Why do Electrons Move?
Most recent answer: 10/22/2007
- David DeCarli
Cromwell High School, CT, USA
Awesome question! (Give your student my compliments for thinking it up!) Naturally, one would think that because protons are positively charged, and electrons are negatively charged, the two should attract and stick together. The reason that doesn't happen can't even begin to be explained using classical physics. This was one of the key mysteries that were cleared up right away by the invention of quantum mechanics around 1925.
The picture you often see of electrons as small objects circling a nucleus in well defined "orbits" is actually quite wrong. As we now understand it, the electrons aren't really at any one place at any time at all. Instead they exist as a sort of cloud. The cloud can compress to a very small space briefly if you probe it in the right way, but before that it really acts like a spread-out cloud. For example, the electron in a hydrogen atom likes to occupy a spherical volume surrounding the proton. If you think of the proton as the size of a grain of salt, then the electron cloud would have about a ten foot radius. If you probe, you'll probably find the electron somewhere in that region.
The weird thing about that cloud is that its spread in space is related to the spread of possible momenta (or velocities) of the electron. So here's the key point, which we won't pretend to explain here. The more squashed in the cloud gets, the more spread-out the range of momenta has to get. That's called Heisenberg's uncertainty principle. It could quit moving if it spread out more, but that would mean not being as near the nucleus, and having higher potential energy. Big momenta mean big kinetic energies. So the cloud can lower its potential energy by squishing in closer to the nucleus, but when it squishes in too far its kinetic energy goes up more than its potential energy goes down. So it settles at a happy medium, with the lowest possible energy, and that gives the cloud and thus the atom its size.
That basically answers your question, although we admit that the answer sounds strange. There really are very definite mathematical descriptions to go along with those words.
You might be interested in some more properties of those electrons in atoms.
If just the right amount on energy is applied, it is possible to knock an electron up to a higher energy orbital (a different shape of cloud, not so close to the nucleus), or even completely off of the atom. If electrons are knocked off of the atoms, they can create electricity. (This is what you see when you look at a VanDeGraff Generator or at lightning.)
If they are only given some energy, but not enough to knock them loose, they will move from one orbital to another (say from the S-orbital to the P-orbital). But if there is no other electron in the lower-energy orbital, they will fall back down again. When they do, they release energy in the form of a photon (light). This is part of the concept that lasers are based on.
Well...I apologize for this answer being so long. Thanks for sticking with me up to here! I hope this answers your question.
(published on 10/22/2007)
Follow-Up #1: Forcing Electrons
- Sally Anne Rosenberg
Park School, Alhambra, Ca. USA
Once you get some atoms or electrons moving around quickly, say with a burner or electrical heater, they can bounce off other atoms, transferring some of their energy.
(published on 10/22/2007)
Follow-Up #2: electron orbits
- John (age 17)
Jamestown, Ohio, United States
Electrons in atoms, like all objects on a small scale, show quantum properties which cannot be pictured in any familiar way. They don't have either a particular wavelength or a particular position.
The explanation of why the electrons don't collapse in further toward the nucleus is more like this. In classical physics, a particle can have any kinetic energy regardless of what position it's at, but not in quantum mechanics. The kinetic energy is determined by the shape of the same 'wave-function' which also represents the probable positions of the particle. If the wave is scrunched in tightly, the kinetic energies it represents are big. So when a wave starts scrunching in close to the nucleus, its kinetic energy goes up more than its potential energy goes down.
The ordinary atomic size minimizes the total energy.
(published on 10/07/2008)
Follow-Up #3: electrons in atom move?
If that sounds mysterious, it is.
(published on 06/10/2009)
Follow-Up #4: source of energy?
- RAGINI (age 15)
MUMBAI, MAHARASHTRA, INDIA
(published on 06/23/2009)
Follow-Up #5: Acceleration of charged particles
- Alex (age 17)
(published on 07/05/2009)
Follow-Up #6: electron relativistic waves
- ??????? (age 18)
For small atoms, relativistic effects aren't very big. The binding energy of the electron in hydrogen is about 13.6 eV. (That's about -27.2 potential, +13.6 kinetic.) The rest energy of an electron is about 500,000 eV. So the kinetic energy is small compared to the rest energy, and thus relativistic effects are small. For the inner electrons in big atoms the energies are large enough for the relativistic effects to be major.
The basic quantum rules (the Dirac equation, for an electron) apply whether or not the particle is in a bound state. The Dirac equation is relativistic and applies regardless of the electron's energy, unlike the approximate non-relativistic Schroedinger equation. The Dirac equation is still a wave equation. If the electron cloud is accelerated, its spatial dimensions change according to the same Lorentz transforms as any other spatial dimensions. If the cloud starts off spherical, it becomes pancake-shaped.
(published on 08/04/2009)
Follow-Up #7: non-question
- John (age 21)
(published on 10/24/2009)
Follow-Up #8: why quantum?
- Bo (age 21)
In almost every modern interpretation of quantum mechanics electrons (and all other small things) show wave-like behavior because they are indeed waves. They are, however, quantum waves, which in some regards behave quite differently from classical waves. For example, when the wave is heading toward many different-looking outcomes, you only see one, not a combination. Sometimes that's reminiscent of how a particle, heading to just one place, would act.
