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We all know (or maybe know) that to move, we need to spend energy. If you want to drive a car, you gotta spend gasoline.

We also know that energy can't be created (first law of thermodynamics, and perpetual motion).

Also, we know that in energy transformation, in real-world almost some part of it is converted into heat produced because of the friction between motion bodies. (for example, part of the gasoline of the car in burned to overcome car's friction with air, and thus would be converted into heat, and won't serve any movement at all).

Now a question has obsessed my mind. How electrons circulate around nucleus for ever? Where does electron get its energy from?

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There is a basic misunderstanding of elementary classical physics in your question.

We all know (or maybe know) that to move, we need to spend energy

The first law of Newtonian mechanics says"

The velocity of a body remains constant unless the body is acted upon by an external force.

So there is no need to spend energy to keep on moving, unless external forces are acting on the body.

Electrons around a nucleus are not a classical problem, but conservation of energy holds also in the quantum states. The electron around the nucleus is in a quantized energy level and can change it only if an external interaction intervenes. It is quantization that guarantees this, since in the classical problem of a charge circulating around an opposite charge there would be continuous radiation which would have made the electron fall into the nucleus. Quantized energy states for the electrons are necessary for atoms to exist and were first proposed by Bohr.

Subsequently quantum mechanics became a full blown theory and needs years of study to assimilate it.

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So, does this mean that if hypothetically, we consume energy of electron little by little, its speed will slow down? – Saeed Neamati Dec 19 '11 at 9:30
Also, does this mean that there is no energy-wasting (like friction) in the level of an electron? Does electron rotate in an absolute empty space? – Saeed Neamati Dec 19 '11 at 9:33
We cannot change the energy of the electron, hypothetically or really, except by the allowed transitions from one energy level to another. That is what quantization means in the microcosm. There is no energy incoming or outgoing in the sysem "nucleus-electron" unless there is external interaction. That is the whole point of quantum mechanics. We only know what the electron is doing if we interact with it. Otherwise we only have probability distributions of where it may be if we interact with it. It is not rotating in the sense of classical physics, like the moon around the earth. – anna v Dec 19 '11 at 10:19
continued: It is a distribution that describes the electrons probable path at the given energy level. One needs the mathematical tools to get an understanding of the situation, words are inadequate. – anna v Dec 19 '11 at 10:22
@annav your explanations were fantastic. Thank you! – Dexters Jan 16 '15 at 4:17

With no friction the angular momentum of the electrons is preserved, just like planets orbits their stars with very very little (general-relativistic) losses.

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To describe elementary particles, classical mechanics is not enough. One needs quantum mechanics, and a lot of study to get what is really going on in the microcosm of the atom. – – anna v Jan 8 '13 at 7:35

i think some energy is always lost through revolution of electron around the nucleus even when we are considering the atom as an individual or when in the "crystal lattice" because over the years the identity of the atom is changed. (think for a while before jumping to a quick questioning) that's why isotopes are there! it may be possible that many of the elements are very much stable and over millions of years they haven't lost their identity,but there is always a CARBON type thing that help us to crack some of the greatest mystery of all.

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-1: Energy is not lost in an orbiting electron the same way as it is lost gradually when I spin a yo-yo around my head. An atom doesn't 'loose its identity' over time and gradually become an isotope. An atom always has a well defined number of neutrons. – DJBunk Jan 7 '13 at 22:59
You're failing in extrapolating everyday experience down to the scale of single atoms. Indeed, every day experience shows that things are constantly running down, but if you keep careful track of the energy you find that it is not disappearing, simply getting spread out among the great number of atoms that make up every day objects. This is called friction, and if you work to eliminate friction you find that energy is not lost in maintaining motion. In addition, friction doesn't operate at the scale of individual atoms as there is nowhere for the energy to dissapate to. – Michael Brown Jan 8 '13 at 1:57

i do not hold any degree in quantum physics (actually in highschool) but as far as i know, the electron revolves around the nucleus in an arbitrary orbit. This means that in different time periods it is in different positions relative to the nucleus so in t1 its electrical force may be z while in t2 it gets 2z so in prder to counteract this increasing force energy is spent but in order to be spent it has to be somehow generated or accumulated. My point is that the electron has got to build up speed in order to avoid colliding to the nucleus. How is this speed built? Maybe it all has to do with inertia and the laws of circular motion or there must be a way the electron uses its spin and charge through converting it into kinetic energy. Anyway it is a well known fact that the electron exchanges energy with the nucleus. My logic may be wrong but this is just an assumption.

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-1 This didn't really answer the question and made little physical sense. In what sense is energy "spent to counteract this increasing force" or "accumulated"? Furthermore, the electron in no way "exchanges energy with the nucleus." – Shivam Sarodia Jan 8 '13 at 3:01
Hi Tommy. Since you are in high school you still have a lot of physics ahead of you to learn . To describe elementary particles, classical mechanics is not enough. One needs quantum mechanics, and a lot of study to get what is really going on in the microcosm of the atom. – anna v Jan 8 '13 at 7:34

protected by Qmechanic Jan 8 '13 at 12:00

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