What is electric force?
Electric force
Our society is dependent on electricity. An electric power failure demonstrates our dependence on electricity. Electricity is an essential ingredient in all the atoms in our bodies and in our environment. The forces that hold the parts of an atom together are electric forces. Furthermore, so are the forces that bind atoms in a molecule and hold these building blocks together in large-scale macroscopic structures, such as a rock, a tree, a human body, a skyscraper. Our immediate environment is dominated by electric forces.
In the following chapters, we will study electric forces and their effects. For a start, we will assume that the particles exerting these forces are at rest or moving only very slowly. The electric forces exerted under these conditions are called electrostatic forces. We will consider the electric forces when the particles are moving with uniform velocity or nearly uniform velocity. Besides the electrostatic force there arises a magnetic force, which depends on the velocities of the particles. The combined electrostatic and magnetic forces are called electromagnetic forces. Finally, we will consider the forces exerted when the particles are moving with accelerated motion. The electromagnetic forces are then further modified with a drastic consequence, which is the emission of electromagnetic waves, such as light and radio waves.
Ordinary matter - solids, liquids, and gases - consists of atoms, each with a nucleus surrounded by a swarm of electrons. At the center of atom, there is a nucleus made of protons and neutrons packed very tightly together - the diameter of the nucleus is only about 6× 10-15m. Moving around this nucleus are ten electrons; these electrons are confined to a roughly spherical region about 1× 10-10m across.
The atom somewhat resembles the Solar System, with the nucleus as Sun and the electrons as planets. In the Solar System, the force that holds a planet near the Sun is the gravitational force. In the atom, the force that holds an electron near the nucleus is the electric force of attraction between the electron and the protons in the nucleus. This electric force resembles gravitation in that it decreases in proportion to the inverse square of the distance. But the electric force is much stronger than the gravitational force. The electric attraction between an electron and a proton (at any given distance) is about 2× 1039 times as strong as the gravitational attraction.
The other great difference between the gravitational force and the electric force is that the gravitational force between two particles is always attractive, whereas electric forces can be attractive, repulsive, or zero, depending on what two particles we consider. The electron-proton electric force is attractive, but the electron-electron-neutron and the proton-neutron electric forces are zero. Table 1 gives a qualitative summary of the electric forces between the fundamental particles.
