h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

What is it? And do I need it?

In ballistic calculations it's what makes
radar aimed artillery unnecessary if you
have a good eye and a sense of maths.
Although much depends on whether your
target is self propelled.

Also applicable in golf and basketball.

Some of the modern astrophysicist types,
even some evo-biologists, rely on it to
fill in their gaps.


~jwf~

"I shot an arrow into the air;
It came to earth I know not where"
- because I know naught about
entropic arcs.


~jwf~

What are you smoking?

It's a Thermodynamic property used in Heat Transfer calculations.

see here http://answers.yahoo.com/question/index?qid=20061217110132AAWv5kQ

no you probably don't need it.

Thanks. I was looking for an explanation that I could understand.

It's closely linked to Enthalpy as i recall from my Heat Transfer Lessons at college.

>>
Science Dictionary
"entropy
A measure of the amount of energy in a physical system not available to do work. As a physical system becomes more disordered, and its energy becomes more evenly distributed, that energy becomes less able to do work. For example, a car rolling along a road has kinetic energy that could do work (by carrying or colliding with something, for example); as friction slows it down and its energy is distributed to its surroundings as heat, it loses this ability."

That example of the car is somewhat confusing because it assumes
that by 'rolling along a road' we understand the car is not running
under its own power but 'coasting' down a hill or with decreasing
momentum after disengaging the engine.

It has also been constricted by the supposed refinement of
calculating 'heat loss' due to friction which was beyond the
ken of early artillery experts who simply knew that if you
shoot something up it will fall in a predictable arc once
its initial energy is dissipated.

As a result of these 'nouveau' heat loss calculations the term
has been almost totally usurped by thermodynamicists and
astrophysicists - but it was also recently utilized in the
lexicon of evolutionary biology.

It was even for a time quite a buzzword in the social sciences
referring to the disintegration of social systems.

Its origins however are in ballistics and refer to the very
predictable arc of a falling cannonball or an arrow.

There is a formula for calculating the arc of a projectile
as it loses impetus and falls toward a predictable target.

Like in basketball or golf or mortar fire.
(But not to rocket propelled grenades - RPGs)

Sorry for the lack of clarity. But let's blame it on
the changing fashion of scientific lingo and not my
smoking habits.


~jwf~

I seem to have a greater capacity for entropy as I growolder.

So, smoking aside, is it more order or more disorder?

It is neither. It is a measure of the order/disorder of a system.

It's absolute value is generally irrelevant at any one moment, and is often hard/impossible to define.

What *is* important is the change in it.

Systems will spontaneously change to greater entropy. So *more* disorder.


It is possible to reduce local entropy but only at a cost of greater entropy elsewhere.


So the analogy I like to give the students I tutor on this is the bedroom of a teenager.

It is very, very easy, nigh on spontaneous for a bedroom to achieve greater untidiness. It is in fact almost the natural state - i.e. it acheives greater entropy very easily.

To achieve lower entropy in that bedroom (i.e. tidiness) requires *huge* effort elsewhere. Great effort upon the owner of the bedroom to actually tidy it, and probably much aggravation upon parent/guardian in terms of arguments, cajoling and bribery to get it done at all.


*You* don't need it, but you may need to understand it to pass your exams and it is a law of nature that exists whether you like it or not.

I am a little bit confused when you say it is order/disorder.

Well order/disorder comes in many different fashions which is why it's hard to define an absolute value.

Water in a glass is 'ordered' but crack that glass and it will disorder itself as much as possible by spreading all over the flooe.

But that glass of water is more disordered than if it was colder.
At 0 °C it will *really* order itself by all the molecules aligning with one another to form ice crystals, lower entropy in the water molecules themselves.

But add a lot of heat and it will get even more disordered by becoming a gas - instead of distributed all over the floor the same number of molecules are disordered all over the room,floor, air, ceiling and walls.


Snooker balls arranged as they are at the beginning of a frame are ordered into a triangle of reds and the colours on their spots. But hit them with the white and they become more disordered. Try and arrange them back to the begining without intervention of the referee

CDs/DVDs arranged in alphabetical order are low entropy, mix them up, still on the shelf and you have greater entropy. Strew the boxes over the floor and you have greater entropy still. Take the discs out and throw them out of the window...


A pile of wood is a nicely, low entropy arrangement of woody type stuff in a pile. Burn it and wow, you've really done some entropy increase, all those carbon, oxygen and hydrogen atoms, distributed around the neighbourhood.

etc. etc. etc.

I thought entropy had something to due with in China and hurricanes in the Atlantic Ocean.

Yep, you could say entropy is the tendency for systems to move from a more ordered to a less ordered state. It's relevant if you build engines or such.

You're thinking of Chaos Theory Witty Moniker but entropy is certainly part of that

>>So, smoking aside, is it more order or more disorder?<<
When entropy increases the disorder increases. So you could say entropy is a measure of disorder. The greater the entropy the greater the disorder. The lesser the entropy the lesser the disorder.

Is that clear?

"I thought entropy had something to do with butterflies in China and hurricanes in the Atlantic Ocean." [Witty Moniker]

That's the Chaos Theory. Chaos and entropy are very similar in my supremely disorded house.As for butterflies and china, I have a nice china pattern that features butterflies.

Next question: How is chaos different from disorder?

The one with an *a* in it is chaos.