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  #801  
Old 24.07.2009, 17:44
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Re: Ask a Scientist

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I'm going to try an experiment tomorrow, I think...

I'm going to be empty of cheese and beer tomorrow morning, and see how
much energy I expend. Then I'm going to fill myself full of cheese and beer
in the afternoon, and measure the effort involved again.

I shall report back with the results on Monday
.
I recommend you wear a tin foil hat, to keep the heat in
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  #802  
Old 25.07.2009, 13:07
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Re: Ask a Scientist

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Assuming doors shut and both fridges in steady state - i.e. have been allowed to run until thermostat kicks in at least once - then the only thing that will happen is that X joules of energy will transfer from surroundings to fridge, then at some point the thermostat will kick in and transfer X joules back out.
I think that assumption makes your analysis unrealistic. A fridge in normal operation will be opened, and cool air will mix with warmer air in the kitchen.

Consider the energy required to cool an empty fridge to a given temperature, with the energy required to cool a fridge that is 90% full of boxes of water. It would require more energy to cool the fridge in the latter case. This is because the specific heat capacity of water is much higher than that of air.

The specific heat of water is 4.186 joule/ gram kelvin
The density of water at 5C is 1000 kg/ m^3

The specific heat of air is 1.0 J/ g K
The density of air at 5C is 1.269 kg/ m^3

Take 250 L as a typical fridge capacity.
250 L = 0.25 m^3

The energy required to cool 250L of air is 0.25 * 1.0 * 1.269 = 0.32 J/ K
The energy required to cool 225 L water and 25 L air = 0.225 * 4.186 * 1000 + 0.03 = 940 J/ K (the contribution from the air is negligible).

So the amount of energy required to cool water is 940 / 0.32 = 2940 per kelvin times greater than the air.

So much for the fridge left alone to cool. In the case where the door is opened and then closed, let's assume all the cool air is exchanged with the room. From the numbers above, we can see it takes 0.32 J/ K to cool the empty fridge down again, and 0.03 J/ K to cool down the full fridge. If we are going from, say, a room temperature of 23 C to 3 C, that's 6.4 J or 0.64 J of extra cooling energy required per opening of the door. If you open the fridge door 10 times per day, then it's 64 J or 6.4 J per day. Expressed as power consumption, that's 7.4 milliwatt for the full of air case vs .74 mW for the full of water case.

This is two or three orders of magnitude less than the power used by a mobile phone charger.

The conclusion is that it is more energy efficient in the long run to keep a full fridge cool, but the practical difference is negligible.

A more realistic model would take into account the efficiency of the fridge cooling system. That is, the energy expended by the fridge to extract a Joule of energy. Also, thermal conductivity is not taken into account. Nor is the effect of removing a cold item, nor adding a warm item considered. As it looks like the energy required to cool a solid object is 1000s of times larger than for air, I suggest the energy costs of running the fridge depend more on the rate at which material is replaced than how full it is.
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  #803  
Old 25.07.2009, 13:46
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Re: Ask a Scientist

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I think that assumption makes your analysis unrealistic. A fridge in normal operation will be opened, and cool air will mix with warmer air in the kitchen.

Consider the energy required to cool an empty fridge to a given temperature, with the energy required to cool a fridge that is 90% full of boxes of water. It would require more energy to cool the fridge in the latter case. This is because the specific heat capacity of water is much higher than that of air.

The specific heat of water is 4.186 joule/ gram kelvin
The density of water at 5C is 1000 kg/ m^3

The specific heat of air is 1.0 J/ g K
The density of air at 5C is 1.269 kg/ m^3

Take 250 L as a typical fridge capacity.
250 L = 0.25 m^3

The energy required to cool 250L of air is 0.25 * 1.0 * 1.269 = 0.32 J/ K
The energy required to cool 225 L water and 25 L air = 0.225 * 4.186 * 1000 + 0.03 = 940 J/ K (the contribution from the air is negligible).

So the amount of energy required to cool water is 940 / 0.32 = 2940 per kelvin times greater than the air.

So much for the fridge left alone to cool. In the case where the door is opened and then closed, let's assume all the cool air is exchanged with the room. From the numbers above, we can see it takes 0.32 J/ K to cool the empty fridge down again, and 0.03 J/ K to cool down the full fridge. If we are going from, say, a room temperature of 23 C to 3 C, that's 6.4 J or 0.64 J of extra cooling energy required per opening of the door. If you open the fridge door 10 times per day, then it's 64 J or 6.4 J per day. Expressed as power consumption, that's 7.4 milliwatt for the full of air case vs .74 mW for the full of water case.

This is two or three orders of magnitude less than the power used by a mobile phone charger.

The conclusion is that it is more energy efficient in the long run to keep a full fridge cool, but the practical difference is negligible.

