1. What is a nuclide? From what I know, it's defined by the mass number, atomic number, and energy content. The mass and atomic number I understand. But I don't get the last part.

2. When total entropy is a reality, what will the Universe look like?

3. How would relativity effect the time between earth and mars dwellers? There will be one right? Since both planets are moving around the Sun at different speeds.

4. Is thermal conductivity directly proportional to density? Do things transfer heat faster just because more of it is there to do so? Or is there another factor involved. It seems like everything that feels warmer or cooler at the same temperature is also denser. Could I say that metal feels hotter or colder at the same temperature as wood because there is more of it touching your hand instead of saying that it conducts heat better?

I will try to answer 2 of these questions

2) Entropy is reality. Although entropy is not directly measurable, by knowing the entropy of a system along with something called the partition function, all measurable quantities are able to be found. This only truley holds for isolated systems, and there is debate over whether the universe is truely isolated (eg is there a flow of energy to the universe from some source outside of our universe)

4)Thermal conductivity is not related to density in the way you think. Thermal conductivity is directly related to electrical conductivity and the reason metal are more conductive is due to their electron structure. In insulators, electrons are confined to atomic or molecular orbitals, and thus one of the only ways energy (heat, electrical)"flows" is via phonons or vibrational modes within the solid. Conductors on the other hand contain electrons have electrons that are free to wander throughout the lattice and thus energy gets propogated more freely. Usually metals are more dense than insulators, thus your density relationship.

If you wish to get more complicated, then you have to look at something called the density of states (DOS) of the material, and certain classes of materials have certain structures of their DOS relating to a quantity called the Fermi energy. For example, a semi conductor has a fermi energy at the top of the valence band of the material, then a gap to the conduction band. This gap in energies is called the band gap energy. A metal on the other hand has a continuous DOS where the Fermi energy lies in the conduction band.

I could blab on all day about this stuff, so if you want to know more, I will add more, but next time in a more structured way. In the mean time get onto google and do some searching. :)

Originally posted by Saith
1. What is a nuclide? From what I know, it's defined by the mass number, atomic number, and energy content. The mass and atomic number I understand. But I don't get the last part.

actually, the mass number A and charge Z are enough to specify the nuclide. there is even a simple formula for calculating the energy content, given A and Z. what is energy content? it is just how much total energy the nuclide has, that is, rest energy of the constituent protons and neutrons, potential energy of the nuclear bonds, and kinetic energy if any of them are moving (and some of them must be moving, at least a little bit, for quantum mechanical reasons). by the formula E=mc^2, this is also the total weight of the nuclide.


2. When total entropy is a reality, what will the Universe look like?


well that depends on whether the universe is going to collapse back on itself, keep expanding at a decelerating rate, or keep expanding at an accelerating rate in the end. this is not quite a resolved question, but these days everyone is thinking it s the last one.

in that case, the universe will just have a bunch of isolated particles, each seperated by a horizon, and the temperature will be 10^-30 K because of hawking radiation from those horizons. as you can imagine, every particle being seperated from every other particle gives rise to quite a great deal of entropy, if there is enough volume.

you can read more about this story here (http://math.ucr.edu/home/baez/end.html)



3. How would relativity effect the time between earth and mars dwellers? There will be one right? Since both planets are moving around the Sun at different speeds.


not noticably. it will be there, but it will be miniscule. the planets are moving fast, but not relativistically fast. there will also be a diference on account of earth being closer to the sun, but again, this will not be noticable. maybe someone will give you a more precise answer. i would expect it to be on the order of seconds difference per decade, or less.


4. Is thermal conductivity directly proportional to density? Do things transfer heat faster just because more of it is there to do so? Or is there another factor involved. It seems like everything that feels warmer or cooler at the same temperature is also denser. Could I say that metal feels hotter or colder at the same temperature as wood because there is more of it touching your hand instead of saying that it conducts heat better?

thermal conductivity is not proportional to density. metals are more conductive because their electrons are essentially free, and they can convect heat. in wood, all the electrons are stuck to their nuclei, so they can t go anywhere.

Originally posted by ryans


2) Entropy is reality. Although entropy is not directly measurable, by knowing the entropy of a system along with something called the partition function, all measurable quantities are able to be found.

entropy is reality? what in the hell is that supposed to mean? entropy is reality and everything else is illusion?

and why isn t entropy measurable? in principle, all you have to do is count the number of states.

