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Name: John
Status: other
Grade: other
Location: FL
Country: N/A
Date: 6/26/2005


Question:
I am an old man and not a high school student. However, you have so much good info on your site you might be able to help.

Atoms are perpetual motion machines. They are constantly expending energy. Without a constant input of energy in some form, an atom would run down quickly. Therefore, they must be dependent on an outside source for power.

Where do they get this power and how? If I were able to build an environment totally devoid of any form of energy, and placed a hydrogen atom in it, would the atom simply dissolve into neutron, proton and electron?


Replies:
Dear John,

Atoms are governed by quantum mechanics, so you have to expect them to behave strangely by Newtonian standards.

Consider the hydrogen atom. This is the simplest atom, consisting of a proton and an electron. When the electron is captured by the proton, it will "see" a large (infinite) number of energy levels. If it gives up enough energy to be captured in a high energy level, it can then lose more energy by emitting photons as it descends to lower energy level. Finally it arrives at the lowest energy level. This is one place where quantum mechanics differs from classical mechanics -- the moon can continuously reduce the radius of its orbit about the earth until it finally crashes into the earth.

When the electron reaches its lowest energy state, it is far from the edge of the proton -- the radius of its orbit is about 1000 times larger than the radius of the proton. The Heisenberg Uncertainty Principle (HUP) then says that if the position of the electron is well determined by being in the lowest orbit, its momentum must be uncertain and so the minimum value of its momentum must be at least comparable to its uncertainty.

This maintains the electron in that orbit forever without any input of energy. If any energy is added, it must push the electron into a higher energy orbit, from which it can again descend to the lowest energy orbit by emitting photons. Its a little like a bowling ball getting trapped in a well. When the ball is at the bottom, it does not need any energy to stay there. It does need energy to get out.

So atoms can last forever without any input of energy just as the bowling ball can stay at the bottom of the well forever without any input of energy.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


You are correct that atoms (let us assume a gas) are in constant motion. (Avoid the word "perpetual" because there are a lot of misconceptions associated with that term.) The "average" energy of a gaseous atom depends solely on its temperature, specifically E = 3/2* R* T where R = 1.987 cal / mol K and T = the temperature in kelvins (K). For an ideal gas (most gases are pretty much ideal under ordinary room temperature conditions and 1 atmosphere), the atoms collide with one another (and the walls of the container the gas is in). These collisions are completely elastic (no friction or stickiness) so that their energy does not need to be replenished.

With regard to the second part of your inquiry, if you place a H atom in a vacuum, totally devoid of any contact with other atoms etc., it stays just like it is, a H atom. Much of outer space is filled with isolated H atoms that stay around for billions of years if they do not encounter other atoms or light of the appropriate energy. Atoms do not "dissolve" into their component sub-atomic particles spontaneously, excluding radioactive atoms. That's just the way they behave.

Vince Calder


John,

If a machine does not put out any energy, the machine will never stop. On a large scale, this is not possible. Friction drains energy form a machine, converting motion energy to heat energy. Air resistance drains energy, transferring motion energy from the machine to the air molecules that the machine must keep pushing out of the way. Also, a machine usually does some sort of work. This too uses up energy. A machine without any such drains would never stop moving.

An atom often has no such drains. There is no friction on the electrons orbiting the nucleus. The electrons do not "rub" against anything. There is no air resistance. The distance between air molecules is extremely large, from the point of view of an electron. If an atom does not do anything, the electron just keeps orbiting. An electron spinning around a nucleus does not exert energy. No energy ever leaves the atom.

When an atom interacts, then energy can be emitted or absorbed. An extreme example of energy exertion is the nuclear energy in the sun. If Hydrogen atoms are hot enough, their electrons are knocked loose. This results in free protons flying all over. Energy has been lost to the electrons. If these protons are packed tightly enough and hot enough, they will collide. Sometimes, they will get close enough to join together. One of the protons can transfer into a neutron, releasing energy in the form of an anti-electron in the process. These may the join with two more protons. You end up with a nucleus of two protons and two neutrons, a Helium nucleus. Such a nucleus has much less mass than the four original protons. This mass is lost as energy: E=mc^2. Some of the energy is released as the anti-electrons. Some is released as light. This released energy is called radiation. Much of this released energy gives a kick to other protons in the sun. Some leaves as sunlight. This will continue in the sun until it runs low on Hydrogen. Afterwards, the star will begin to loose power, eventually dying.

Even atoms use the standard energy relation: Energy In + Energy Lost From Within = Useful Energy Out + Worthless Energy Out. With atoms, however, Energy Lost From Within and Worthless Energy Out seldom show up.

Kenneth E. Mellendorf
Physics Instructor
Illinois Central College



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