Tuesday, November 11, 2008

Magnetic Field

Magnetic fields are produced by electric currents and by magnets at all points in the space around it. It can be either macroscopic currents in wires, or microscopic currents associated with electrons in atomic orbits. The magnetic field B is defined in terms of force on moving charge in the Lorentz force law. The interaction of magnetic field with charge leads to many practical applications. Magnetic field sources are essentially dipolar in nature, having a north and south magnetic pole. The SI unit for magnetic field is the Tesla, which can be seen from the magnetic part of the Lorentz force law Fmagnetic = qvB to be composed of (Newton x second)/(Coulomb x meter). A smaller magnetic field unit is the Gauss (1 Tesla = 10,000 Gauss).

When a conductor is exposed to a magnetic field an electrical current is produced. By constantly reversing its polarity, the electrical current fluctuates back and forth in the opposite direction. This fast and constant reversal of the electrical current direction is called alternating current (AC).

Permanent magnets are objects that produce their own persistent magnetic fields. All permanent magnets have both a north and a south pole. Like poles repel and opposite poles attract. The magnetism in a permanent magnet arises from properties of the atoms, in particular the electrons, that compose it. Each atom acts like a little individual magnet. If these magnets line up, they combine to create a macroscopic magnetic effect. For more details about what happens both microscopically and macroscopically, see the article ferromagnetism.