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Q & A: Why do electrons move?

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Q:
One of my students asked me, "Why does the electron move at all?" I admitted I didnít know and would like to find out for myself and for her. Thanks
- David DeCarli
Cromwell High School, CT, USA
A:
David -

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.

-Tamara

(republished on 07/21/06)

Follow-Up #1: electrons in atom move?

Q:
How do electrons move around the nucleus?
- Anonymous
A:
The easiest case to describe is a hydrogen atom. It has just one electron. That electron exists in a spherically symmetric cloud around the nucleus. It's not going anywhere at all. However, the cloud has the potential to show movement in any direction if something comes along to 'measure' that movement. Likewise the electron can be found in any position in that little cloud if something comes along to measure the position to that accuracy.
If that sounds mysterious, it is.

Mike W.

(published on 06/13/09)

Follow-Up #2: electron orbits

Q:
I've heard that electrons don't collapse in on the nucleus because the trajectory that they take around the nucleus must be an integer of their wavelength, or else they will destructively interfere with themselves. Thus their wavelength, which is proportional to their energy, prevents them from collapsing, because in order to radiate energy, the energy must be given off at a certain rate, which would cause the electron wave to destructively interfere with itself . . . My question is where do electrons get their kinetic energy, and thus their wavelengths from, and, if that wavelength theory is true, how can they be lowered back to their orininal energy level after being elevated by a photon, since that would cause destructive interference?
- John (age 17)
Jamestown, Ohio, United States
A:
John- Those things you've heard are often taught in school and pictured in popular science shows. Nonetheless they are false or too vague to be useful.

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.

Mike W.

(published on 06/13/09)

Follow-Up #3: source of energy?

Q:
BUT THE QUESTION REMAINS, WHY DOES THE ELECTRON START MOVEMENT AT ALL? WHERE FROM DOES IT GET THE ENERGY?
- RAGINI (age 15)
MUMBAI, MAHARASHTRA, INDIA
A:
That would be a big problem if somehow there was a way for the electrons to start with zero energy. If an electron is floating around on its own, its kinetic energy can be very low. However, there is then a lot of electrostatic energy associated with its electric fields.That can be lowered by bringing it closer to a positive charge, like a proton.  That can form a simple hydrogen atom. The electron will now have more kinetic energy, but less potential energy.  The extra energy will radiate away as an electromagnetic field.

Mike W.

(published on 06/26/09)

Follow-Up #4: Acceleration of charged particles

Q:
What is the property of a particle that enables particles to be accelerated by a potential difference
- Alex (age 17)
Aus
A:
It's electric charge.  A potential difference is measured in Volts.  A particle with charge equal to that of an electron will experience a 1 eV increase in energy for a 1 V increase in potential.  An alpha particle, with charge twice that of an electron, would get 2 eV increase in energy, etc.   Since this quantity has an algebraic sign you can both accelerate and decelerate charged particles.  By the way, one electron volt is equal to 1.602x 10−19 joules.

LeeH

(published on 07/07/09)

Follow-Up #5: electron relativistic waves

Q:
what is the speed, with what the electron orbits around the atom, close enough to c, so relative effects apply or not ? when the electron is a lonely walker in vacuum, does it behaves like a ball or like a wave( i mean is it nessesary to orbit around a nuclies, to think of it like wave-cloud-etc, while not observing ) . and if it behaves like ball/wave, if we accelerate it to close-c-velocities what would happen according lorenzs' contraction;
- димитър (age 18)
българия
A:
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.

Mike W.

(published on 08/17/09)

Follow-Up #6: non-question

Q:
I'm not here to ask a question but make a statement on this subject. See, the problem with Quantum Physics is that they don't take into account the universe. The universe is moving and so is the matter inside of it for one basic reason; we are in a black hole. The reason the electron is moving, because we are moving in a shape of a vortex. The most important question that most teachers and professors don't address when it comes to atoms is, why do positive charges stick together? The answer is: Gravitational Singularity. If you think of earth as being an atom and people as the electrons, the people are pulled towards the core of the earth, but due to the distance the force is weak, and we are able to move around and continue with our daily lives. It's the same case with the atom; gravitational singularity at the core of the atom is what's holding the protons together, and allowing electrons to stay in orbit.
- John (age 21)
NJ
A:
Ok, since you don't have any questions, I do. Do you have any shred of evidence for any one of those assertions? Are you able to calculate any actual measurable physical quantity, like those which are calculated using quantum theory, including the attractive nuclear force?


Mike W.



(published on 11/04/09)

Follow-Up #7: why quantum?

Q:
Why do electrons move like a wave? instead of a line. is there a hidden force making them move like a wave?
- Bo (age 21)
NYC
A:
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.

Mike W.