Entropy is definately not a measurable quantity. You cannot stick an entropmeter into something and measure entropy. If you want to count states, you need to measure say the number of particles in say an energy state thus measure energy.

I was trying to address question 2 which I think really is hard to understand.

Entropy is not measurable.

In systems with a near continuum of states, entropy is indeed hard to measure. In greatly simplified systems, entropy can be measured rather directly. We'll define the inability to measure entropy in complex systems a technological obstacle -- in principle, it is possible to measure entropy.

In the broadest terms, entropy can always be calculated for any system, even those in which it would be "impossible" to measure empirically.

- Warren

Lets take to this to a new thread, it could be an interesting discussion.:)

If a nuclide can be defined by just the atomic and mass number, then what makes it different from an isotope?

Since thermal conductivity isn't proportional to density, then should density be mentioned? If someone wants to know why a metal pole, or an icecube, or a puddle of water feels colder than air, wood, or styrofoam at the same temperature, then you should mention thermal conductivity along with density right? I figured if their proportional then you would only have to mention one. But if they both contribute....

Is "heat capacity" what Im thinking about? Something with more heat capacity I would imagine is proportional to how dense it is and something at the same temperature, with the same thermal conductivity but more heat capacity would feel hotter.

Originally posted by Saith
Is "heat capacity" what Im thinking about? Something with more heat capacity I would imagine is proportional to how dense it is and something at the same temperature, with the same thermal conductivity but more heat capacity would feel hotter.

yes, heat capacity and thermal conductivity both contribute to how cold something feels. heat capacity is also not proportional to density though.

Originally posted by Saith
If a nuclide can be defined by just the atomic and mass number, then what makes it different from an isotope?


isotopes and nuclides are kind of synonomous. chemists usually use the term isotope to mean different nuclide with the same number of protons. nuclear physicists don t care as much about what chemical element the nuclide is.

Originally posted by lethe
isotopes and nuclides are kind of synonomous. chemists usually use the term isotope to mean different nuclide with the same number of protons. nuclear physicists don t care as much about what chemical element the nuclide is.

Then there are:
Isobars: nuclei of equal mass number (# of protons+# of neutrons)
Isotones: nuclei of equal neutron numbers
and
Isodiaspheres: nuclei of equal neutorn excess (# of neutrons - # of protons)

Originally posted by Saith
3. How would relativity effect the time between earth and mars dwellers? There will be one right? Since both planets are moving around the Sun at different speeds.


I have a suggestion on this. Since were talking about different planets, then we are talking about orbits. An orbit is a geodesic, so I don't think that the distance from the Sun will contribute to the temperal variation. The overall curvature of the space-time for each orbit is compensated by the curvature of the space-time at each planet due to the centripetal acceleration (principle of equivalence). The tidal force will be greater at the Earth's orbit, but this is only going to affect the temporal difference from one side of a given planet to the other, and it will be very miniscule so far away from a mass so small.

I think that the only thing to contribute to the time difference would be the relative motion of Earth to Mars, not each planet's motion with respect to the stars (or whatever frame you prefer). Also, it is not just a constant motion that will dilate time, it is the deviation from the geodesic. Therefore, if someone traveled from Earth to Mars, they would think that Earth time was slower, but, if someone traveled from Mars to Earth, they would think Mars time was slower. When, actually, on average (meaning, over the course of a comparative cycle between the planets, not sidereal orbit), there would be no difference in the elapse of time.

I am not 100% sure of this, though; it is just a suggestion.

Thermal conductivity is not necessarily related to electrical conductivity. It is true that most electrically conductive materials are good thermal conductors, this isn't always the case. For example, diamond, which is a pretty crappy electrical conductor, makes an excellent thermal conductor. Another example is my pet ceramic pyrolytic boron nitride. It is a decent thermal conductor, however it has a high resistance (Resistivity, ohm-cmc 10^15). I can't find or remember any examples of stuff that's electrically conductive but not very thermally conductive right now, and I've got to go report my bike stolen, so I'll leave it at this.

I think maybe electrolytic solution would be good electical conductor and pour thermal conductor.

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