Sure thing. A long-term trend in medicine has been to reduce LDL scores in patients. Nowadays, we're trying to increase HDL scores as well. You're up.
Science & Technology Quiz 2: The one with the catchy title.
- Thread starter brennan
- Start date
- Status
Mise
isle of lucy
Boo
!
!
nonconformist
Miserable
Open floor.
Sowwy.
Sowwy.
Perfection
The Great Head.
By quantifying entropy and energy and whatnot flow in and out of it with quantities.
- Joined
- Oct 5, 2001
- Messages
- 30,080
The second law is for an isolated system, where it will tend to maximum entropy. I presume by 'open' system you mean one that is not isolated?
In that case, the second law if fairly meaningless - it probably implies that energy changes over the system boundary can cause entropy to decrease.
I guess you could say that the second law for an open system is actually the third law.... entropy tends to zero at 0 K.
Mise
isle of lucy
Is it to do with reversibility? I.e. processes in open systems are irreversible or something?
Yes, an open system is a system that is not isolated. But the second law is not "meaningless" for an open system. Often the second law is formulated in a way that only applies to isolated systems. However that is only a special case of the second law that can be applied to all systems.
@Perfection: That's a bit vague, and you don't have the right quantities, but that's the right track.
@Mise: Processes in open system can be reversible. If you have a reversible process you can use another special case for the second law, but the general form is also valid for irreversible processes.
- Joined
- Oct 5, 2001
- Messages
- 30,080
I'm not sure what you're driving at here. Regarding open vs closed system, I still assert that for an open system, the concept of entropy is rather meaningless; to decrease entropy, you need a change in energy, which is either heat or work. Yes, this can be a localised change within a system (which could be described as a change within an 'open' system), but in that case it is more useful to simply redraw the boundaries around the affected area to show the energy transfer across the boundary.
The 'open system violation of the second law' is an argument that people like to use to obfuscate that evolution somehow breaks the second law. Is that the answer you're looking for?
I would suggest that the true second-law representation for an open system is simply that you can't decrease entropy without a localised change in heat or work.
The 'open system violation of the second law' is an argument that people like to use to obfuscate that evolution somehow breaks the second law. Is that the answer you're looking for?
I would suggest that the true second-law representation for an open system is simply that you can't decrease entropy without a localised change in heat or work.
I don't get why you think the concept of entropy should be meaningless. It is defined and it is useful. Why should we abandon the concept of entropy just because it is a bit more complcated to apply?
And redrawing the boundaries might not always be easy, so it can be useful if you don't have to do that but still have the second law at you disposal.
No. Although I am annoyed that some people tend to use entropy as an argument without having any idea what it is, that has nothing to do with my question.
Close, but wrong.
I was looking for something more specific, but I guess your answer cannot be argued with, so its your turn.
The second law in its general form can be formulated as dS >= dQ / T, or in words: The change of entropy has to be at least the change of heat divided by temperature. In case of a closed system the change of heat is zero, so the law takes the more popular "change of entropy has to be greater than zero"-form. In case of a reversible process the >= can be substituted by a = sign. In an open system dQ can be smaller than zero, so the entropy can decrease, but this decrease is limited by the amount of heat flowing out of the system. And also note that entropy cannot be decreased by work.
Mass, spin and axis of spin? (Complete guess)
Mise
isle of lucy
I think a black hole can only really spin on 1 axis.
I guess the faster it spins, the greater its radius perpendicular to its axis is in comparison with the radius parallel to its axis?
I guess the faster it spins, the greater its radius perpendicular to its axis is in comparison with the radius parallel to its axis?
Well, that covers two : mass and angular momentum. I consider angular momentum to be a single vector-valued parameter. That's not all however. Try to think of other things that are generally conserved.
As for the singularity I'm just interested in the geometric shape. I.e. is it a sphere, a torus a line? Note that I'm actually referring to the thing inside where curvature goes to infintiy not everything up to the Event Horizon.
Mise
isle of lucy
Parameter guesses: Charge? Energy?
Shape guesses: Sphere? Torus? Line? Point? Two balloons tied together? Funnel? Vagina-shaped?
Shape guesses: Sphere? Torus? Line? Point? Two balloons tied together? Funnel? Vagina-shaped?
Okay, I'll stop here since all the correct answers are now sort of distributed over the answers. It are three parameters, and those are mass, angular momentum and indeed charge.
The shape does only depend on whether the angular momentum is zero or not.
It's a point-shaped singularity if the angular momentum is zero and ring-shaped otherwise.
Your turn.
Mise
isle of lucy
Open Floor!
(i.e. if someone can think of something before I can, ask away!)
(i.e. if someone can think of something before I can, ask away!)
A somewhat lame question:
What colour are a zebra's stripes?
(And why do we consider this the case)
What colour are a zebra's stripes?
(And why do we consider this the case)
- Status
