Although conceived of as distinct phenomena until the 19th century, electricity and magnetism are now known to be components of the unified field of electromagnetism. Particles with electric charge interact by an electric force, while charged particles in motion produce and respond to magnetic forces as well. Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. A moving electric charge generates a magnetic field. A magnetic field induces electric charge movement, producing an electric current. Electricity and magnetism are essentially two aspects of the same thing because a changing electric field creates a magnetic field, and a changing magnetic field creates an electric field. (This is why physicists usually refer to "electromagnetism" or "electromagnetic" forces together, rather than separately.) Electricity and magnetism are two essential topics in the science of physics. We use electricity to power computers and to make motors go. Magnetism makes a compass point north and keeps notes stuck onto our refrigerators.