Re: What force keeps electrons spinning?

Hello-

   Thank you for your question!  To begin, an electron does not really 
spin, it is a point charge.  This means that it has an infinitesimal 
radius (assuming a sphere).  A spin implies that this spherical point 
charge has an associated angular momentum.  Angular momentum can be 
defined as the object's moment of inertia multiplied by the angular 
velocity, which is the velocity obtained if you pick a point on the sphere 
perpendicular to the axis of rotation and measure the magnitude of the 
angle swept out per unit time.  Since the size of an electron is 
infinitesimal, the radius of the "sphere" is virtually zero.  This gives a 
zero angular momentum, and thus an actual spin of zero.  
   Quantum mechanics gives the result that angular momentum is quantized, 
i.e. it can only assume certain values.  P.A.M. Dirac showed that 
the "spin" of an electron is actually a quantum number, known as intrinsic 
spin.  The number associated with intrinsic electron spin is 1/2.  Another 
result in quantum mechanics states that no two particles in the same 
orbital can have the same quantum numbers.  This is known as the Pauli 
Exclusion Principle.  Dirac's result along with the Pauli Exclusion 
Principle give us a way to describe the electron in its orbit.  For 
example, in a helium atom the first electron orbital can hold two 
electrons.  The quantum numbers of those two electrons cannot be the same, 
thus they are assigned a spin of +1/2 and -1/2 or "spin up" and "spin 
down."  It becomes convenient to assign the value of "spin" to the 
electron for this purpose, not because it has actual angular momentum.
   As for the electrons being sucked into the nucleus, it does seem that 
classically the Coulomb force between the negatively charged electron and 
the positively charged protons in the nucleus would cause the electron 
to "fall" inward into the nucleus.  In addition to this, as the electron 
is orbiting the nucleus, it would produce electromagnetic radiation or 
energy.  This is the result of accelerating a charged particle.  As this 
radiation was emitted and the electron lost energy, it would spiral in 
towards the nucleus.  
   Quantum mechanically, we know that the angular momentum of the 
electron's orbit (as opposed to the angular momentum of its rotation) is 
quantized, as is the amount of energy that the electron possesses.  If the 
amount of energy is set to a certain amount, this implies that it cannot 
be losing energy due to electromagnetic radiation as implied by classical 
theory.  The orbital angular momentum is also quantized, which implies 
through Bohr's model that the radial distance from the nucleus is also 
quantized, so the electron cannot exist in the nucleus.  It must maintain 
a certain radius from the nucleus.  Both of these quantizations give us a 
good picture of why the electron stays in orbit around the nucleus.
   The reason that a planet stays at the same constant rate of rotation 
can be found in Newton's first law.  It states that an object in motion 
tends to stay in motion unless acted upon by another force.  The process 
through which the matter which formed the earth coalesced gave it an 
initial rotation.  This rotation has yet to be acted upon by another force 
to change it.  It theoretically could be changed, say by a collision with 
an asteroid that had enough energy.  
   The planets are held out from the sun simply by the fact that they are 
at the right distance.  The definition of the force of gravity is the 
inverse of the distance between two objects.  For the mass of our sun, and 
the mass of the earth, it formed at precisely the correct distance to stay 
in orbit.  This is relevant to the current excitement of the discovery of 
new planets.  The topic addresses the fact that planets are rare 
considering the amount of stars that exist.  If the matter of a potential 
planet began to fuse at the wrong distance from a star for its mass, then 
it would either spiral into the star and be consumed, or continue on a 
trajectory away from the star, though its path would be curved due to the 
gravitational force that did act upon it.
   The nucleus does have a gravitational force like the sun, but compared 
to the attraction it is extremely small.  The ratio of the attraction due 
to the Coulomb force to attraction due to gravity is somewhere around 
10^39.  That is why the attraction due to gravity in atomic physics is 
negated.
   I hope this helps!

Matt