electromagnetism - Magnetic Field and Current Relationship - Physics Stack Exchange
An electric current results from the collective movement of free charges under the effect of an electric field. The free charges may be electrons in a metal (current. The tesla is a fairly large unit of magnetic field, so we Above, you were told that a loop of current-carrying wire produces a magnetic field For most purposes, the difference is inconsequential. The formulas derived for the magnetic field above are correct when dealing with the entire current. A magnetic.
The breakdown process forms a plasma that contains enough mobile electrons and positive ions to make it an electrical conductor. In the process, it forms a light emitting conductive path, such as a sparkarc or lightning.
Electric current - Wikipedia
Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules or atoms. A plasma can be formed by high temperatureor by application of a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current.
However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons or ions through either field electron emission or thermionic emission. Thermionic emission occurs when the thermal energy exceeds the metal's work functionwhile field electron emission occurs when the electric field at the surface of the metal is high enough to cause tunnelingwhich results in the ejection of free electrons from the metal into the vacuum.
Externally heated electrodes are often used to generate an electron cloud as in the filament or indirectly heated cathode of vacuum tubes.
- Magnetic field
Cold electrodes can also spontaneously produce electron clouds via thermionic emission when small incandescent regions called cathode spots or anode spots are formed. These are incandescent regions of the electrode surface that are created by a localized high current. These regions may be initiated by field electron emissionbut are then sustained by localized thermionic emission once a vacuum arc forms.
These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field. Vacuum tubes and sprytrons are some of the electronic switching and amplifying devices based on vacuum conductivity.
Superconductivity Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. Like ferromagnetism and atomic spectral linessuperconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effectthe complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state.
The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics. Semiconductor In a semiconductor it is sometimes useful to think of the current as due to the flow of positive " holes " the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron.
This is the case in a p-type semiconductor. A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator. In the classic crystalline semiconductors, electrons can have energies only within certain bands i. Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material.
The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy closer to the nucleus are occupied, up to a particular band called the valence band. Then, mark each location with an arrow called a vector pointing in the direction of the local magnetic field with its magnitude proportional to the strength of the magnetic field.
An alternative method to map the magnetic field is to 'connect' the arrows to form magnetic field lines. The direction of the magnetic field at any point is parallel to the direction of nearby field lines, and the local density of field lines can be made proportional to its strength. Magnetic field lines are like streamlines in fluid flowin that they represent something continuous, and a different resolution would show more or fewer lines.
An advantage of using magnetic field lines as a representation is that many laws of magnetism and electromagnetism can be stated completely and concisely using simple concepts such as the 'number' of field lines through a surface.
These concepts can be quickly 'translated' to their mathematical form. For example, the number of field lines through a given surface is the surface integral of the magnetic field. Various phenomena have the effect of "displaying" magnetic field lines as though the field lines were physical phenomena.
For example, iron filings placed in a magnetic field, form lines that correspond to 'field lines'.
Field lines can be used as a qualitative tool to visualize magnetic forces. In ferromagnetic substances like iron and in plasmas, magnetic forces can be understood by imagining that the field lines exert a tensionlike a rubber band along their length, and a pressure perpendicular to their length on neighboring field lines. The rigorous form of this concept is the electromagnetic stress—energy tensor.
Magnetic field and permanent magnets[ edit ] Main article: Magnet Permanent magnets are objects that produce their own persistent magnetic fields. They are made of ferromagnetic materials, such as iron and nickelthat have been magnetized, and they have both a north and a south pole. Magnetic field of permanent magnets[ edit ] Main articles: Magnetic moment and Two definitions of moment The magnetic field of permanent magnets can be quite complicated, especially near the magnet.
The magnetic field of a small [nb 7] straight magnet is proportional to the magnet's strength called its magnetic dipole moment m. The equations are non-trivial and also depend on the distance from the magnet and the orientation of the magnet. For simple magnets, m points in the direction of a line drawn from the south to the north pole of the magnet. Flipping a bar magnet is equivalent to rotating its m by degrees. The magnetic field of larger magnets can be obtained by modeling them as a collection of a large number of small magnets called dipoles each having their own m.
The magnetic field produced by the magnet then is the net magnetic field of these dipoles. And, any net force on the magnet is a result of adding up the forces on the individual dipoles. There are two competing models for the nature of these dipoles. These two models produce two different magnetic fields, H and B. Outside a material, though, the two are identical to a multiplicative constant so that in many cases the distinction can be ignored.
This is particularly true for magnetic fields, such as those due to electric currents, that are not generated by magnetic materials. Magnetic pole model and the H-field[ edit ] The magnetic pole model: It is sometimes useful to model the force and torques between two magnets as due to magnetic poles repelling or attracting each other in the same manner as the Coulomb force between electric charges.
Maxwell's Equations: Electric Current Density
This is called the Gilbert model of magnetism, after William Gilbert. In this model, a magnetic H-field is produced by magnetic charges that are 'smeared' around each pole.
These magnetic charges are in fact related to the magnetization field M. The H-field, therefore, is analogous to the electric field E, which starts at a positive electric charge and ends at a negative electric charge. Near the north pole, therefore, all H-field lines point away from the north pole whether inside the magnet or out while near the south pole all H-field lines point toward the south pole whether inside the magnet or out.
Too, a north pole feels a force in the direction of the H-field while the force on the south pole is opposite to the H-field.