Entropy

[geeksoo]

Hi whiteholes,

Thermodynamics is the study of macroscopic behaviour, and in particular to your example, by employing an approach known as statistical mechanics. Essentially you are considering the total number of microstates in a system, and assigning a probability to that particular microstate. You are corect to say that there exist a situation whereby all the energy is in the pendulum albeit a low probability. However, entropy denoted by S, is the SUM of all the product of the probabilities of all microstate and its natural logarithm, mathematically S=-k sigma( p ln p ). where p is the probability of each individual microstates. Hence what you have done is only considering one of the possible microstates and the summation explains where all your "missing" entropy has gone.

But do you use such powerful telescopes? Or do you have time to study thousands of photos to discover a planet? In order to get accurate data, telescopes have to be sent to space as the atmosphere of the earth will distort light coming into the earth.

On a side note, I happen to be quite informed about developments in this field. Currently, there are about 3-4main planet search groups out there, one based primarily in the california region, led by Prof. Geoff Marcy from UC Berkeley who has discovered the most no. of planets and is also the discoverer of 51 peg B, the first extrasolar planet found. They do not take photographic plates to detect planets in general. However they do take precise measurement of the parent stars radial velocity as well as photometric analysis using the keck interferometers. Direct imaging of planets has only been done recently using the VLT on 2M1207b and GQ lupi b, but it is worth ot note these systems are extremely hard to come by. Many amateurs are indeed contributing to the discovery of extrasolar planets by collecting photometric results, you do not need to send a telescope in space or have very "powerful telescopes" to participate in the search. A commercial grade CCD or photon-meter with a regular 5" newtonian will do fine. (probably an 8 " for catadioptrics, wat most would have here at singastro). Furthermore it is also exactly because these professional astronomers have no time to collect and proccess these data that they "outsource" such stuff to amateurs

[zong]

Very well explained geeksoo, but i don't think half the forum understands half the formulae you wrote here..

Or actually, maybe it's more like we really are too lazy to figure out how that formula was derived. But anyway you have interpreted and explained the topic of entropy well, better than i have done in here. Good job!

[Fuzzball]

I also don't understand.
:?
2nd law of thermo dynamics states: In a spontaneous process in a close system, entropy occurs”

Entrophy can be defined as a measure of randomness or disorder eg a messy room has alot of entrophy, the natural tendency is for the room to go from order to disorder unless an outside force clean it up. my understanding is that entrophy will only increase or decrease in the process of the break down. But entrophy will occur regardless of the rate.

btw, still don't get the point of the discussion.

[annechan]

What is entropy exactly? After reading the whole thing, all I get is:
"entropy is the measure of disorder in a close system"
and
"contrary to popular belief, entropy IS NOT the measure of disorder" or so following by something I can't seem to understand
Can anyone explain to me please?

[geeksoo]

Thermodynamics in general is the study of the relationship between macroscopic quantities, quantities that only have a meaning when you consider a collection of particles, for example temperature(T), pressure(P) and volume(V).(i.e It is meaningless to talk abt the T P or V of a single particle). In the early days, people are not concerned with the microscopic behaviour that are the underlying causes of such macroscopic quantities .This approach of studying such relationship is known as Classical Thermodynamics, in contrast to what we now have, Statistical (Modern) Thermodynamics. Hence the meaning of Entropy in Classical Thermodynamics is slightly different from when it is used in the context of Statistical (Modern) Thermodynamics.

So in classical thermodynamics, entropy often refers to the (qoute from wiki) "measure of the amount of energy in a physical system that cannot be used to do thermodynamic work." which in simple terms is unusable heat energy. This is the definition that most of us are familiar with, and it is also somewhat the same meaning as "measuring the disorder" of the system. Measuring the disorder is just some pretty way of saying heat energy, furthermore to be precise you can't exactly measure(quantify) disorder.

Conversely in statistical thermodynamics, people "measure the disorder" in some other way. They do so by adding up in a certain way all the associated probabilties of each configuration of energy distribution among the particles. So essentially, both are trying to measure the disorder of a system , and it just happens that Statistical thermodynamics is a more acurate description of "measuring the disorder" of a system.I really hope this helps clear up some stuff.

btw, still don't get the point of the discussion.

:roll: Haha I guess theres not much point to it. But if you are refering to the relevance of thermodynamics and entropy to Astronomy, then there is alot of point. Thermodynamics is especially important in studying the interiors and transport phenomenon in stars. For the more theoretically inclined, like whiteholes , you may be intrested to read up on polytropic models, the saha equation and the lane-emden equation for stellar interiors. I wouldn't want to further extend this post to put people to sleep.