A more realistic model would take into account the efficiency of the fridge cooling system. That is, the energy expended by the fridge to extract a Joule of energy. Also, thermal conductivity is not taken into account. Nor is the effect of removing a cold item, nor adding a warm item considered. As it looks like the energy required to cool a solid object is 1000s of times larger than for air, I suggest the energy costs of running the fridge depend more on the rate at which material is replaced than how full it is.
I think that is the same conclusion that I arrived at with my educated guess
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  #804  
Old 25.07.2009, 15:21
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Re: Ask a Scientist

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I think that is the same conclusion that I arrived at with my educated guess
Now you have some support

I enjoyed working it out.
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  #805  
Old 19.09.2009, 14:33
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Re: Ask a Scientist

A new question (or is it two separate questions?).

I know that heat rises. Also, the almosphere is thinner at altitude and the suns rays are stronger there.

In that case, why does snow form on the top of mountains instead of in the valleys, and why does ice form on the top of water, rather than the bottom?
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  #806  
Old 19.09.2009, 18:59
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Re: Ask a Scientist

The second part is easy and nothing to do with the other parts. Ice forms on the top of water because it is one of the rare solids that is less dense than its liquid form. So it floats. Maybe it is more accurate to say that is why it STAYS on top. It FORMS on top because air cools down faster than water. So the coldest part of the water is always at the top, next to the air.
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  #807  
Old 19.09.2009, 19:03
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Re: Ask a Scientist

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A new question (or is it two separate questions?).

I know that heat rises. Also, the almosphere is thinner at altitude and the suns rays are stronger there.

In that case, why does snow form on the top of mountains instead of in the valleys, and why does ice form on the top of water, rather than the bottom?
Not sure about the first question, but the reason for the second is that ice is less dense than (liquid) water.
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  #808  
Old 19.09.2009, 19:18
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Re: Ask a Scientist

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The second part is easy and nothing to do with the other parts. Ice forms on the top of water because it is one of the rare solids that is less dense than its liquid form. So it floats. Maybe it is more accurate to say that is why it STAYS on top. It FORMS on top because air cools down faster than water. So the coldest part of the water is always at the top, next to the air.
The density of water is temperature related. Ice can only start to form after the entire body of water has been cooled down below 4°C, the temperature of maximum density of water. Before that happens, the colder, and heavier, water at the top just sinks and blends with the warmer water below.
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  #809  
Old 19.09.2009, 19:23
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Re: Ask a Scientist

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why does snow form on the top of mountains instead of in the valleys
Because mountains are further away from the seething molten magma that lies beneath our feet, and which sometimes erupts in the form of volcanoes, steam baths and geezers.

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and why does ice form on the top of water, rather than the bottom?
Because ice floats. If it was heavier, it would form at the bottom of the ocean, and polar bears would have nowhere to go.

Really, you don't need to ask one of them scientists. Just ask me: The friendly, well-informed bloke in the pub...
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  #810  
Old 19.09.2009, 19:27
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Re: Ask a Scientist

Another new question, sorry Deep Purple, I couldn't even answer the second part, let alone the first.

Can anybody tell me, why do cooked carrot rounds have such a distinctly different ( which in my opinion,is rather unpleasant) taste to say, diced, julienned or other cooked carrot preparation?

I suspect it has to do with the fact that the center piece has been cut and therefore all those ( I think) toxins or whatever chemical it is they have, have been released, to be dispelled into the cooking water, so are less concentrated, taking away "that taste".
Still, that is only my theory, I'm sure there is a more qualified and sure explanation.

Beg pardon that this query has nothing to do with the quantum theory or the likes, but I have long wondered and I'm sure there must be a scientific reason and I sure would like to know it.
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  #811  
Old 19.09.2009, 19:28
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Re: Ask a Scientist

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Really, you don't need to ask one of them scientists. Just ask me: The friendly, well-informed bloke in the pub...
Oh PLEASSSSSEE start a new threat! I have so many questions for such a person

Gal x
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  #812  
Old 19.09.2009, 19:32
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Re: Ask a Scientist

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Because mountains are further away from the seething molten magma that lies beneath our feet, and which sometimes erupts in the form of volcanoes, steam baths and geezers.



Because ice floats. If it was heavier, it would form at the bottom of the ocean, and polar bears would have nowhere to go.

Really, you don't need to ask one of them scientists. Just ask me: The friendly, well-informed bloke in the pub...
Having met this old fellow in a pub I can verify that he is a fountain of information.
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  #813  
Old 19.09.2009, 19:35
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Re: Ask a Scientist

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Can anybody tell me, why do cooked carrot rounds have such a distinctly different ( which in my opinion,is rather unpleasant) taste to say, diced, julienned or other cooked carrot preparation?
Because they are a sign of a lack of imagination on the part of the cook.

Any monkey with a knife and a chopping board can slice a carrot into rounds, and proceed to boil it into submission, leaving nasty soggy discs of blandly vague and faintly offensive carrothood.

