In the nineteenth century physics was divided into relatively independent branches, four of which were electricity and magnetism, heat, light, and sound — four different manifestations of energy. The fifth classical branch was mechanics, which treats of force and energy and their effect on matter and so serves as a base for the other four. In 1864 James Clerk Maxwell (1831)* developed a theory which showed that light and radiant heat were electromagnetic fields moving through space, and shortly after this experiments began to reveal the complex structure of atoms, leading to a new branch of physics sometimes called “modern” physics or atomic physics, which attempts with increasing success to give a unifying explanation to all the classical divisions of physics (and to all of chemistry) in terms of sub-atomic particles. The explanation rarely causes replacement or even major modification of classical physics, and when dealing with large numbers of particles, as in any visible piece of matter, the methods of classical physics usually remain the more useful. Although the particle approach gives the satisfaction of a deeper understanding, it has remained more useful in beginning the study of physics to follow the classical divisions and to appeal occasionally to atomic physics for explanations. Accordingly, we start with mechanics.
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