SCATTERING

in physics, a change in the direction of motion of a particle because of a collision with another particle. As defined in physics, a collision can occur between particles that repel one another, such as two positive (or negative) ions, and need not involve direct physical contact of the particles. Experiments with subatomic particles indicate that the electric repulsive force between the particles satisfies Coulomb's law, which states that the force varies as the inverse square of the distance between the particles; i.e., if the distance is halved, the force is quadrupled. Experiments show, as in the Figure, that the trajectory of the scattered particle, whatever the angle of deflection, is a hyperbola and that as the bombarding particle is aimed more closely toward the scattering centre the angle of deflection increases. In probing the interior of the atom, the physicist Ernest Rutherford passed a stream of alpha particles through a thin sheet of gold foil. The alpha particles were emitted by a radioactive material and had enough energy to penetrate an atom; although most passed right through the gold foil, some were deflected in a way that indicated that the scattering was produced by a Coulomb force. Because the alpha particles are positively charged and the electrons in the atom are negatively charged, it followed that there must be a large positive charge inside the atom to create the Coulomb force by interacting with the alpha particles. In this way the nucleus of the atom was discovered.