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This question already has an answer here:

I was reading this answer by madame anna v:

You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is accelerating continuously and would thus radiate away its energy and fall into the nucleus.

One of the reasons for "inventing" quantum mechanics was exactly this conundrum.

The Bohr model was proposed to solve this, by stipulating that the orbits were closed and quantized and no energy could be lost while the electron was in orbit, thus creating the stability of the atom necessary to form solids and liquids. It also explained the lines observed in the spectra from excited atoms as transitions between orbits.

But I don't understand one assumption: Why would an electron in an orbit be accelerating continuously, and would thus radiate away its energy and fall into the nucleus in a classical model?

Also in a planetary model, would planets also be accelerating and thus fall into the sun?

I mean for example: a planet would accelerate towards the sun, but because it will then have bigger velocity, it will escape a little bit the sun, and then it will accelerate towards the sun... and the whole process repeats again. But, why wouldn't that be true for an electron and proton?

Thank you.

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marked as duplicate by dmckee Dec 30 '13 at 18:18

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  • $\begingroup$ When a charge is accelerating, it radiates energy in the form of EM waves. $\endgroup$ – user29727 Dec 30 '13 at 12:35
  • $\begingroup$ No, that's not what I want. I mean for example a planet would accelerate towards the sun, but because it will then have bigger velocity, it will escape a little bit the sun, and then it will accelerate towards the sun... and the hole process repeats again. But why wouldn't that be true for an electron and a proton? $\endgroup$ – electron-in-my-body Dec 30 '13 at 12:51
  • $\begingroup$ In the planetary model the planets do indeed radiate gravitational waves. But the radiated power is unbelievably small: see en.wikipedia.org/wiki/… . The Earth radiates about 200W (a lightbulb's worth) and at that rate would take about $10^{13}$ times the age of the universe to spiral into the Sun! Planetary gravitational models are slightly different from a classical charge in a Coulomb potential: gravitational waves can only have quadrupole radiation, whereas the charge system has dipole radiation. This makes grav waves much weaker. $\endgroup$ – WetSavannaAnimal Dec 30 '13 at 13:40
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I suspect you have misunderstood what is meant by acceleration in the context of circular motion. Acceleration is the rate of change of velocity, but remember that velocity is a vector so it has both direction and magnitude. You are probably thinking that acceleration means a change in the magnitude of the velocity, e.g. speeding up from 1 m/s to 2 m/s, but it can also mean a change in the direction. For a planet orbiting a sun, or a classical electron orbiting the nucleus, the direction of its motion is continually changing so it is accelerating even when the magnitude of its velocity stays constant. So when you say ...

I mean for example a planet would accelerate towards the sun, but because it will then have bigger velocity, it will escape a little bit the sun, and then it will accelerate towards the sun

... this isn't true because in a circular orbit only the acceleration is only changing the direction of motion. The planet stays at the same radial distance from the sun.

Incidentally, a planet orbiting a star, and therefore accelerating towards it, does radiate energy in the form of gravitational waves. However because gravity is a vastly weaker force than electromagnetism the rate of energy loss is vastly slower.

There is more about this in the answers to the question Uniform Circular Motion

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I think the answer to this is fairly simple, an electron undergoing acceleration will emit a photon, thus losing energy. So if we assume a planetary model involving an orbit the electron would have to emit photons continuously, lowering its energy state and collapsing the atom.

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  • $\begingroup$ But why if an electron accelerate it will emit a photon? For example normal matter even if it accelerate or not will emit a photon, but it isn't collapsing. $\endgroup$ – electron-in-my-body Dec 30 '13 at 13:34
  • $\begingroup$ @elec: Look up something called Bremsstrallen (although I may have misspelled it a bit). $\endgroup$ – Olin Lathrop Dec 30 '13 at 13:57
  • $\begingroup$ Don't bother with Bremstrahlung, just look up Maxwell's equations. $\endgroup$ – dmckee Dec 30 '13 at 18:15

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