If you ever thought physics was my strong point, this post will surely dispel that notion. Anyway, what I understand about atoms is that they have one or more electrons perpetually waltzing around them – at least for temperatures above absolute zero. So where do the electrons get the energy to do so, and why don’t they ever run out of it?
Just because something moves forever and doesn’t dissipate energy doesn’t make it a perpetual motion machine. That is perpetual motion, but it’s not a perpetual motion machine. For something to be a perpetual motion machine, you have to be able to take energy from it to do something without the system losing energy. Perpetual motion doesn’t violate any laws of physics, but perpetual motion machines do.
In the absence of friction, tidal forces, and the like, you can have a planet orbiting a star with its energy always staying the same. That doesn’t violate any laws of physics. But if you somehow harness the orbital energy of the planet to run some sort of machine, the amount of energy in the planet’s orbit has to decrease. It might not decrease much, or even measurably, but it will decrease. When we use a gravitational slingshot around a planet to speed up a space probe, the space probe gains energy, which is taken from the orbital energy of the planet. It’s not noticeable, because the planet is so much more massive than the space probe, but there is a change in the planet’s orbit.
[bolding mine]
You answered your own question. The motion within an atom is dependent on an outside heat source, and thus not perpetual.
It is important to note that electrons do not “move” in the sense most people think they do. They are not like little planets orbiting a tiny sun (how they are often depicted). An electron is a wave function. A smudged out probability smear.
Also an atom is effectively in a vacuum. Just like anything else an object in motion will continue in that motion. So even if they were tiny little balls orbiting the nucleus there is no issue here. They would merrily continue to do so since nothing is taking energy from them.
RNATB, just to reconcile these two answers, are you saying that the motion of the electrons is dependent on (or caused) by the outside heat source? Per Anne Neville’s answer, does that mean that the atom is taking energy from the heat source, leaving the heart source (whatever it is) with less?
Yes.
If it helps, keep in mind that the “heat source” is not whatever produces the heat but wherever the atom gets it from- surrounding atoms, for example.
Heat energy added to an atom will put the electrons in a higher energy state and is one of the reasons you can combine atoms into molecules.
And why, on a macro level, you can take apples and make apple pie.
Mmm… Pie…
Electrons don’t go around the nucleus of an atom. They aren’t like little solar systems. For some reason, atoms are often explained this way, especially in low-level science classes, but it isn’t so.
The way an electron behaves in an atom isn’t influenced by heat, either. That is, the electron doesn’t exhibit thermal motion.
If the electron did run circles around the nucleus, you should expect it to export energy in electromagnetic radiation because there is acceleration of charge.
Electrons exchange energy with outside sources by absorbing or emitting energy quanta. The name for a quantum of electromagnetic energy is a photon.
I thought that perpetual motion machines were machines that exhibited perpetual motion, and that the further requirement that you be able to keep extracting energy from the system had to do with something being even more useful than a perpetual motion machine, perhaps being a “perpetual energy generating machine”. That is, perpetual motion machines are not only apparently impossible, they wouldn’t even be useful.
[
No it isn’t. According to the Bohr model, electrons can only orbit a nucleus at discrete (quantized) energy levels, and they can only change levels by absorbing or radiating the energy difference. There is a temperature associated with this based on how far above the vacuum state an electron is, but it isn’t the same as random motion in an aggregate or any other concept of temperature as experienced in the everyday world, and the electron neither needs or can take on additional energy while remaining at the same orbital state. An undistrubed electron in orbit of an atomic nucleus will continue to orbit in perpetuity, or at least until the proton decays (which takes a very, very long time–much longer than the current lifespan of the universe–if it occurs at all).