(published on 11/04/09)

Follow-Up #8: quantum energies

Q:
Okay, I've read the answers that have been proposed to the questions. If I get it correctly, the reason why electrons do not collapse on the protons is because the potential and kinetic energies settle into a happy medium and thus do not send the electron pinwheeling into the nucleus. But one thing I would like to ask is how do the potential and kinetic energies prevent the electron from careening into the nucleus? What are the mechanics of these energies that act on the electron?
- Ze Xuan (age 13)
Singapore
A:
I think there's one key ingredient here that isn't close to what one would guess based on classical mechanics. It's that the kinetic energy of the electron wave depends on its shape. Specifically, it goes as the second derivative of the wave function with respect to spatial coordinates. That means that the only way to get a small kinetic energy is to have a wave which varies only slowly as a function of position. However, in order to be concentrated in a small region (needed to lower the potential energy) the wave obviously must vary rapidly as a function of position. That's why there is a trade-off.

Mike W.

(published on 11/15/10)

Follow-Up #9: why quantum electron?

Q:
How is it that as you said in a hydrogen atom the electron exists in a cloud around the nucleus? Isn't it just one electron? Why is it a "cloud" around the nucleus and not like a planet around a star?
- Abstract1 (age 21)
detroit, mi
A:
Yes, it is just one electron. The point, however, is that an electron is not what you think it is.  It really is a smear, not a dot. That smear can, under some conditions, by pulled into a small region or under other conditions expanded out to a large region. So far as we can tell, that's all there is to it. As for the idea that there's really a dot-like position hiding in there, as we discuss above, violations of the Bell Inequalities show that such pictures are false.

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.

Mike W.

(published on 03/06/11)

Follow-Up #10: quantum facts

Q:
From the previous answers given I see no explanation for the perpetual motion of the electron? As far as I can tell Quantum Mechanics does not explain the motion of any particle? It says that electrons are smears or clouds. I find this abstact mathematical explantion very unsatisfatory. I think that the most honest answer that can be given by a physicist is they don't really know what causes the motion of particles. So I will rephrase the question. Why is an electron not stationary? What is the mechanical explanation for its movement? Equally important is it really a cloud of probabilites or an actual physical object that is impossible to measure in our laboratories. Since at this time in history we cannot determine the actual location and momentum of an electron why have we have settled for the pseudo explanation of a probability cloud as the explantion of electron motion?
- mmfiore
Florida
A:
You're nostalgic for a world of mechanical parts. Any mechanical model- in fact, any model which has any local realist description- must obey mathematical relations called the Bell Inequalities. Unfortunately, a wide variety of experiments give results flatly inconsistent with these inequalities.

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 http://en.wikipedia.org/wiki/Anomalous_magnetic_dipole_moment)

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.

Mike W.

(published on 09/07/11)

Follow-Up #11: Why do electrons move?

Q:
Why do atoms (or electrons) move? The thread I'm on has explanations of how but not why. Ragini from Mumbai and mmfiore from Florida asked the same question but I did not see an answer.
- Ray Lavey (age 65)
Bristol, RI
A:
We've been somewhat avoiding this because the answer is a bit technical. An electron exists as a wave function. The velocity of the wave function is not separate from how the wave is distributed about in space. In fact, the momentum is a function of how rapidly the quantum wave changes from place to place. Any electron in a confined space must have a wave function that changes from near zero far away to something else in the central region. Since the wave function must change from place to place, the wave is made of components with momentum.

I know this sounds pretty abstract. All I can do is recommend studying some beginning quantum mechanics.

Mike W.

(published on 09/19/12)

Follow-Up #12: quantum chance and determinism

Q:
As a science enthusiast, but not professional I found your statement "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" refreshingly surprising. From what I have been reading (admittedly a smorgasbord of books written mainly for lay readers), I had come to think of the quantum world as dicey and the classical physical world as easier to predict. Can you flesh out your descriptions a bit? Thanks
- Heather (age 44)
USA
A:
Great question. Things are usually described in the way you say, but that's left over from the confusion of early quantum mechanics. I'll try to bring you up to speed with our current confusion.

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".

Mike W.



(published on 10/30/12)

Follow-Up #13: electron motions

Q:
has any physicist theorized that the reason for electrons' inability to be observed for any 'duration' longer than an instant, is that they move at the speed of light? and my real question is; if you were able to see the stream of electrons moving through a copper coil (I don't know if you can) would they be like a stream of water clinging to the outermost edge of the coil due to centripetal force?
- dan (age 28)
mn
A:
To start with your last question, there is a slight tendency for the electrons' momentum to cause them to concentrate toward the outside of a coil as they flow through it. That effect is very small, however, because the electrical forces between them make the electron fluid nearly incompressible. The current pattern in the wire is slightly affected by the magnetic forces due to the field created by the current itself.

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".

Mike W.

