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Author Topic: Did Chris and Kat blow it?  (Read 2748 times)

Offline Eric A. Taylor

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Did Chris and Kat blow it?
« on: 04/11/2009 07:21:44 »
I just finished with this week's podcast and I think Chris and Kat made a mistake. The question was, "Does the Earth gain weight from plants growing?" or something close to that. Chris argued that the sun's energy contributes a little weight to the plant's in energy under E=MC2. The amount of energy/mass the Earth gets is really really really tiny. If you convert 1 gram (about .0353 oz.) of matter to energy you'd get about the same energy as 20,000 English tons of TNT or about the same as a 20 megaton nuclear explosion. So you would need that much energy to get 1 single gram of weight. That's a lot of energy, far more than the amount the entire Earth gets in 24 hours.

On top of that the Earth loses about the same amount of energy at night that it got that day, otherwise every day would be hotter than yesterday. Further the amount of heat energy contained INSIDE the Earth is much higher. This heat comes not from the sun but is left over energy from the formation of the Earth (plus maybe some radioactive decay). Every time a volcano erupts (about 5 per day world wide) the Earth cools a bit.

As far as living things growing: Earth is a closed system (well mostly closed) so when a plant grows it uses stuff in the soil where it's growing. If nothing eats the plant or there is no fire the plant dies and returns the stuff to the soil. If an animal eats it the animal will take some of the plant stuff into it's body then deposit the rest someplace else, but still on Earth. Then eventually the animal will die and all it's stuff is returned to the Earth.

There is a very fun little story from Hamlet that illustrates this. Hamlet is returning to Denmark from England where his Uncle has tried to have him executed and finds a grave digger who tells Hamlet how a King can pass through the gut of a commoner. The King is buried and eaten by a worm. A fishmonger uses the worm as bait then sells the fish he catches with it. The commoner eats the fish and thus, indirectly, eats the King. It's possible that you today could have atoms in your body that were once atoms in Shakespeare's body (Or James I or Jane Grey, or Egbert...)
« Last Edit: 04/11/2009 07:28:24 by mountaineirc1969 »


 

Offline Don_1

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Did Chris and Kat blow it?
« Reply #1 on: 04/11/2009 07:31:21 »
Being no physicist, I stand to be corrected here, but I don't think the energy absorbed from the Sun by day is lost (into space) overnight, but rather dissipates and thus remains trapped in Earth's confines.
 

Offline Mr. Scientist

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Did Chris and Kat blow it?
« Reply #2 on: 04/11/2009 07:48:52 »
The earth constantly looses energy and gains energy. Night and day are simultaneous around the world. The world does not simply stop to give way to energy because night time has arrived for us.
 

Offline Eric A. Taylor

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« Reply #3 on: 04/11/2009 08:01:27 »
I'm afraid your wrong here, and I don't need fancy math to prove it. If you add energy to a system in pulses, as the Earth gets energy from the sun, at first the system's average temperature will rise but only until equilibrium is reached. Equilibrium is reached when the amount of heat lost between pulses (at night say)is equal to the amount added during a pulse (during the day). Earth, on average, is in equilibrium. Some days more heat is added than lost, sometimes less. This is why some days are warmer than others. If heat is retained in the Earth system then the Earth will become warmer every day.

It does seem that right now Earth is getting warmer. This however is simply short term. On average, on time scales of millions of years, the temperature stays very much the same.
 

Offline graham.d

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« Reply #4 on: 04/11/2009 09:54:49 »
I think you are right Mountaineirc1969. The slight complication is that the earth is not chemically inert so that chemical reactions, like photosynthesis, use some of the received energy. However, this energy, over a long enough period, will also be released to give a net equilibrium. An issue with man-made global warming is that we are releasing the energy, trapped over millions of years as carbon and carbon compounds, in a very short time. This changes the equilibrium, albeit on a temporary basis.

Just to be clear though, this energy release is not directly responsible for the global warming - it is a tiny amount compared with the radiation from the sun - but the change in the chemical make up of the atmosphere that can cause a net temperature change (Greenhouse effect). This will also change the equilibrium, and the threatened danger is that it could settle at a different balance point with the earth's surface at a higher average temperature.
 

Offline Mr. Scientist

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Did Chris and Kat blow it?
« Reply #5 on: 04/11/2009 13:17:16 »
I'm afraid your wrong here, and I don't need fancy math to prove it. If you add energy to a system in pulses, as the Earth gets energy from the sun, at first the system's average temperature will rise but only until equilibrium is reached. Equilibrium is reached when the amount of heat lost between pulses (at night say)is equal to the amount added during a pulse (during the day). Earth, on average, is in equilibrium. Some days more heat is added than lost, sometimes less. This is why some days are warmer than others. If heat is retained in the Earth system then the Earth will become warmer every day.

