The "Quantum Physics" page has a basic physics error

Thread Starter

Ingeniir

Joined Jun 29, 2011
19
Orbiting negative electrons are therefore attracted toward the positive nucleus, which leads us to the question of why the electrons do not fly into the atom's nucleus. The answer is that the orbiting electrons remain in their stable orbit because of two equal but opposite forces. The centrifugal outward force exerted on the electrons because of the orbit counteracts the attractive inward force (centripetal) trying to pull the electrons toward the nucleus because of the unlike charges.
This is not correct. The so-called "centrifugal outward force" does not exist. In this model, the ONLY force on the electrons is the attractive inward force. On a force diagram, in which you draw arrows representing forces, the only arrow you would draw is one extending from the electron and pointing towards the nucleus. The reason the electrons remain in stable orbit in this model is because they experience a uniform acceleration radially inwards towards the nucleus, and objects that undergo such inward radial acceleration moved in an orbit with constant radial velocity. The forces on the electron are, in fact, UNbalanced, which results in its acceleration. Remember, objects subjected to unbalanced forces experience acceleration, while objects subjected to balanced forces experience no acceleration and will maintain their velocity. If the forces on the electron were balanced, (and the electron had some initial velocity), it would travel in a STRAIGHT line, straight out orbit and away from the nucleus.

Here is a good diagram: http://wapedia.mobi/en/File:Centripetal_force_diagram.svg

A good way to visualize how the radial acceleration results in circular motion is this: On a force diagram, such as the one above, that shows both velocity arrows and force arrows, you can use the directions of the arrows to visualize how they interact. Acceleration is the rate of change of velocity. In terms of arrows, that means if you translate the acceleration arrow (simply slide it, don't rotate it about any point) to the point of the velocity arrow, the new line created between the base of the velocity arrow and the tip of the translated acceleration arrow is the direction of the new velocity arrow. See the first diagram on this page: http://www.lhup.edu/~dsimanek/scenario/centrip.htm (the acceleration arrow is labeled ΔV). If you repeat this process, you may be able to visualize the acceleration arrow as constantly "pulling in" the velocity arrow , which results in a velocity arrow that is always perpendicular to the line between the object and the center of the orbit (a radius), which means the object is traveling in a circle.

Sorry for the physics-heavy post on an electronics forum! But the error on that page in the E-book is one of the most common physics misconceptions, and I felt compelled to explain it away.
 

Thread Starter

Ingeniir

Joined Jun 29, 2011
19
i'm embarrassed, the next paragraph explain what this post is. Give me a few minutes to figure out how to delete this post.
 

Wendy

Joined Mar 24, 2008
21,914
After 10 posts a lot of features kick in. Basically we had problems with homework hit and run types, and in general we don't like to delete posts. If the mods have mercy it will just disappear, rendered invisible.
 

ErnieM

Joined Apr 24, 2011
8,007
However... electrons don't orbit the nucleus. They can't as they would be radiating energy due to their motion, and the only energy they have is their momentum. So if they were orbiting they would radiate until they hit the nucleus.

An electron is huge compared to the nucleus and just sits there in place. However, it's a completely quantum mechanics thing that cannot be discussed in classical terms.
 

Wendy

Joined Mar 24, 2008
21,914
The OP has stated he made a mistake, and you need to read the section in question, because it doesn't say anything like what you are stating. The point of this thread was to correct a perceived error in the book. The chapter in question goes through old theory first, then moves on to current theory, which is accurate as far as I can tell.

Given time, it will need updating again, but not today.
 
Top