following up on the references section of the article may help to provide specific detail. This article I cannot access the full text: Kotaro Konnoi, Chikara Hirayamai, Masatoshi Nakamurai et al. Papain protects papaya trees from herbivorous insects: role of cysteine proteases in latex. The Plant Journal. Volume 37 Issue 3 Page 370 - February 2004. doi:10.1046/j.1365-313X.2003.01968.x. The url is to an abstract. This is more pay to get science: Andreas Schaller, A cut above the rest: the regulatory function of plant proteases. Planta, Volume 220, Number 2 / December, 2004. DOI 10.1007/s00425-004-1407-2. I hope it helps, one grows weary of sifting through enzyme ads targeting the public gullibility about life saving foods:-) --Seejyb 04:07, 23 December 2006 (UTC)[reply]

I would guess that it's a protection from insects. StuRat 15:12, 23 December 2006 (UTC)[reply]