Lightsabers and Thermodynamics – Poll Below
Posted by Jeff Id on June 4, 2011
UPDATE: Ids poll answers –
Question 1 – Would a minus 180 C block of ice the size of the moon and in lunar orbit warm or cool the Earth.
Ids Answer – It would warm the Earth. No matter what you assumed the definition of lunar orbit to mean for the majority of the time, the ice chunk would find itself blocking a view of the blackness of space at 3K or minus 270C. In place of that minus 270C radiation which continually strikes the earth, a small section of the sky has a hotter, brighter minus 180C emission. Therefore – the Earth Warms.
Question 2 – Would it warm the Sun.
Ids Answer – It would warm the Sun for the same reasons as above. The tiny little pinpoint in the distance would probably add as much as a distant star, but it would add a little and that is all it takes.
Now, if you get all caught up in how much and could it be measured etc. etc. and so on, you are probably lousy at story problems. The answers are actually quite simple. It will be interesting to learn where I am wrong in the inevitable follow up comments :D.
Sometimes I’m too short with people. It comes from some of the pressures of the rest of life, I really should show more patience though, after all we’re all just trying to understand.
There are a group of people on who have published articles on global warming (on the internet only) who claim to have disproven the effect that CO2 has on surface air temperatures. There are a variety of these sorts of things but one, which is most annoying, is the claim that backradiation from a colder source (air at higher altitude) to a warmer one (the ground) somehow violates the second law of thermodynamics. This argument is false. Now I could spend time with the equations and demonstrate that the net heatflow is always from warmer to colder, which it is, but I think those who misunderstand this point are missing the fundamental understanding of what temperature is and how energy can also flow from cold to hot. Therefore, we only need words today.
This first section discusses the physical mechanisms of thermodynamic conduction.
Thermodynamic laws are based on a bulk material reactions to temperature. When two objects come in contact with one another, one hot the other cold. Heat energy will transfer from the hot object to the cold one – this is basically the simplest conceptual form of the second law of thermodynamics. What would you say if I claim that on a microscale, some heat goes the other way?
Temperatures taken in bulk are a combination of the motion of individual atoms, mass of the atoms and the density of the atoms. When different temperature objects of equal density and material type are considered, the difference in temperature is in the physical velocity (vibration, rotation or linear) of the molecules. If the hotter object is more dense than the other, it is possible for the individual atoms of each mass to have the same physical kinetic energy, but since the hot object is denser, it has a higher probability of transferring its kinetic energy at any particular moment in time. Note though, that this second situation does not preclude the occasional transfer from the cooler object to the hotter one. This is because not every atom in the material is vibrating/spinning/or translating by exactly the same amount. In such collisions, an individual atom can be said to be cooler or warmer but cool and warm are truly bulk concepts rather than microscopic. This whole situation can get far more complicated so I’ll leave it there, but the point is on average, heat ALWAYS will flow from hot to cold at a pre-determined rate. The bulk rate is based on the probability of energy transfer on an individual atom from a micro-scale analysis.
Before moving on to radiation transfer, imagine a container of Nitrogen gas (N2) at room temperature. Two balls on a spring. Each molecule can spin like a dumbell, have linear velocity like a car or spring back and forth. If you instantly compress the bulk volume of gas to half the volume, the temperature rises instantly. What happened to the velocity of the individual molecules?
The answer is nothing. All you did was push them closer together so that the temperature increase you measure is simply an increase in probability of collision and energy transfer to the outside walls of your imaginary container. The atoms are closer together so more of them hit the wall each second. Eventually the energy will be transferred out of the container until room temperature is met and the actual molecules of Nitrogen in your container will individually have slowed until the probability of energy transfer into the container (on a microscale) is exactly equal to the probability of energy transfer out.
This concept deleted above is incorrect. Serioso kept saying so in the thread below and after some review, I have to agree that he is right.
So when we were discussing the temperature of Venus on the previous two threads, and I made the comment that it was a bad comparison because of pressure, a side discussion started with ScienceofDoom assuming that I believed the high pressure caused the high temp. This is also a common fallacy on the internet which is prone to raising my blood pressure. Unfortunately (or rather fortunately), I was staring at a pile of biofuel in the woods while comforting a cold can of ale against the blazing IR heat and didn’t see any of the discussion all last weekend. It might be interesting though to explain from a conceptual level the reason that the huge pressure on Venus doesn’t create ANY of the temperature gradient.
Take a column of gas in a perfectly insulated and very long tube (hundreds of miles). Place the tube in an earth-like gravity field such that the bottom of the tube achieves 90 atmospheres, the top is 0.1 atmospheres. The tube contains only gas and no precipitates form and no convection occurs. No heat is added or removed from the tube and the gas is inserted at a single temp until the tube is full. In the beginning, the gas in the tube base will be far hotter than that of the rarefied gas at the top due to compression which causes the molecules to be close together at the base.
