Magnetic Field Topics

• edward purcell
• electricity and magnetism
• magnetic induction
• magnetic torque
• bubble chamber
• mechanical motion
• iron filings
• electromagnets
• geophysicist
• astrophysicist
• electromagnetism
• stator
• electrical current
• electric generator
• permeability
• electrical energy
• magnetic fields
• trajectories
• polarity

Related Images

Source

del.icio.us Tags

• physics
• magnetic
• science
• tesla
• research
• magnetism
• toread
• magnet
• f.o.r.c.e.sofn.a.t.u.r.e
• energy
• reference
• geomagnetics
• sciences
• and
• electronics
• math
• perpetual
• motion
• mathematics
• fields
• field
• wikipedia

Magnetic Field, Magnetic Field

A changing electric field results in a magnetic field, and a changing magnetic field also generates a electric field (see electromagnetism).

Hence, Edward Purcell, in Electricity and Magnetism, McGraw-Hill, 1963, writes, Even some modern writers who treat ' B as the primary field feel obliged to call it the magnetic induction because the name magnetic field was historically preempted by H . This seems clumsy and pedantic. If you go into the laboratory and ask a physicist what causes the pion trajectories in his bubble chamber to curve, he'll probably answer "magnetic field", not "magnetic induction." You will seldom hear a geophysicist refer to the Earth's magnetic induction, or an astrophysicist talk about the magnetic induction of the galaxy. We propose to keep on calling B the magnetic field. As for H , although other names have been invented for it, we shall call it "the field H " or even "the magnetic field H ." In a similar vein, says: So we may think of both B and H as magnetic fields, but drop the word 'magnetic' from H so as to maintain the distinction As Purcell points out, 'it is only the names that give trouble, not the symbols'. although there are many alternative names for both (see sidebar). To avoid confusion, this article uses B -field and H -field for these fields, and uses magnetic field where either or both fields apply.

By continuously switching the electrical current through each of the electromagnets and thereby flipping the polarity of their magnetic field the stator keeps like poles next to the rotor; The resultant magnetic torque is then transferred to the shaft. The inverse process, changing mechanical motion to electrical energy, is accomplished by the inverse of the above mechanism in the electric generator.

The use of iron filings to display a field presents something of an exception to this picture: the magnetic field is in fact much larger along the "lines" of iron, due to the large permeability of iron relative to air.

When moving along the current, to the left the magnetic field points up while to the right it points down. (See figure to the right.) The strength of the magnetic field decreases with distance from the wire.

The inverse process also occurs: a changing magnetic field, such as a magnet moving through a stationary coil, generates an electric field (and therefore tends to drive a current in the coil). (These two effects bootstrap together to form electromagnetic waves, such as light.) This is known as Faraday's Law and forms the basis of many electric generators and electrical motors.

Petrus Peregrinus mapped out the magnetic field on the surface of a spherical magnet. Noting that the resulting field lines crossed at two points he named those points 'poles' in analogy to Earth's poles.

In this model, B = 0 ( H + M ) is an effective magnetization which includes the H -field term to account for the energy of setting up the magnetic field in a vacuum.

When two like magnetic poles repel each other, the magnetic lines of force spread outwards from each other in the space between the two poles. Maxwell considered that magnetic repulsion was the consequence of a lateral pressure between adjacent lines of force, due to centrifugal force in the equatorial plane of the molecular vortices. When deriving the equation for magnetic force in part I of his 1861 paper, Maxwell used a quantity which was closely related to the circumferential speed of the vortices. This quantity was therefore a measure of the vorticity in the magnetic lines of force, and Maxwell referred to it as the intensity of the magnetic force. In the 1861 paper, the magnetic intensity which we denote as v, was always multiplied by the term as a weighting for the cross sectional density of the lines of force. The quantity v corresponds reasonably closely to the modern magnetic field vector H , and the product v corresponds very closely to the modern magnetic flux density B , where is referred to as the magnetic permeability.

In the solar dynamo model of the Sun, differential rotation of the solar plasma causes the meridional magnetic field to stretch into an azimuthal magnetic field, a process called the omega-effect . The reverse process is called the alpha-effect.

A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was conceptualized by Nikola Tesla, and later utilized in his, and others', early AC (alternating-current) electric motors. A rotating magnetic field can be constructed using two orthogonal coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and so, in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees create the rotating magnetic field in this case. The ability of the three-phase system to create a rotating field, utilized in electric motors, is one of the main reasons why three-phase systems dominate the world's electrical power supply systems.

Source: Wikipedia > Magnetic Field