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  #781  
Old 26.06.2009, 09:20
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Re: Ask a Scientist

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Could be that in environments like desserts, having much scarse food, those who had poison could hunt better and feed themselves, beside higher chance of survival if a fight needed .
I thougth of that, but that's the opposite problem to that faced by jungle dwellers, who are also often poisonous.
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  #782  
Old 26.06.2009, 22:17
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Re: Ask a Scientist

Interesting question.

I couldn't find any surveys of the distribution of all poisonous animals. But I thought about what little I know, and maybe I have half an idea why I couldn't find one.

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Does anyone know why you find more and nastier poisonous animals generally in hot climates.

Is this just because there's greater diversity of life in those places anyway?
Poison/ venom used for defense or predation?

More as in an absolute number, or as a proportion?

And are we counting species or biomass or individuals?

Also, poisonous to what?

I would include invertebrates in the list of animals.

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This wouldn't seem to hold true for deserts, where there are often extremely poisonous creatures.
Australia, essentially a large desert, certainly seems to have a high proportion of venomous animals.

I am guessing that poison-for-predation makes sense in an arid region like a desert, because you don't want your prey to get away. If you get enough venom in it, it won't get far.

Poison-for-defence doesn't make sense for the individual if it dies while killing its prey.
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  #783  
Old 29.06.2009, 19:23
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Re: Ask a Scientist

Totally unrelated question... why is there always a breeze around big waterfalls and fountains? For example, if you walk up to the base of the big fountain in Geneva then there's quite a gale blowing.

I guess the answer is something to do with fluid displacement and the air rushing in to fill in where the water was. However in a fountain the water's always there... for example the Geneva fountain is always the same size and displacing the same amount of air, and hence it's effectively static...

A flagpole that's the same height as the fountain would also be static, but there's no wind effect around that merely due to its presence...
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  #784  
Old 29.06.2009, 19:39
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Re: Ask a Scientist

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Totally unrelated question... why is there always a breeze around big waterfalls and fountains? For example, if you walk up to the base of the big fountain in Geneva then there's quite a gale blowing.

I guess the answer is something to do with fluid displacement and the air rushing in to fill in where the water was. However in a fountain the water's always there... for example the Geneva fountain is always the same size and displacing the same amount of air, and hence it's effectively static...

A flagpole that's the same height as the fountain would also be static, but there's no wind effect around that merely due to its presence...
You can also feel air flow in the vicinity of a fast-moving river.

Air can only flow across a pressure gradient, from high to lower pressure. So there must be a region of lower pressure near the fountain. Your observed that a "moving pole" - the fountain's jet causes a wind, whereas a flagpole - which is static - does not.

The effect is, I believe, due to Bernoulli's principle. Your question reminded me of an item in New Scientist's Feedback from 8 years ago.
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  #785  
Old 30.06.2009, 11:25
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Re: Ask a Scientist

Yup, a bit of Bernouilli effect, a bit of boundary layer adhesion/skin friction, a bit of
form and interference drag and a lot of vortices, large and small!

And as noted by the Beast, a fountain is not solid - the Geneva Jet d'Eau, for example,
has about 7000 litres of water (7 tonnes!) in the air at any time, which sets off from
the base of the jet at approx 200kph, which is Quite Fast.
By the time the water reaches its apex, it's shed most of its energy to the surrounding
atmosphere by imparting motion to the surrounding air, causing wind and noise :-)
(The pumps burn a Megawatt, and even tho' not all of that power makes it into
movement in the water, that's still a vast amount of energy for the water jet to shed!)

And, yes indeed , it's also why a cold damp shower curtain will inevitably waft itself
into the shower and wrap itself round one.
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  #786  
Old 03.07.2009, 17:03
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Re: Ask a Scientist

Thanks guys
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  #787  
Old 18.07.2009, 16:43
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Re: Ask a Scientist

Another qu. related to fountain or waterfall:
Why water becomes quite milky and a few meters further again normal?
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  #788  
Old 18.07.2009, 17:28
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Re: Ask a Scientist

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Another qu. related to fountain or waterfall:
Why water becomes quite milky and a few meters further again normal?
It could be the "milkiness" is due to turbulent mixing of water and air at the bottom of the waterfall. As the water flows away, the air bubbles rise and the water clears.
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  #789  
Old 24.07.2009, 14:10
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Re: Ask a Scientist

Does a full fridge use more, less or the same amount of energy as an empty one? The obvious assumption is that all the food in the fridge is already chilled and thus the system is in a steady state.
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  #790  
Old 24.07.2009, 14:17
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Re: Ask a Scientist

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Does a full fridge use more, less or the same amount of energy as an empty one? The obvious assumption is that all the food in the fridge is already chilled and thus the system is in a steady state.
I have always understood that it is more economical to keep the fridge full. If it is near to empty, each time it is opened, cold air will escape to be replaced by warm air that needs cooling. If full, there must be less air to escape.

