Union University
Union University Department of Physics

The Science Guys



Science Guys > September 2001

September 2001

The nucleus of an atom has several protons, all of which have positive electric charge. Since like electric charge repels, why doesn't the nucleus fly apart?

Imagine two beams of atoms on a collision course, each moving at better than 99% the speed of light. These beams collide, causing the central cores (the nuclei) of several atoms to get so close together that they blast each other into smaller bits of matter and energy. This is, in the parlance of physics, a scattering experiment, and analyzing such scattering experiments has helped reveal the answer to our question.

The nucleus of an atom (the core) consists of protons, which carry a positive electric charge, and neutrons, which carry no electric charge. Electric charges which are alike, do indeed repel each other, much like poles of two magnets can repel. This repulsion is referred to as Coulomb repulsion, after the French physicist, Charles Coulomb (1736-1806) who made many pioneering measurements of this force. The repulsive force becomes huge when the electrically charged particles are very near one another, and in the nucleus protons are very close indeed. The size of the nucleus is on the order of 0.00000000000004 inches (or 10-15m). This is a number so important in physics that it has its own name- the femtometer (fm).

Scattering experiments have revealed the presence of another force in the nucleus called the nuclear strong force. The strong force acts on both neutrons and protons, so it isn’t a force affiliated with the electric charge. The strong force has also been found to be attractive- that is, it always pulls particles together, like gravity pulls us to the Earth. However it is much stronger than gravity (gravity is the weakest force in nature).

Another curious feature of the strong force is the range over which it functions. The repulsive Coulomb force between protons is long-ranged. This means that two protons will be repelled from one another over relatively large distances. However, the nuclear strong force is short-ranged. Particles must be within about one fm of each other before the strong force "turns on." However, once the strong force becomes active, it overcomes all other forces. When the strong force gets its claws on a proton, it pulls the proton firmly to the nucleus. The strong force is approximately 100 times larger than the Coulomb repulsion.

You may have heard about subatomic entities called quarks. These are the tiny constituents of neutrons and protons. Quarks come in several types, and three of them are present in each proton and each neutron. Quarks have a variety of interesting properties, but what's important for our discussion is how they interact. They interact via the strong force! In fact, the strong force interaction "leaks out" of each proton and neutron, producing the nuclear strong force that binds the nucleus together. So, at its root, an atomic nucleus is held together by a force that reaches out from the very quarks within the neutrons and protons.

Armies of physicists smashing atoms together with tremendous energies have verified all of this. Physicists call these "atom smashers" accelerators. There are linear accelerators, ring accelerators, and colliding beam accelerators, to name a few. The results of these collisions are recorded and examined in painstaking detail. It’s a bit like finding out what makes a watch tick by firing one at a brick wall at several hundred miles per hour and then examining the pieces that result. It is hoped that one day, scattering experiments will enable us to determine more fundamentally the nature of the origin of force itself.