Magnetism

Dynamics

Work, Energy, and Power

Gravitation

Electricity

Both the force of gravity and the electromagnetic forces (electrostatic force and magnetic force) are action at a distance force, as opposed to ' contact' forces ('contact' force, in itself, is just a convenient concept for surface electrostatic repulsion). In terms of Newton's Laws, being able to exert force means being able to affect the velocity of distant objects. To describe the ability of an object to participate in force interactions at a distance, it is convenient to employ the concept of the field. The electric field E at a point in space tells us how many newtons of electric force per coulomb of charge would be experienced by a test charge placed at that point (N/C). Likewise, the gravitational field g tells us the gravitational force per unit mass that would be experienced by a mass introduced at that point (N/kg). The presence of magnetic field B at a position in space relates the capacity of a charged particle to experience magnetic force at that position, but the charged particle must be moving. F = qv X B

There are important differences between the electric force and magnetic force:

1) While the electric force is always parallel to the electric field, the magnetic force is perpendicular to the magnetic field.

2) The electric force is independent of particle velocity, while the magnetic force is quite dependent on particle velocity, i.e. magnetic force is proportional to the component of the particle's velocity perpendicular to the magnetic field.

3) When an electric force displaces a particle, it is performing work. Because magnetic force is always perpendicular to particle velocity, a steady magnetic field does no work when a particle is displaced. (A corrolary is that a magnetic field alone cannot alter the kinetic energy of a particle).