By the second law of thermodynamics, temperature inside the tube will equalize over time until it is exactly the same from top to bottom. This is incorrect as Paul Lindsay points out below — the correct answer is that with height, velocity is lost in a gravitational field so the top will be cooler — see the correct answer in the comments.
Heat always flows from hot to cold – buckle up it’s the 2nd law!! After stabilization, the base will be at a higher pressure but the temperature will have balanced throughout the tube. For the second law of thermo to hold, the lower density gas molecules need to transfer energy to higher density molecules at an equal rate per unit time — on a bulk scale only — for stable temps.
This means that the low density molecules MUST have a higher kinetic velocity! Interesting, no? If they didn’t, the temperatures wouldn’t be balanced yet, and heat energy would still be flowing.
Again, the velocity (kinetic energy) can be spin, vibration or linear motion, but the probability of energy transfer throughout the entire tube length must balance for the thermodynamics, which drive so much of our lives, to be functional. Therefore, the temperature gradient (lapse rate) on Earth and Venus must be caused by something other than the pressure of the gas. PV = mRT is absolutely NOT the direct cause of lapse rate, those who tell you it is, are wrong. Those who tell you pressure has nothing to do with lapse rate are also wrong, but that is due to convection and that is another story.
On to radiation.
On the previous thread, someone else linked an article which claimed that global warming is false because the principle of back radiation violates the second law of thermodynamics. ‘ Back radiation’ is what occurs in climate science when cooler air at a higher altitude spontaneously emits a photon in a random direction which happens to go downward. It goes upward with equal probability but that wouldn’t be ‘back’ radiation. This is exactly equivalent to shining a light in a random direction. Now there really isn’t such a thing as an expert in electromagnetic radiation as the stuff isn’t fully understood, but lets say I’ve spent a good deal of time with it and am reasonably educated for a human so bear with me.
Electromagnetic waves are governed by the principle of superposition. Where two lasers intersect, the beam fields add together and continue propagating as though they had never met. It is incredibly interesting in that if you re-create the field at any point before during or after the intersection, you exactly recreate the pattern of the original beams. Holography is based on this principle. It doesn’t matter if the beams are different frequencies, or traveling in opposite directions, they have no effect on each other outside of the local interference patterns created.
Like ducks in a lake, the waves add up and keep propagating in their own directions independently of each other.Electromagnetic radiation is emitted by all objects having a temperature. It is a spontaneous occurrence which again, when taken in bulk can be approximated by an equation. Planck’s blackbody equation approximates both the spectrum and the energy of the emission of various materials at specific temperatures. If you’ve made it this far, you probably aren’t one of the most technical readers here (unless you’re trying to catch me in an error) so I’ll put the wiki link in. There are simpler forms, but I’m lazy by nature so the reader is left to find simpler links. Planck’s law is as correct as Einstein and Newton, it governs the color of your incandescent light bulb as well as the sun and even the burner on your electric stove. The color of the output of a visibly hot object is mostly based on temperature. Ever heard the phrase – red hot, or white hot? That is what Planck’s law defines. I am familiar with it so when I first studied global warming, the basic theory was hit you in the head obvious correct within minutes. Actually, it was so obvious I don’t remember considering anything else. Another point is that all of the technical AGW bloggers and climate scientists I know of, agree on these points.
Putting together the principle of superposition and the basic physics of Planck radiation, it becomes apparent that colder objects also emit electromagnetic energy and this energy cannot be stopped (or even altered) by other electromagnetic energy. If the lightsabers of StarWars were actually light, they would simply pass through each other unaffected. That would make a lousy sword fight though!
According to Planck, things above absolute zero (Kelvin) always emit this radiation. Ice cubes, the dark side of the moon, light bulb filaments, glass, liquid nitrogen, and even the background blackness of space has a temperature of 3 Kelvin. That means that staring at an ice cube with a temperature of 273 K (0C) will warm your eyes when compared to the blackness of space. The ice emits IR which strikes you and warms you. You can’t stop it with electromagnetic energy, even if you are the sun.
Now some claim this violates the second law of thermodynamics, but it does not. While the ice is heating my eye, my eye is at 98.6 (or higher if I’m mad at the internet) and emits more energy/unit area back at the ice!! The net direction of energy flow is still always from hot to cold, the god of physics is happy and thus my Id’s universe is complete.
So when people send you links claiming that backradiation from the atmosphere to ground is a violation of the second law, perhaps the question back to them should be – have you seen my lightsaber?