What factors depend on the magnetic field that causes the current to flow through them?
A) Direct conductor
B) Carrier coil
C) Solenoid
Answers
Answer:
Our thinking about magnetic sources has changed considerably over the centuries. The only form of magnetism known until the 19th century was ferromagnetism. Certain materials, when "magnetized", would attract certain other materials. The only materials attracted by a magnet were those that could become magnetized themselves. Since only certain materials exhibited magnetic properties, scientists concluded that magnetism was an inherent property of materials.
Then, in the 19th century, scientists studying the relatively new field of electrical currents discovered that moving charges produce magnetic effects. A current traveling through a loop of wire creates a magnetic field along the axis of the loop. The direction of the field inside the loop can be found by curling the fingers of the right hand in the direction of the current through the loop; the thumb then points in the direction of the magnetic field. With this discovery, magnetism appeared to occur in two different manners: ferromagnetism depending on the material, and electromagnetsim caused by currents.
Classical views of the (a) orbital motion
and (b) spin of an electron. As atomic physics and chemistry began to explain the periodic table with the help of the Bohr model of the atom in the early 1900s, magnetic properties were assigned to the electrons in atoms. Electrons appeared to exhibit two types of motion in an atom: orbital and spin. Orbital motion referred to the motion of an electron around the nucleus of the atom. Since a charged particle was moving, a magnetic field was created. But electrons (and protons and other particles) also appeared to be spinning around their centers, creating yet another magnetic field. The magnetic field due to the orbital motion and the magnetic field due to the spin could cancel or add, but expressions for the exact coupling between the two are too complicated to go into here. Since electrons were moving and spinning within atoms, ferromagnetism could now be explained by the motion of charges within different materials. If all of the electrons in an object line up with their spins in the same direction, the spins will add and create an observable field.
That last sentence is slightly unrealistic. Solids contain incredably large numbers of electrons, and they will never all completely line up. Instead, a solid generally consists of magneticdomains. In a domain, the majority of electrons which can (unpaired valance electrons) will have spins aligned. Adjacent domains will generally not be oriented in identical directions. In magnetized materials, some domains will cancel, but the average domain orientation will be in one direction, producing a net magnetic field. In unmagnetized materials, the domains are randomly oriented and cancel, so no observable field is created. The figure to the right illustrates these concepts.
The concept of magnetism being entirely due to the motion of charges has been modified significantly in the 20th century, thanks to quantum mechanics. The Bohr model of the atom must be modified to include uncertainty. We can never determine exactly the trajectory of an electron or say for certain where it will be found. The uncertainty principle requires that we instead say only where the electron is most likely to be found. Until we measure the position of the electron, its wave function is spread out over all space, with a higher probability of finding the electron in the classical orbit described by Bohr.
(a)
Sample electron spins
in a solid. Not all are
aligned, but . . .
(b)
. . . when canceling spins
are accounted for, a net
magnetic field remains.
(c)
This residual field in a
region is the net magnet-
ization of the region, or
domain.
(d)
Solids contain several
such domains, which
are generally not
aligned completely, but
(e)
The fields of the individual
domains in a magnetized
solid don't completely cancel
but leave a net field
(f)
In an unmagnetized solid
the fields of nearby
domains completely
cancel, leaving no net field
Our concept of spin must also be adjusted to fit with the discoveries of the 20th century. Electrons are thought to be "point particles," which means they have no spatial extent. Which means they can't be physically spinning around their centers. While the word "spin" has survived, it now refers to an intrinsic property of a particle rather than to any physical rotation through space. Since electrons and other particles have intrinsic spin, they create magnetic fields automatically. After considering quantum mechanics, we are once again left with two types of magnetism: intrinsic magnetism due to the "spins" of electrons, and electromagnetism due to the motion of electrons.