Science questions not worth a thread I: I'm a moron!

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I sort Of understand how in the dying stages of a stars life the heavier elements are formed . Now right before a star goes supernova it's "making" iron right ? But arent there elements with a greater atomic mass than iron ? How and when are they formed ?
 
When the star runs out of hydrogen to fuse into helium at the core, there isn't enough energy being produced to push against the force of gravity of the star, so it collapses, and all the heavier elements are formed during the collapse of the star via fusion, but it isn't enough to prevent the collapse, IIRC. After the collapse the momentum throws all the heavier elements out in a supernova, unless the star was so massive as to become a neutron star or a black hole.
 
ParadigmShifter is right that it is theorized to happen during during a supernova, but the process is not fusion. Instead it is rapid neutron capture. When a big enough star collapses because fusion has topped, neutrons (and neutrinos) will be created from protons and electrons. When the star blows apart due to gravitational energy, a part of these neutrons will be blown out and all nuclei will be exposed to a very high intensity neutron flux. Those nuclei have a chance to capture those neutrons. And although the resulting isotopes are very unstable and would decay in a short amount of time, the neutron intensity is so high, that another neutron is absorbed in an even shorter amount of time. This way, very heavy elements can be formed that are very neutron heavy, until either the element becomes heavy enough to show spontaneous fission, or the neutron stream dies down and the isotope can decay, mainly via beta-decay, to the stable isotopes.
 
U is for potential energy; I can only assume they used those letters because they were next to each other.

Sent from a phone, apols for any mistakes.
 
A quantum well is a potential that looks like this:

Code: 
________        _________
        |      |
        |      |
        |      |
        |      |
        |      |
        |______|

with the size of the structure being on the order of the wavelength of the particles. In that case, the particles' energy is quantized so that they can only occupy discrete energy levels (in the direction of the quantum well).

A double well is just two of those placed close together:
Code: 
________        ____        _________  
        |      |    |      |
        |      |    |      |
        |      |    |      |
        |      |    |      |
        |      |    |      |
        |______|    |______|
To be interesting, the distance should be small enough so that there is some kind of interaction between them, e.g. particles tunneling from one well to the other.
 
uppi said:
A quantum well is a potential that looks like this:

Code: 
________        _________
        |      |
        |      |
        |      |
        |      |
        |      |
        |______|

with the size of the structure being on the order of the wavelength of the electrons. In that case, the electrons' energy is quantized so that they can only occupy discrete energy levels (in the direction of the quantum well).

A double well is just two of those placed close together:
Code: 
________        ____        _________  
        |      |    |      |
        |      |    |      |
        |      |    |      |
        |      |    |      |
        |      |    |      |
        |______|    |______|
To be interesting, the distance should be small enough so that there is some kind of interaction between them, e.g. electrons tunneling from one well to the other.
Click to expand...

Are the wells physical?
Could you try to put this into layman's terms?
 
ShahJahanII said:
Are the wells physical?
Could you try to put this into layman's terms?
Click to expand...

This requires heavy invocation of quantum physics, but I'll try:
The wells are features in the potential energy of a structure. If we were talking about gravitation, the wells would be two trenches in a plane. If you would put a ball on the plane it would move freely, but once it gets into one of the trenches, it would fall down and be stuck there.

But with quantum wells we are (usually) talking about the electromagnetic force acting on a charged particle, e.g. an electron. As long as the electrons stays in the part of the structure where the potential is flat, it can move around freely. But if it gets into the part with the trenches it can emit energy and be stuck in the area were the wells are in the potential.

If the electron would be behaving classically, it would behave as before with the only exception that it cannot leave the well. But if the well is small enough, the fact that the electron behaves as a wave comes into play. The electron does not behave as a ball anymore, but is described by a probability distribution behaving as a standing wave in the well. If you solve the Schrödinger equation for that situation, you see that there is only a finite number of energies the electron can have. If we go back to the analogy of a gravitational potential it would be if a ball could only hover at heights of 1m, 4m and 9m, but not at some height in between (but along the trench it could still move freely).

In a double well, another quantum effect becomes relevant: If the wells are close enough together, the electron can tunnel from one well to the other. That means, the electron can "jump" over the barrier between the wells and can be found in the other well than it was put in initially. This would be like a ball lying in one trench, which is then suddenly found in a parallel trench without being thrown over the barrier separating them.
 
uppi said:
This requires heavy invocation of quantum physics, but I'll try:
The wells are features in the potential energy of a structure. If we were talking about gravitation, the wells would be two trenches in a plane. If you would put a ball on the plane it would move freely, but once it gets into one of the trenches, it would fall down and be stuck there.

