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-   -   Science quiz (http://www.gamingforce.org/forums/showthread.php?t=26342)

RacinReaver Dec 9, 2007 04:50 PM
Hmm, not sure about my answer.

Spoiler:
Put the milk in now. The same amount cold milk will always cool your tea by the same amount since your ratio of specific heats and whatnot isn't going to change, but things cooling don't cool at a linear rate, it's faster. So if the tea is very far away from ambient temperature, during ten minutes next to boiling it will lose more heat than thirty degrees over room temperature. Of course, I'd try to let the water sit in my mug for long enough that it heats the mug up a little, that way there'd be a bit of stored heat to keep the tea warm while the air's trying to cool it down. :p

katchum Dec 9, 2007 04:58 PM
Spoiler:
Right! But I don't understand the "warming up the mug thing". Why would you want to warm up the mug? That would cool down your tea. I would just add the milk directly so that the tea would be colder immediately. So the mug will be colder too, which means less warmth will be lost because of the temperature difference between mug and cold air.

Unless you mean that the mug is warmer than the fluid? This is only possible if the air has enough convection. => natural convection between air and fluid + fluid going to gas. And I think you're right because the fluid will cool down faster than the mug wall.

So 100% right. Great!

RacinReaver Dec 9, 2007 05:07 PM
Spoiler:
If you warm the mug up with the water, then when you add the milk to cool it off, the mug will actually try and heat the water back up.

I suppose in the end it wouldn't actually matter if you heated the mug with the water or not, since there should be about the same energy transfer both ways. Forgot when I was writing it that the water had to lose some of its own energy to heat the mug up. :p)

katchum Dec 9, 2007 05:10 PM
Spoiler:
Yeah it's more complex than that if you take in account convection and conduction of the mug wall and all. And we're not going into that area are we!

Fluffykitten McGrundlepuss Dec 11, 2007 06:43 AM
Spoiler:
If you left the tea bag in there for ten minutes the tea will be pretty horrible anyway. Why can't you take the tea cup and milk with you to the phone? Or even bring the phone back to the tea? Who actually still uses landlines?

A bit of common sense more often than not negates the need for scientists.

katchum Dec 11, 2007 04:07 PM
I'm not sure about this one:

I'm on earth now and see a big glass box (10 meter x 10 meter). Inside the box is vacuum.

Now someone inside the box (probably a robot) holds a feather in one hand and a metal ball in the other hand. He releases both materials. What happens?

Spoiler:
I just can't imagine a feather falling so fast. I just can't imagine it...

RacinReaver Dec 11, 2007 05:08 PM
Didn't they actually do this on the moon (or one of similar style)?

Spoiler:
They fall at the same rate because there's no air resistance. Gravity gives the same amount of pull downwards to both things, so they'll fall at the same rate.

packrat Dec 11, 2007 05:13 PM
Spoiler:
Actually it was a hammer and a feather.
YouTube Video

Hard to believe, but there you go, concrete video evidence.
Of course if you still don't want to believe it, in the related links there are several videos explaining how all the moon missions are hoaxes and this experiment never happened. ='D

katchum Dec 11, 2007 05:53 PM
We have a flame. (coming out of the cooking place with gas fuel)

You'll see a little bit of a blue color and much of the yellow color. Which one is the hottest part?

And if you have a flame, is it hotter at the bottom, at the top or in the middle?

When you warm up a metal in the dark, which color does it have? Red? Can it also turn into blue?

Fluffykitten McGrundlepuss Dec 12, 2007 11:44 AM
Spoiler:
Flames are tricky things and there's a bunch of factors that determine the hottest part. I think though that in a gas hob type flame, the hottest part is towards the middle or base, in this case the blue bit.

I don't think you can heat metal blue. I'm auditing an iron foundry at the moment and what comes out of their furnace is white to yellow. That's not much of a scientific answer though. :(

Locke Dec 12, 2007 03:17 PM
Quote:
Originally Posted by packrat (Post 549567)
Spoiler:
Actually it was a hammer and a feather.
YouTube Video