(published on 01/17/13)

Follow-Up #14: electron compressibility

Q:
Um, first of all, i dont know anything much about the theories you are talking about....but,from what i've read, u r describing the electron as a cloud whose spread depends on the possible velocity of it. Something like, the higher the force with which a rubber-band is stretched, the more it "spreads" and disfigures from its original shape. But then in the follow up #13 u r describing it as a some sort of fluid and that it is incompressible. that is one thing that's confusing. the other that i dont understand is ur description of the potential and kinetic energy settling in a happy medium. i hope u dont give me some fancy named theorems or equations cz i'm only a 10th grader.
- Vishnu (age 15)
Chennai,TN,India
A:

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.

Mike W.


(published on 02/08/13)

Follow-Up #15: quantum superpositions

Q:
In your answer to Q # 12: "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." I think someone has. Andew Cleland and his team at Santa Barbara University recently (2010) demonstrated a Quantum Paddle which can exist in two diferent states at the same time: http://www.nature.com/news/2010/100317/full/news.2010.130.html Any thoughts?
- Lakhi (age 50+)
Ann Arbor, MI
A:
Superpositions of what seem like different possibilities have been observed on increasingly large scales, including Andrew Cleland's paddles. Note that the paddle is in one state but that state includes components with different classical properties, e.g. position in this case.

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.

Mike W.

(published on 02/11/13)

Follow-Up #16: non-classical superpositions

Q:
Thanks for your answer in Follow-up Question # 15: "Interference effects provide the evidence for the coexistence of the non-classical superpositions of different classical possibilities." 1. How can two different classical states (positions of Cleland's paddle correspond to one quantum state? 2. Are there some examples of these interference effects at the non-classical (quantum) level and how they then relate to classical possibilities? Thanks!
- Lakhi (age 50+)
Ann Arbor, MI
A:
1. Let's say that state |A> represents one position of the particle and state |B> represents the other. State (0.8|A>+0.6|B>) is one example of the countless quantum states that include both classical possibilities.

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. (double-slit-C60.pdf)

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.

Mike W.


(published on 02/11/13)

Follow-Up #17: electron wave and lightning

Q:
If electrons move in waves how come when you see electrons for example lightning, why does in not look like a classic ocean wave?
- David (age 16)
NJ
A:

Hello David,
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,

Lingyi


(published on 06/04/13)

Follow-Up #18: Where is an electron?

Q:
this is a great post by the way...as for my question..from what i read i think that when you say electrons are smears..you're talking about how we currently see them trough experimentation..thourgh mathematical functions....creating probabilty boundaries..because in the end..there'll have to be something..a defined particle....it's there..as a particle..but its too fast..too small..too unpredictable for us to measure exatly..thats why we create those smears and orbital shapes and stuff...am i even partly correct in this..?....and also....when you say that the elctron is spread out like a cloud..that cloud simply represents different points where it could be..if we are ever able to actually see the electron..through any way..the actual electron..will we find it standing there..or moving around...? i'd really appreciate it if you would also throw some light on quantum world's reality vs. what we calculate and state....i mean more about the theory and what it could actually be..and less about what ambiguous experiments have shown so far. Thank you
- anonymous
A:

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:

http://van.physics.illinois.edu/qa/listing.php?id=3210

http://van.physics.illinois.edu/qa/listing.php?id=11959

http://van.physics.illinois.edu/qa/listing.php?id=5834

http://van.physics.illinois.edu/qa/listing.php?id=23017

Mike W.


(published on 06/30/13)

Follow-Up #19: why do electrons move?

Q:
I have read all answers but I think the question still remains unanswered.. why electron have to move..? according to Rutherford electrons was moving and radiating continuously.. According to Bohr electron is moving in circular orbits or you may say cloud is circular or elliptic, but my question is: Why not electron just not move in straight line toward the nucleus and collapse if really opposite charges attract.. Or answer to this is electron is falling freely just like a satellite..
- Zeeshan khan (age 22)
Pakistan
A:

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.

Mike W.


(published on 08/03/13)

Follow-Up #20: Forcing Electrons

Q:
In answering the question "Why do "Electrons Move", you say "With a strong enough force, it is possible to give an electron enough energy to knock it up to a higher energy orbital, or even completely off of the atom (if the force which is giving it the energy to move around is stronger than the electric force holding it near the nucleus). If electrons are knocked off of the atoms, they create electricity" My question is, where do these forces come from? How do we create these forces? I’d like to be able to explain this to my students, but mostly I just want to know. Thanks, Sally Anne Rosenberg
- Sally Anne Rosenberg
Park School, Alhambra, Ca. USA
A:
The main forces that accelerate the electrons are (no surprise) electrical forces, from electrical fields. There are all sorts of ways of getting big electrical fields on atoms. One is to shine light on the atoms, since light consists of an electromagnetic wave. Another way is to combine chemicals which react (say by burning some gas). As the electrons rearrange, electromagnetic energy can be released.
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.

(republished on 07/21/06)

Follow-up on this answer.