If it wasn't opened, I don't think that there would be much difference.
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  #791  
Old 24.07.2009, 16:13
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Re: Ask a Scientist

I personally believe, food stuff need more energy than same volume air to be kept cold/frozen, specially as insulation is not ideally perfect.
So I wouldn't fill the fridge unnecessarily!
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  #792  
Old 24.07.2009, 16:25
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Re: Ask a Scientist

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I have always understood that it is more economical to keep the fridge full. If it is near to empty, each time it is opened, cold air will escape to be replaced by warm air that needs cooling. If full, there must be less air to escape.

If it wasn't opened, I don't think that there would be much difference.
You can also take into account the different properties of materials when it comes to heat retention. As far as I'm aware, solids (in general) tend to retain their temperature better than air, so while they'll take longer to cool, once at that temperature, they'll stay at it longer, and help with cooling the fresh warm air.

I think...
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  #793  
Old 24.07.2009, 16:37
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Re: Ask a Scientist

Another scientist on forum.

Expert on: nanomedicine.
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  #794  
Old 24.07.2009, 16:40
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Re: Ask a Scientist

You could do a test! :-)
Turn off the fridge and without opening, check after 2-3 days how stinky the stuff are!
And after clean up, try again with empty fridge after the same time, and then compare the result
Since Insulation is not perfect, Fridge needs regularly to cool down and spend energy.

Or check power consumption with a meter and compare for both cases.
Or check how often and the time you here click of the condenser turning on and compare.
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  #795  
Old 24.07.2009, 16:42
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Re: Ask a Scientist

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Does a full fridge use more, less or the same amount of energy as an empty one? The obvious assumption is that all the food in the fridge is already chilled and thus the system is in a steady state.
First Law of Thermodynamics:
"The change in a system's internal energy is equal to the difference between heat
added to the system from its surroundings and work done by the system on its
surroundings."

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.

The value of X is unaffected by the contents of the fridge, be it empty,
the remaining half of last night's pizza, or crammed with beer.

So, as long as you don't want to get at the beer'n'pizza, the answer is
"over time, the same"
.
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  #796  
Old 24.07.2009, 16:43
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Re: Ask a Scientist

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You could do a test! :-)
Turn off the fridge and without opening, check after 2-3 days how stinky the stuff are!
And after clean up, try again with empty fridge after the same time, and then compare the result
Because an empty fridge will smell after 3 days?

Might be better to log and compare the ambient temperature inside the fridge over time...
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  #797  
Old 24.07.2009, 16:48
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Re: Ask a Scientist

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You could do a test! :-)
Turn off the fridge and without opening, check after 2-3 days how stinky the stuff are!
And after clean up, try again with empty fridge after the same time, and then compare the result
I don't recommend it - I tried it once, but the cat died...
...
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  #798  
Old 24.07.2009, 16:58
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Re: Ask a Scientist

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First Law of Thermodynamics:
"The change in a system's internal energy is equal to the difference between heat
added to the system from its surroundings and work done by the system on its
surroundings."

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.

The value of X is unaffected by the contents of the fridge, be it empty,
the remaining half of last night's pizza, or crammed with beer.

So, as long as you don't want to get at the beer'n'pizza, the answer is
"over time, the same"
.
But we're discounting the heat capacitance of the food, my guess is that it is a lot easier to keep a fridge full of food at a steady temperature than a fridge full of air, ergo less clicking of the thermostat and less energy wasted.
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Old 24.07.2009, 17:06
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Re: Ask a Scientist

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But we're discounting the heat capacitance of the food, my guess is that it is a lot easier to keep a fridge full of food at a steady temperature than a fridge full of air, ergo less clicking of the thermostat and less energy wasted.
Note the judicious use of the phrases "steady state" and "over time" previously...

Yes, we could look over a shorter period, and factor in Specific Heat
Capacities, inefficiencies and hysteresis characteristics of thermostat,
heat pump etc, but in the longer term i think you'll find that it all balances
out...
i.e. steady state is not the same thing as steady temperature!
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Old 24.07.2009, 17:20
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Re: Ask a Scientist

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
.
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