PDA

plant
2010-Jun-04, 01:39 PM
Hi all,

A) How is it that the force between 2 magnets decreases as the cube of distance- whereas gravity, electrostatic charge etc decrease with the square of distance?

B) Do the weak and strong nuclear forces obey the inverse square law?

thanks!

grant hutchison
2010-Jun-04, 03:08 PM
The inverse square law would apply to magnetic monopoles. But magnets always come as dipoles, and this causes the deviation from inverse-square.
The nuclear forces do not follow inverse-square: they have a limited range, for one thing.

Grant Hutchison

grapes
2010-Jun-04, 03:25 PM
As an example of what grant is talking about, consider an electron paired with a proton--in between them, there is a strong tendency for another electron to move towards the proton, because it would be repelled by the electron and attracted by the proton. But, far away from the pair, the effect of the pair is reduced considerably because their individual effects tend to cancel each other.

Eta C
2010-Jun-04, 04:24 PM
The question on the difference between magnetism (a dipole) and electric charge (a monopole) has already been answered. The case with the strong and weak forces is a bit more complicated.

Current theories such as quantum electrodynamics (QED), quantum chromodynamics (QCD), and electro-weak characterize forces through the exchange of a virtual particle. In the case of QED this particle is a photon. For QCD (which is the modern name for the strong, or color force) it is one of a set of gluons. In the case of the weak force two massive particles (the W+/- and Z0) carry the force. To first order, the range of the force is limited by the amount of time the virtual particle can allow a temporary violation of the uncertainty principle. Since photons are massless, the EM force, as described in QED, has an infitite range. The W and Z are massive (on the order of 8 GeV/c2). The amount of time a virtual W or Z can exist is very short, so the distance one travel in that time is also short. This makes the weak force very short-ranged.

The color force is more complicated. Quarks are bound by virtual gluons which, like W and Z, are limited by the uncertainty principle. However, they also carry the color charge themselves (Think of the situation if photons were electrically charged). This means that virtual gluons can exchange virtual gluons and bind to themselves. This helps limit the range of the color force the the inside of a particle such as a proton. The so called "strong" nuclear force is a second order effect between otherwise color-neutral particles like protons and neutrons. Think of the Van der Waals force between electrically neutral atoms. That it remains strong enough to bind particles in a nucleus gives you an idea of how strong the actual color force is.

In a quantum theory of gravity the force is carried by massless gravitons. Like photons, a virtual graviton has an infinite lifetime allowing gravity to have an infinite range and to follow an inverse-square law for its strength.