It does seem that right now Earth is getting warmer. This however is simply short term. On average, on time scales of millions of years, the temperature stays very much the same.

Was this directed at me?
 

Offline Bored chemist

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« Reply #6 on: 04/11/2009 19:23:02 »
"If you add energy to a system in pulses, as the Earth gets energy from the sun, "
The earth does not receive energy from the sun in pulses.
The sun doesn't switch on and off.
 

Offline yor_on

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« Reply #7 on: 04/11/2009 20:32:05 »
The Earth spends its energy at its highest layer, and only there. That mechanism is what defines the energy that will be kept inside our atmosphere. For any energy to leave Earth it will have to find its way to that layer. The sun radiates energy down at earth and it's been a balanced system until recently where we've gone in and added to the energy in the atmosphere due to coal, oil, methane, fertilizers etc. That extra energy inherent in the molecules will be radiated out by them and mostly directed downward and outwards due to that the reflected energy from those molecules will be in a frequency that can't be absorbed by other molecules once radiating. That they still add energy to themselves is mostly due to the kinetic energy they get when colliding with each other. So those molecules will collect energy from each other. It is also so that the higher you come the more heat the molecules seem to be able to keep. All that together creates an added heat and that heat can only be balanced at that utmost atmospheric layer. As for energy leaving Earth through clear desert nights that is as far as I know true. But it doesn't matter that much as that heat is radiated exciting air molecules that then will 'bump' each other collecting the kinetic energy. That is called our global warming, there is also differences due to different molecule weight, ability to store energy, for example methane have around sixty times larger ability to store heat than CO2, convection etc etc involved in those mechanisms.

--

forgot to point out that we are in fact creating more molecules in our atmosphere due to use of 'hidden storages' in our earth that we let out due to combustion. Also, the heat stored in our atmosphere 'pushes' constantly out that utmost layer meeting space which add to the disturbed equilibrium we have today. Earth is not a linear system.
« Last Edit: 04/11/2009 20:39:13 by yor_on »
 

Offline LeeE

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« Reply #8 on: 05/11/2009 15:51:23 »
Hmm... a quick calculation gives the cross sectional area of the Earth as 1.275 x 1014 square meters, and (quoting from Wikipedia) "The World Meteorological Organization defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 watts per square metre."

...so we get a total of roughly 1.530 x 1016 watts of energy falling upon the Earth.

There are 86400 seconds in a 24 hour day, and one Joule is one Watt per Second, so the Earth intercepts 1.322 x 1021 Joules of energy per day.  If we then divide this by 'c2' to get the equivalent mass we end up with around 14689 kg.

Hmm... someone had better check my maths.

But in any case, not all of the light that is intercepted by the Earth is absorbed as much of it is immediately reflected back into space, otherwise we wouldn't be able to see the Earth from space at all.  Much of what is initially absorbed is subsequently re-radiated back in to space too, so the amount that is actually retained by the Earth for any period of time does seem to be pretty small.
 

Offline chris

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Did Chris and Kat blow it?
« Reply #9 on: 05/11/2009 23:57:04 »
The question was actually, "is the Earth gaining weight through plant growth..."

In answering the question, transcribed here - http://www.thenakedscientists.com/HTML/content/questions/question/2455/ - I made the point that, as an isolated system, the biosphere is gaining weight because photosynthesis is capturing energy from the Sun and converting this to chemical energy. Since E=mc^2, if the energy in the system increases then the mass must increase.

However, I also pointed out that, relative to the amount of weight added to Earth in the form of in-falling dust and space-debris exceeding 40,000 tonnes per year, this is a drop in the ocean.

Chris
 

Offline Eric A. Taylor

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Did Chris and Kat blow it?
« Reply #10 on: 06/11/2009 00:31:11 »
"If you add energy to a system in pulses, as the Earth gets energy from the sun, "
The earth does not receive energy from the sun in pulses.
The sun doesn't switch on and off.


I'm talking locally. Like the song says "it's ALWAYS 5 o'clock somewhere" The Earth is so big that there isn't time for the energy on one side to make it around before local sunrise. Venus spins much more slowly so it's nearly the same temperature globally.
 

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Did Chris and Kat blow it?
« Reply #10 on: 06/11/2009 00:31:11 »

 

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