But with quantum wells we are (usually) talking about the electromagnetic force acting on a charged particle, e.g. an electron. As long as the electrons stays in the part of the structure where the potential is flat, it can move around freely. But if it gets into the part with the trenches it can emit energy and be stuck in the area were the wells are in the potential.

If the electron would be behaving classically, it would behave as before with the only exception that it cannot leave the well. But if the well is small enough, the fact that the electron behaves as a wave comes into play. The electron does not behave as a ball anymore, but is described by a probability distribution behaving as a standing wave in the well. If you solve the Schrödinger equation for that situation, you see that there is only a finite number of energies the electron can have. If we go back to the analogy of a gravitational potential it would be if a ball could only hover at heights of 1m, 4m and 9m, but not at some height in between (but along the trench it could still move freely).

In a double well, another quantum effect becomes relevant: If the wells are close enough together, the electron can tunnel from one well to the other. That means, the electron can "jump" over the barrier between the wells and can be found in the other well than it was put in initially. This would be like a ball lying in one trench, which is then suddenly found in a parallel trench without being thrown over the barrier separating them.
Click to expand...

So, a well for an electron would be similar to a magnet or something else that would exert a force on it indirectly? I think I'm getting there, could you provide a real-world example?
 
ShahJahanII said:
So, a well for an electron would be similar to a magnet or something else that would exert a force on it indirectly? I think I'm getting there, could you provide a real-world example?
Click to expand...

In principle a magnet might be able do the trick, but the properties of magnetic fields are not really suited to engineer such a potential on the required scale. Real-world examples are usually semiconductor structures. In a very simple picture, the atoms exert forces on the electrons and the force depends on the specific semiconductor. By putting thin layers of different semiconductors on top of each other, the force is varied over the layers and you can get the wells.
 
ShahJahanII said:
So, a well for an electron would be similar to a magnet or something else that would exert a force on it indirectly? I think I'm getting there, could you provide a real-world example?
Click to expand...

Not exactly indirectly, because gravitation is exerting a force directly on an object. Something indirect will be me throwing a ball at someone to indirectly apply a force onto them. Though you can argue that forces are just exchange of force carrier particles, but that's beside the point.

Try thinking of a capacitor, or the charge imbalance you can feel just before lightning strikes.

Though the double well example is best represented with things like ammonia molecules if you are slightly familiar with chemistry. Ammonia has an arrangement like a Calthrop (Tetrahedral), with three of the points occupied by hydrogen atoms, the center by Nitrogen, and the other point by the Nitrogen atom's unshared electrons.

There are two configurations for this atom without physically breaking it and re-arranging the positions with respect to each other. The Ammonia electron pair pointed up or down (or left and right, but that's just a rotation by 90 degrees of the thing). These correspond to two different potential wells, close enough such that occasionally the molecule will spontaneously flip its orientation (think of the electrons as suddenly popping up the other side, and the hydrogen atoms flipping up or down to reform the tetrahedral shape). That is tunneling in the double well.
 
uppi said:
In principle a magnet might be able do the trick, but the properties of magnetic fields are not really suited to engineer such a potential on the required scale. Real-world examples are usually semiconductor structures. In a very simple picture, the atoms exert forces on the electrons and the force depends on the specific semiconductor. By putting thin layers of different semiconductors on top of each other, the force is varied over the layers and you can get the wells.
Click to expand...

Bluemofia said:
Not exactly indirectly, because gravitation is exerting a force directly on an object. Something indirect will be me throwing a ball at someone to indirectly apply a force onto them. Though you can argue that forces are just exchange of force carrier particles, but that's beside the point.

Try thinking of a capacitor, or the charge imbalance you can feel just before lightning strikes.

Though the double well example is best represented with things like ammonia molecules if you are slightly familiar with chemistry. Ammonia has an arrangement like a Calthrop (Tetrahedral), with three of the points occupied by hydrogen atoms, the center by Nitrogen, and the other point by the Nitrogen atom's unshared electrons.

There are two configurations for this atom without physically breaking it and re-arranging the positions with respect to each other. The Ammonia electron pair pointed up or down (or left and right, but that's just a rotation by 90 degrees of the thing). These correspond to two different potential wells, close enough such that occasionally the molecule will spontaneously flip its orientation (think of the electrons as suddenly popping up the other side, and the hydrogen atoms flipping up or down to reform the tetrahedral shape). That is tunneling in the double well.
Click to expand...

Really helps, thank you. :goodjob:
 
If I had a bucket of water at its triple point, what would it look like?
By the same line of reasoning, what would a bucket of super-critical water look like?
 
Triple point looks like slush with bubbles coming out of it. The thing is, I don't think you can tune the volume fast enough to hold it at triple point so the water transitions to ice. I think.

Supercritical water... you know on a hot day when you see the ground shimmering from uneven pockets of air? Kinda like that. But flowing.
 
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