Electricity and magnetism are often discussed as separate physical phenomena: The first describes the forces and flow of electric charge (e.g., from electrons, the negatively charged particles around atomic nuclei); the second describes the attractive or repulsive forces between certain materials. However, electricity and magnetism are actually closely related: The motion of electric charge creates a magnetic field—an area in which magnetic forces can be observed—and a magnetic field affects how a charge moves. In this way, electricity and magnetism are unified in a single fundamental force known as electromagnetism.

The strength of the magnetic field through a given area is called magnetic flux. Magnets have two poles (north and south); in the case of a simple bar magnet, one pole is located at each end and the magnetic field can be visualized as flux lines that loop from one end of the magnet to the other. By convention, the lines are drawn as arrows flowing from the north pole to the south pole. The magnetic field is stronger near the ends of the magnet, and here the flux lines are most dense.

When electrically charged particles travel through a magnetic field, they experience a force from the field. This phenomenon can be used to generate electricity. When a wire made of an electrically conductive material (one containing atoms whose outer electrons can move easily from one atom to the next) passes through a magnetic field, the magnetic field knocks electrons loose from their atoms to create a difference in electric potential, or voltage, in the conductor. In a closed circuit, where there is an uninterrupted path, this voltage drives a flow of electrons, or electric current. The voltage pushes electrons along the conductor from one atom to the next much like a difference in water pressure pushes water through a pipe. This process is called electromagnetic induction.

As a conductor moves through a stationary magnetic field, electromagnetic induction generates electric voltage. It is the changing strength of a magnetic field on the conductor, and not the movement of the conductor itself, which induces the voltage. This means that a stationary conductor in a changing magnetic field can also produce voltage and, subsequently, electric current. The amount of voltage generated is related to the change in magnetic flux. This relationship was discovered by an English scientist named Michael Faraday and is known as Faraday's law: The amount of voltage generated is equal to the change in magnetic flux divided by the change in time.

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