Hard to believe, but there you go, concrete video evidence.
Of course if you still don't want to believe it, in the related links there are several videos explaining how all the moon missions are hoaxes and this experiment never happened. ='D
To be specific, it was a geologist's hammer too iirc :P

packrat Dec 12, 2007 03:47 PM
Quote:
Originally Posted by katchum (Post 549586)
When you warm up a metal in the dark, which color does it have? Red? Can it also turn into blue?
Spoiler:
I believe this is more a characteristic of metals, but as you heat the metal up, yes it will start off looking red. Its been a while since I've looked into this, but I think the reason it is red is because when you add heat, the electrons of the atoms that make up the metal move to a higher energy level, and then drop down again into the lower energy states. The first electrons to step into the higher energy levels predominantly have larger orbital radii, and thus the photons released when they step back down tend to have a wavelength that JUST HAPPENS to be around the red region of the spectrum. When you add more heat energy to the metal, electrons in the lower energy orbitals start to emit photons as they step up and down, and since they come from smaller orbitals, the photons they emit are subsequently of a smaller wavelength. Unfortunately, you cannot see blue light being emitted from the heated metal because the electrons that would ordinarily do so (those in the lower energy levels) are merely joining the crowd of the rest of the spectrum, and the resultant light would at best be white. Following this line of reasoning (and I fear I'm making an ass of myself because I might be wrong) you can get different colors of light by heating different materials which have fewer electrons than those of metal, because the wavelengths of the photons first emitted will be from lower energy states, and thus a smaller wavelength.</essayquestion>

katchum Dec 12, 2007 03:58 PM
To be honest I don't know the answer this time... I always thought blue was warmer.

I also think the color you see is a function of the temperature. For a perfect black emittor. But yeah, metals are all but perfect black emittors.

YO PITTSBURGH MIKE HERE Dec 12, 2007 04:04 PM
The color of a flame can change due to a variety of things, one of the most prevalent being the material being burned. Adding certain chemicals to a flame can radically alter its color. I'm sure temperature has an influence, but it certainly isn't the sole factor.

packrat Dec 12, 2007 04:31 PM
Spoiler:
The rules for black body emissions definitely operate separately from diffusion flames. Otherwise, your typical candle flame would be burning at a couple thousand degrees kelvin, rather than a couple hundred.

As I understand, scientists are just now coming to grips with what makes flame act the way it does. I've been told since I was a wee-lad that blue flames are hotter than the rest, though I have also never heard a satisfactory explanation for why.

I would imagine that the areas of most perfect combustion will be the hottest, and subsequently this will be the areas where the mixture of oxygen and fuel are most ideal. This is of course at the bottom of the flame, since as hot air goes up oxygen-rich air from around the base is whisked in to replenish what was displaced. This air has the highest concentrations of oxygen, so fuel burns at the highest efficiencies in this region.
The color of the flame aside, I reason that the regions closest to the base of combustion are the hottest.

katchum Dec 14, 2007 06:33 AM
Two glasses of water are at the same temperature. In one of them you dissolve 2 spoons of salt.

Then you throw in both of the glasses some ice cubes. In which glass will the ice cubes melt the fastest?

Fluffykitten McGrundlepuss Dec 14, 2007 07:47 AM
Spoiler:
Adding salt to water lowers the freezing point, therefore the cube in the salty water will dissolve faster.


I've got one. If the time is 1:11, how long in minutes and seconds will it be until the hands cross again on a standard, analogue clock?

katchum Dec 14, 2007 08:30 AM
I'm not sure about that. It seems that the water will be colder in the glass with salt. I'm going to do the experiment myself, as I don't know the answer. :eagletear:

I believe you, but I'm also sceptical.

And: one hour and six minutes? I don't understand the question really. Otherwise it also could be one minute.

EDIT: I CHECKED IT!!!!

The glass with salt water melts SLOWER!!!! Anyone who can give an explanation?

RacinReaver Dec 14, 2007 05:41 PM
I think the question is posed incorrectly. I looked around a bit, and found this slightly analogous situation. It would have to be the same weight in both cups. One containing, say, 1 kg of water, and the other containing of 1 kg of water + salt.

Quote:
Freezing point of salt water « The Official MartinZ Blog

Does water boil faster if you put salt in the water?

Yes and no. If you look at how fast water boils when you add a small amount of salt to it, such as when cooking your noodles, the change is insignificant between pure water and the salted water. However, if you take two identical pots and add one gallon of pure water to one pot and one gallon of 20 percent salt water to the other and heat the two pots on identical stoves, the pot containing the salt water will come to a boil first. Surprised?

To truly answer the question, one must look at what it takes to boil a container of water.

The time it takes a bucket of liquid to boil is controlled by essentially three things. The first is how much heat or energy you put into the bucket. The second is how fast the temperature rises in response to the heat input (the liquid’s heat capacity), and the third is the boiling point of the liquid. Assuming that we can control our stoves and add the same amount of energy to each pot, this variable becomes insignificant.