One interpretation, due to David Bohm, claims that these quantum objects are actually point-like coordinates influenced by a wave. So in this interpretation an electron is a wave plus a coordinate dot. That's the closest to the picture you have in mind.
So far as I know, the Bohm interpretation adds nothing (except some hassle with relativity) to the simpler interpretation that the wave is all there is.
(published on 10/24/2009)
Follow-Up #9: quantum energies
- Ze Xuan (age 13)
(published on 11/14/2010)
Follow-Up #10: why quantum electron?
- Abstract1 (age 21)
From the point of view of the fundamental equations of physics, the mystery (if any) is not how an electron can be spread out but rather how the planet can be not spread out.
(published on 03/03/2011)
Follow-Up #11: quantum facts
You may believe that relativistic quantum mechanics is a "pseudo-explanation". Nonetheless it is spectacularly successful at predicting experimental results. For example, it predicts the electron's gyromagnetic ratio (the ratio of an electron's magnetic moment to what would be expected in a simple classical picture) to within ten decimal place accuracy. (see )
There are indeed mysteries concerning the relation between the facts of the definite quantum behavior of small things and the chancy behavior of large things. We can't get around those mysteries by pretending we don't know what we know experimentally about the small scale.
(published on 09/07/2011)
Follow-Up #12: Why do electrons move?
- Ray Lavey (age 65)
I know this sounds pretty abstract. All I can do is recommend studying some beginning quantum mechanics.
(published on 09/18/2012)
Follow-Up #13: quantum chance and determinism
- Heather (age 44)
To the best of our knowledge the quantum state of anything changes following a purely deterministic (and also linear) equation. The first version of that, the Schroedinger equation, described the behavior of a single particle in a fixed classical environment. However, those basic features (determinism, linearity) have remained exact properties of all subsequent quantum field theories, describing many interacting quantum objects. Quantum field theories have been confirmed countless times in an enormous variety of experiments.
So that leads to a big problem, often described by the Schroedinger cat story. The output state from any linear theory is exactly the same as the sum of the output states of the components of the input states. That means that the outcomes of typical quantum starting states include wildly different large-scale events, such as a live cat and a dead cat, summed up or "superposed".
The problem is that no one has ever seen any superposition of such different large-scale realities. You only see one or another, following probability rules. Somehow randomness comes in on the road from simple quantum rules to big-scale events.
There are a variety of ways, all unpleasant, of trying to explain how this happens. They go by the name of "interpretations of quantum mechanics".
(published on 10/29/2012)
Follow-Up #14: electron motions
- dan (age 28)
As for your first question, electrons don't move at the speed of light. I'm not sure what you meant about the part about "inability to be observed".
(published on 01/09/2013)
Follow-Up #15: electron wave and lightning
- David (age 16)
The fact that electrons are waves is somewhat counter-intuitive. as suggested by De Broglie in around 1923, electrons are waves with wavelength given by . This equation implies that the typical De Broglie wavelength of electrons is so small that it's not observable by visible light. Also, the wave-property of electrons propagating in some direction doesn't have the sorts of crests and troughs that ocean waves have. What's waving (a quantum phase) is less directly observable than a density. Thus, even though comes from a collection of moving charges, their quantum wave-patterns are not visible.
Hope this helps,
(published on 01/29/2013)
Follow-Up #16: electron compressibility
- Vishnu (age 15)
Those are subtle questions.
Nothing is entirely incompressible. The near-incompressibility of the electrons we mentioned above wasn't a quantum effect. It's just that the many electrons in the metal strongly repel each other and therefore are hard to push together. There is also a quantum resistance to compression, harder to explain, that affects even single electrons.
The 'happy medium' story involved that quantum springiness. Take a hydrogen atom. The positive proton in the middle is pulling the electron cloud in. But the quantum springiness pushes the electron cloud out. As the electron cloud gets pulled in more, both those effects grow but the quantum springiness grows more. When the cloud is just atom-sized, the two effects balance each other.
(published on 02/07/2013)
Follow-Up #17: quantum superpositions
- Lakhi (age 50+)
Ann Arbor, MI
Interference effects provide the evidence for the coexistence of the non-classical superpositions of different classical possibilities. It gets harder and harder to detect these superpositions on larger scales, because processes called decoherence cause loss of interference between the different possibilities.
So far we've never seen the loss of interference except when ordinary decoherence effects are expected. That makes us suspect that the basic quantum rules apply on all scales. If so, then the superpositions would continue on all scales, but without the telltale interference effects. That would mean that our minds exist in decoherent superpositions of different states seeing all the different outcomes of quantum processes. That's called the "Many Worlds" interpretation of quantum mechanics.
(published on 02/10/2013)
Follow-Up #18: non-classical superpositions
- Lakhi (age 50+)
Ann Arbor, MI
2. There are many, many such examples. Here's a favorite: little carbon soccerballs, buckyballs, show interference when shot through two slits, indicating that the state included the same buckyball simultaneously in both slits. ()
One might think that some fancy theory without such weird superpositions could account for the interference effects. The many experiments which show violations of the Bell Inequalities (search this site and the web) show that we really have no choice but to admit that these weird superpositions are real.