Anne Neville is correct; simple (or simplified) systems involving only clearly delineated fundamental forces (or interactions) are conservative. As such, the energy never disappears from a closed system; however, energy can be lost from use because it is too randomized to access. As an example, if you inflate a balloon with air (or heat the air), there is energy stored within the excess pressure inside. However, if you are inside the balloon you can’t make use of this pressure because it is the same (at equilibrium) everywhere in the balloon; there is nowhere for the energy to come from and go to, and thus no work can be done. If you poke a small hole in the balloon, however, the air will flow out of the high pressure environment of the balloon and into the low pressure ambient environment; by mounting a fan or turbine in the path of the escaping air you can capture some (but not all) of the energy to do work. Macro world systems lose energy to heat (randomized energy) due to friction, internal hysteresis, thermodynamic limitations, et cetera, and the only way to recover this energy is to find a volume that has less energy than the area containing the rejected heat, which like the high pressure air in the balloon example, flows into the low energy area. Thermodynamicists idealize these as high temperature and low temperature “reservoirs”.
Isolated quantum systems don’t experience thermodynamic limitations; however, interactions between systems of quantized particles follow an analogue of the laws of thermodynamics based upon statistical uncertainties of QM. An electron in orbit of a nucleus will remain in orbit and will be localized as a non-time dependent function, but interactions between two electrons and their resultant complementary properties (say, position and momentum) will vary statistically from the time of interaction, contributing to increasing to “disorder” or uncertainty between the systems.
Stranger
Eh?
At absolute zero all molecular motion stops, no?
Now, there’s an answer I can run from!
But thanks – I guess I’m beginning to get it. I didn’t realize the atom was a vacuum. And now I don’t think I want to ask why it’s a vacuum.
Nope. Even at T=0 is at the ground state, but it still has energy and thus “motion” (albeit in the QM sense rather than in classical mechanics). In reality, there are qualitative limits to how close any real system can get to the ground state for a given interval of time.
I’m not quite sure what you mean about the “atom [is] a vacuum”, but a vacuum can be defined as the state at which you cannot extract energy. There is energy even in the vacuum state, and attempting to create a sub-vacuum state (see Casimir effect) results in negative (attractive) energy creation and the creation of “virtual” particles which interact to oppose the creation of a lower energy state.
As Napier says, if electrons were orbiting the nucleus in the classical sense they should be radiating away energy and spiraling into the nucleus, making all matter extremely unstable. The fact that this doesn’t occur gave rise to the Bohr model of quantized energy levels and ultimately the modern valence shell model of atomic structure and quantum chemistry, which is one of the pillars of modern QM.
Stranger
Keep in mind that the OP is talking about movement within the atom, not movement of the atom. Atoms (or molecules) moving around = heat, and this is what you are talking about. But looking at an atom, by itself, and the energy associated within that system is what the OP is talking about.
That’s just me remembering (and distorting) something from upthread.
They’re explained that way because the Bohr model of the atom (the technical term for the “miniature solar system” model) is a lot easier to understand than what is actually going on, and is useful for some problems. You can use the Bohr model to understand what is going on in ionic bonding, or when atoms radiate at certain wavelengths when electrons move from one level to another. And you can understand it without the kind of advanced math or quantum mechanical concepts that you need to understand a more accurate model. Most high school students would have problems with the math required for matrix mechanics (if you are a math-phobe, for the love of God and your sanity, do not click that link), and most people have trouble visualizing what’s going on in any kind of quantum-mechanical system. A miniature solar system, on the other hand, is easy to visualize.
What he (she?) meant by that is that there are no drag forces on an electron. There wouldn’t be air resistance, obviously- air is made up of molecules, which are bigger than electrons. There aren’t random subatomic particles around that collide with electrons and make them slow down.
You know, this never occurred to me, though it’s pretty obvious when you think about it. So that dead space between the molecules that make up air – does it have a name? Does it have any properties to speak of? (And forgive me if I sound a little like Chance the Gardener here.)
Yep: vacuum.
Okay, that was a little too obvious. I guess I should have asked if it had a brand name.
Yes, what Stranger and John and Anne said.