It takes an experienced and imaginative chef to cut carrots into strips, cubes or dodecahedral nuggets, which may then be prepared in a thousand and one original and delicious ways, with seasoning, sauces and spices beyond the dreams of the kind of person who'd cut their carrot into round slices.

The poor flavour, therefore, is not a consequence of the cutting process itself: Rather, it is an indicator of the uselessness of the cook.

Anyone for carotte peruvienne with a fine jus d'araignee?
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  #814  
Old 19.09.2009, 19:44
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Re: Ask a Scientist

You may be onto something, submissive carrots taste just plain ick!

If your theory is anything to go by, I must not be the worst in the kitchen, good to know.
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  #815  
Old 19.09.2009, 19:55
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Re: Ask a Scientist

For the carrots, the answer seems to be terpenoids.

From the classic paper "Water blanching effects on headspace volatiles and sensory attributes of carrots": "Most volatiles, in particular terpenoids (sabinene, β-pinene, β-myrcene, limonene, trans-caryophyllene, α-humulene, β-bisabolene and α-farnesene) decreased by at least 50% within 60 seconds of blanching."

I reckon that the unpleasant flavour, due to these terpenoids with funny names, get extracted from the small carrot bits faster than from the big bits.

Still, good question. You should ask France's molecular gastronomer Hervé This, who loves this kind of stuff.

PS Carrots contain no poisons. All flavours are chemicals.

PPS Thanks to you I discovered the web site of the "World Carrot Museum".
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Old 19.09.2009, 20:08
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Re: Ask a Scientist

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Another new question, sorry Deep Purple, I couldn't even answer the second part, let alone the first.

Can anybody tell me, why do cooked carrot rounds have such a distinctly different ( which in my opinion,is rather unpleasant) taste to say, diced, julienned or other cooked carrot preparation?
You go right on with your round carrot slices- don't give in to the julienne peer pressure.

If you want to improve their taste, bring them to a boil and cook for just a couple of minutes. Discard the water and then finish cooking with fresh water.
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  #817  
Old 19.09.2009, 20:51
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Re: Ask a Scientist

Thank you Frank Zappa and Mud.

Terpenoid is my new word of the week, the name sounds just like the 'taste'.

Mud,I will certainly give that method a try, though on thinking about it (I keep getting logged out even with the remember me box ticked), I wonder if by soaking the carrot rounds in some lightly salted water might also help? ( I have found by doing this with late season zuccini, it takes away that bitterness), as, well, there are definitely times where the carrot rounds just look right, depending on what I want to do. No snobbishness here, unless you want to count that taste.

Thank you very much all, am now off to discover the joys of the World Carrot Museum...

I think I am a bit too shy to ask `Mr Herve, but will be looking him up and if I get the nerve to ask, will post back.
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  #818  
Old 19.09.2009, 21:45
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Re: Ask a Scientist

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The second part is easy and nothing to do with the other parts. Ice forms on the top of water because it is one of the rare solids that is less dense than its liquid form. So it floats. Maybe it is more accurate to say that is why it STAYS on top. It FORMS on top because air cools down faster than water. So the coldest part of the water is always at the top, next to the air.
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Not sure about the first question, but the reason for the second is that ice is less dense than (liquid) water.
That makes sense. It would appear to be a different reason for the mountains.

DB, I would so like to believe you but I fear that there may be more to it.

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Because mountains are further away from the seething molten magma that lies beneath our feet, and which sometimes erupts in the form of volcanoes, steam baths and geezers.
Would that geezer be the well-informed bloke in the pub? Perhaps he is just spouting hot air.
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  #819  
Old 19.09.2009, 22:16
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Re: Ask a Scientist

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A new question (or is it two separate questions?).

I know that heat rises. Also, the almosphere is thinner at altitude and the suns rays are stronger there.
Nobody tackled this part. To say that "heat rises" is not quite correct. Heat doesn't rise or sink. What that refers to is hot air rising. Hot air is less dense than cold air, so it's actually more accurate to say cold air sinks.

If you really want to get pedantic, according to the 2nd law of thermodynamics, heat goes wherever it's not; IE hot flows to cold.
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  #820  
Old 19.09.2009, 23:13
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Re: Ask a Scientist

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Nobody tackled this part. To say that "heat rises" is not quite correct. Heat doesn't rise or sink. What that refers to is hot air rising. Hot air is less dense than cold air, so it's actually more accurate to say cold air sinks.

If you really want to get pedantic, according to the 2nd law of thermodynamics, heat goes wherever it's not; IE hot flows to cold.
OK, I just about understand that bit; but it still doesn't seem to explain why the tops of mountains are colder than the valleys. If the cold air sinks, shouldn't the reverse be true. Shouldn't Switzerland have valleys full of snow and mountain top resorts?
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