(published on 02/11/2013)
Follow-Up #19: Where is an electron?
Nice question. The answer may be simpler than you expected. No, you are not even partly correct.
As we discuss in various questions, if that picture of the electron really being somewhere were true (and the same for other smeared quantum properties), then some mathematical relations called the Bell Inequalities would have to be obeyed. They are not obeyed. Therefore those properties (exact electron position, etc.) do not exist. These experiments are the exact opposite of "ambiguous". They compare a definite prediction with a definite result, and find that the prediction is wrong. Therefore the basis of the prediction, the claim that those exact positions and so on exist, is false. It's not just that the values change rapidly, or are hard to measure, or anything else like that. They really don't exist.
You ask what comes out if we do a measurement which gives some much narrower range for the position of the electron. (No experiment gives an actual point.) We can say what happens if we do another experiment like that a little later. The electron might be anywhere over a big range. That's just what quantum mechanics predicts, because a narrow electron cloud has a big range of velocities. If you do the experiment many times, you find that quantum mechanics predicts the range very precisely.
You also ask for some more explanation of quantum reality. Here we are on less certain ground, almost in the slippery realm of philosophy. I suggest having a look at these old answers:
(published on 06/30/2013)
Follow-Up #20: why do electrons move?
- Zeeshan khan (age 22)
Rutherford and Bohr did say those things but they were wrong. The entire picture underlying those descriptions is mistaken. As described in the other answers, electrons and all other small objects are not classical things. The attempt to picture them as classical things, with definite positions and velocities, leads to false predictions.
Fortunately, there's another way to describe them, modern quantum mechanics. In quantum mechanics both the range of positions and the range of velocities are described by the same wave function. That means that some properties of the distribution of positions are connected with properties of the distribution of velocities. That's unlike classical physics, where the position and velcoity can be described independently. One of the implications of quantum mechanics is then that a narrow range of positions is always accompanied by a big range of momenta, with a corresponding big kinetic energy.
(published on 08/02/2013)
Follow-Up #21: do electrons spin around the nucleus?
- Promotor (age 28)
By "spin" I assume you mean "orbit", since you specify "around the nucleus". The answer is sometimes.
In the ground states of H and He, the electrons have no orbital angular momentum and cannot be said to orbit in any reasonable sense of the word. In the standard states with well-defined non-zero orbital angular momentum, e.g. one of the 2P states of H, you could sort of say that the electron orbits the nucleus. That would be misleading, however, since the distribution of positions of the electron doesn't change in time, in sharp contrast to what you probably mean by "spin around the nucleus".
You can make states with superpositions of states of higher energy with different angular momenta that do have lumps of wave-function that do orbit around the nucleus before decaying via photon emission. Since, so far as we know, the world is made of quantum objects and you do see things in classical-like orbits, you shouldn't be surprised that something like that is at least possible for electrons in atoms.
I guess I should mention one exotic possibility that would alter those answers. There's a proposed new version of quantum mechanics, not yet shown to be consistent with all the basic effects, called "Many Interacting Worlds". In that picture any one of the worlds does have particles at particular positions. If somehow that picture develops, is consistent with known effects, and manages to correctly predict any as yet unknown effects, our answers will change.
(published on 03/11/2015)
Follow-Up #22: electron smears
- Paul (age 16)
We've discussed electron smears before (), so maybe it's good to start there and then follow up.
On tunneling, we can measure rates of tunneling in a variety of circumstances. These rates are just what is predicted by the quantum picture, with the tail of a wavefunction falling off following the wave equation. Since the same wave equation predicts the properties of atoms, the types of chemical bonds that form, etc. It wouldn't make sense to try to put together a special classical mechanism for tunneling.
(published on 04/11/2015)
Follow-Up #23: photons as particles
- Jeremy (age 44)
Nothing much has changed in this business since Feynman wrote. Both light and electrons are quantum waves. These are different from classical waves in several ways, including:
1. You can set up counters that count individual blips, which is why we say these are "particles".
2. The blip pattern is partially random, not predictable from the wave, or from any prior local property of the universe.
(published on 05/23/2019)
Follow-Up #24: how to reach atomic ground state
- Ilya Kamens (age 29)
You can make a list of the fixed-energy states that the electron can be in for say a hydrogen atom. We described how the lowest energy "ground" state has a size determined by a balance of minimizing kinetic and potential energy. So you ask how the electron manages to reach that ground state, if it starts of in some other state, or in some combination of other states.
Those other states have more energy. You're exactly right that the electron can't end up in the ground state without ditching that extra energy. The extra energy leaves in the form of photons.
At high temperatures there are lots of photons around, so an atom in the ground state can pick up energy and end up in a different state. It's only when things are cool enough that almost all the atoms are in the ground state.
p.s. Historically, realizing how much kinetic energy would be required for an electron to localize in a nucleus was a motivation for proposing the existence of neutrons, rather than just combinations of protons and electrons, to expain the different charges of nuclei with almost the same mass.
(published on 12/29/2019)