Magnetic field
A magnetic field is a vector field that describes the magnetic influence of electrical currents and magnetized materials. In everyday life, the effects of magnetic fields are often seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges (electric currents) such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field varies with location. As such, it is an example of a vector field.
Eddy current
A magnet induces circular electric currents in a metal sheet moving past it. See the diagram at right. It shows a metal sheet "(C)" moving to the right under a stationary magnet. The magnetic field "(B, green arrows)" of the magnet's north pole "N" passes down through the sheet. Since the metal is moving, the magnetic flux through the sheet is changing. At the part of the sheet under the leading edge of the magnet "(left side)" the magnetic field through the sheet is increasing as it gets nearer the magnet, formula_1. From Faraday's law of induction, this creates a circular electric field in the sheet in a counterclockwise direction around the magnetic field lines. This field induces a counterclockwise flow of electric current "(I, red)", in the sheet. This is the eddy current. At the trailing edge of the magnet "(right side)" the magnetic field through the sheet is decreasing, formula_2, inducing a second eddy current in a clockwise direction in the sheet.