|Electricity · Magnetism|
The magnetic field is the area around a magnet in which there is magnetic force. Moving electric charges can make magnetic fields. Magnetic fields can usually be seen by magnetic flux lines. At all times the direction of the magnetic field is shown by the direction of the magnetic flux lines. The strength of a magnet has to do with the spaces between the magnetic flux lines. The closer the flux lines are to each other, the stronger the magnet is. The farther away they are, the weaker. The flux lines can be seen by placing iron filings over a magnet. The iron filings move and arrange into the lines. Magnetic fields give power to other particles that are touching the magnetic field.
In physics, the magnetic field is a field that passes through space and which makes a magnetic force move electric charges and magnetic dipoles. Magnetic fields are around electric currents, magnetic dipoles, and changing electric fields.
When placed in a magnetic field, magnetic dipoles are in one line with their axes to be parallel with the field lines, as can be seen when iron filings are in the presence of a magnet. Magnetic fields also have their own energy and momentum, with an energy density proportional to the square of the field intensity. The magnetic field is measured in the units of teslas (SI units) or gauss (cgs units).
There are some notable specific kinds of the magnetic field. For the physics of magnetic materials, see magnetism and magnet, and more specifically diamagnetism. For magnetic fields made by changing electric fields, see electromagnetism.
The electric field and the magnetic field are components of the electromagnetic field.
The law of electromagnetism was founded by Michael Faraday.
Physicists can say that the force and torques between two magnets are caused by magnetic poles repelling or attracting each other. This is like the Coulomb force repelling the same electric charges or attracting opposite electric charges. In this model, a magnetic H-field is produced by magnetic charges that are 'smeared' around each pole. So, the H-field is like the electric field E which starts at a positive electric charge and ends at a negative electric charge. Near the north pole, all H-field lines point away from the north pole (whether inside the magnet or out) while near the south pole (whether inside the magnet or out) all H-field lines point toward the south pole. A north pole, then, feels a force in the direction of the H-field while the force on the south pole is opposite to the H-field.
In the magnetic pole model, the elementary magnetic dipole m is formed by two opposite magnetic poles of pole strength qm separated by a very small distance d, such that m = qm d.
Unfortunately, magnetic poles cannot exist apart from each other. All magnets have north/south pairs which cannot be separated without creating two magnets each having a north/south pair. Also, magnetic poles do not account for magnetism that is produced by electric currents nor the force that a magnetic field applies to moving electric charges.
Hans Christian Ørsted, Der Geist in der Natur, 1854
One of the first drawings of a magnetic field, by René Descartes, 1644, showing the Earth attracting lodestones. It illustrated his theory that magnetism was caused by the circulation of tiny helical particles, "threaded parts", through threaded pores in magnets.
Magnetic field for Kids. Kiddle Encyclopedia.