An electromagnetic field is an area in which electric and magnetic forces are interacting. It arises from electric charges in motion. Electromagnetic fields are directly related to the strength and direction of the force that a charged particle, called the "test" charge, would be subject to under the electromagnetic force caused by another charged particle or group of particles, called the source.

An electromagnetic field is best understood as a mathematical function or property of spacetime, but may be represented as a group of vectors, arrows with specific length and direction. For a static electric field, meaning there is no motion of source charges, the force F on a test charge is F = qE, where q is the value of the test charge and E is the vector electric field. For a static magnetic field (caused by a moving charge inside an overall neutral group of...