The boiling point of water does rise if you add salt to it, but only by about 2°C (4°F) to 102°C (216°F). Remember, water boils at 100°C (212°F). This is an insignificant change for adding such a large amount of salt. For you science nerds out there, the boiling point increase is calculated using the “ebullioscopic” constant of water. This leads us to the important variable, how fast water or salt water heats up, or the solution’s heat capacity.

The heat capacity of water is very high. What this means is that it takes a lot of energy to raise the temperature of water 1°C; in fact, the calorie is defined as the amount of energy that it takes to heat one gram of water to 1°C. Not to digress, but the high heat capacity of water is good, especially if you live on a planet where two-thirds of the surface is covered by water - it helps regulate the global temperature.

Now back to the question. If you look at the heat capacity of salt water, you will find that it is less than pure water. In other words, it takes less energy to raise the temperature of the salt water 1°C than pure water. This means that the salt water heats up faster and eventually gets to its boiling point first.

Why does salt water have a lower heat capacity? If you look at 100 grams of pure water, it contains 100 grams of water, but 100 grams of 20 percent salt water only contains 80 grams of water. The other 20 grams is the dissolved salt. The heat capacity of dissolved salt is almost zero when compared to the high heat capacity of water. This means that the heat capacity of a 20-percent salt solution is 80 percent that of pure water. Twenty percent salt water will heat up almost 25 percent faster than pure water and will win the speed race to the boiling point.

Please note that this will not hold true if you take two identical pots containing one gallon of water each and add the salt to one pot because then the volume of liquid in the salted pot will be greater than the one gallon starting point.

katchum Dec 14, 2007 05:47 PM
I'm sorry but you're looking at the wrong direction. I'll explain the answer.

The salt in the water wouldn't make any huge difference on the temperature, so don't search in that area.

What you have to think about is convection!

When you put an ice cube in the salt water, the cold water stays above and the warm water stays underneath, because salty water has a higher density! => ice cube doesn't melt fast.

When you put an ice cube in normal water, the cold water goes down and the warm water rises up => ice cube melts faster.

RacinReaver Dec 14, 2007 05:59 PM
Actually the convection wouldn't matter, I think what would matter is in salt water more of the ice cube would float over the top of the surface (less would be submerged), so less water touching it means it melts slower. I think for normal water it's something like 4/5 of the ice cube would be submerged (actually just had that value on the final I took yesterday), so when it's salt water, maybe only 3/5 would be submerged. Since the side of the ice cube touching the water would transfer heat faster than the side touching the air, the one with more ice in the water would melt faster.

I think the convection currents within the water itself should be approximately the same between the two.

pyrrhus Dec 15, 2007 07:06 AM
I found this explanation satisfying:

General Chemistry Online: FAQ: Solutions: Why can adding salt to ice water make the ice melt slower?

katchum Dec 15, 2007 07:46 AM
No pyrrhus, that experiment is done while stirring! In my case I just put the ice cube in the salt water, without stirring.

See:

"Watch how the temperature of the ice water falls after the initial addition of salt. What does a plot of temperature vs. time look like? (Stir constantly and completely!) "

I still think it's convection. Because you can easily imagine that this ice water layer around the ice cube will be motionless, therefore it gives isolation around the ice cube. (when put in the salt water)

In normal water there is natural convection, so no isolation around the ice cube.

When you do the experiment yourself you can even SEE it! There are these little transparant lines flowing all over...

Here is my reference, it's in flemish though. Pyrrhus will confirm it's true!:

NWO - De Quiz van 2000

Quote: "In het bekertje met zoetwater smelt het ijs. Het koud smeltwater is zwaarder dan het oorspronkelijke water, zodat het naar de bodem zakt. Het warme water stijgt juist op, waardoor er weer ijs zal smelten. Kortom, er treedt circulatie op in het glas. Er wordt steeds warm water aangevoerd dat het ijs snel doet smelten.

Anders gaat het toe in het glas met zout water. Daar is het smeltwater van het ijsblokje zoet en het omgevingswater is zout. Zoetwater is veel lichter dan zout water. Het koude zoetwater zal niet zakken en het ijsklontje blijft omgeven door het koude smeltwater. Er treedt geen convectie op en daarom zal het ijs in het glas met het zoute water veel langzamer smelten."



I'll post some more nice questions from that site next time.

RacinReaver Dec 15, 2007 03:23 PM
Shouldn't there be even more currents in the salt water one since you'll be getting a gradient in the concentration of salt with less salt surrounding the melting ice cube and more salt towards the bottom of the glass?

katchum Dec 15, 2007 05:32 PM
You mean like diffusion? There is, but the effect of density is bigger, I think.


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