Head (Physics)

A manager , also conductor is, in physics a substance , the different types of energy or particles capable of transporting between different locations. There are conductors for electricity , heat , light and magnetism . A non-conductive material is called an insulator .

Electrical conductor

An electrical conductor is a medium that has a high density of freely moving charge carriers and therefore good electrical conductivity and the lowest possible electrical resistance , making it suitable for transporting charged particles; this transport is called electric current . The synonymous, but ancient term for an electrical conductor, conductor , describes in the narrower sense a charge collector made of metal in the form of a can or ball on electrostatic devices.

For electrically conductive connection wires for the power supply, see Electrical wiring .

1st class leader

1st class conductor, copper cable

Note: First class and second class conductors are to be distinguished from the electrotechnically standardized conductor classes 1 to 6 !

First class conductors do not experience any material change from the electrical conduction .

Metals , graphite and some other chemical compounds such as niobium (II) oxide are so-called 1st class conductors. The conductivity of metals (e.g. measured as specific resistance ) is not based on the number of electrons on their outer shell ( valence electrons ), but is primarily determined by the lattice structure . Metals form a crystal lattice structure in which the electrons are only weakly bound and can be viewed as electron gas ; that is, the electrons can move more or less freely.

The best electrical conductor is silver , copper is hardly inferior to it, but it is lighter and much cheaper. This is even more true for aluminum , which has the best mass-specific conductivity. This is why copper (cables, conductor tracks, coils) and aluminum (voice coils from loudspeakers) are used as electrical conductors in technology.

The conductivity also depends on the material temperature. In the case of metals, the specific resistance increases slightly with an increase in temperature (see electrical conductivity # temperature dependence ); in the case of carbon and semiconductors , the resistance can also decrease as the temperature increases.

With some (partly also insulating ) materials, the specific resistance can jump to zero at very low temperatures. This condition is called superconductivity .

Quantum mechanical consideration

If you look at metals in terms of quantum mechanics ( Bloch wave function , Fermi-Dirac statistics ), the result is that the electrons cannot accept any energy , but can only exist in certain energy bands - the shape of these bands depends on the crystal lattice of the material.

The Fermi energy (the energy of the most energetic electron at a temperature of 0 Kelvin ) enables a differentiation:

If the Fermi energy is in a permitted band ( conduction band ), it is called a conductor .
If the Fermi energy lies between the permitted bands, it is a

Insulator when the energetic distance between the valence band and the conduction band is large compared to the thermal energy ;
otherwise it is a semiconductor .

Semiconductors are a special form: in their pure state, their crystal lattices can build up stable electron bonds. The electrons can rise into a conduction band at higher temperatures; therefore, compared to metals, semiconductors conduct better at higher temperatures.

An interesting effect in semiconductors is hole conduction (also known as fault conduction ): the electron that has risen into the conduction band leaves a hole in the bond that behaves like an electron with a positive charge and also contributes to conductivity.

Foreign atoms can also be introduced into semiconductors - this is called doping . The foreign atoms either serve to introduce additional electrons - one then speaks of n-doping (e.g. nitrogen in silicon crystal ) - or contain fewer electrons to introduce holes, which is called p-doping (e.g. boron in Silicon crystal).

Models

Superconductivity

Superconductivity can occur at low temperatures. The resistance of the superconducting material jumps to zero below a limit temperature, which can be explained quantum mechanically . This limit temperature depends on the alloy : While the first investigated superconductors required temperatures close to absolute zero , so-called high-temperature superconductors are also known today, in which this effect also occurs at higher temperatures. However, these are still very low temperatures (below −130 ° C).

Applications

for highly sensitive sensors for electromagnetic fields
to reduce resistance losses in electrical systems
loss-free transport of electricity

Head of 2nd class

Second class conductors are materially changed by the management process .

So-called ion conductors are 2nd class conductors . The conductivity results from the dissociation (splitting) of the (ionic) crystal lattice structure with the formation of electrically charged , mobile ions in the so-called electrolyte . This can be done by dissolving in a polar solvent (such as water ) or by melting .

Salt solutions are a classic example . During the dissolving process, soluble salts are broken down into solvated (as surrounded by the solvent) positive and negative ions; these cause the conductivity . The positive ions migrate in the direction of the negative cathode and are therefore called cations ; the negative anions migrate to the positive anode . The respective ions are discharged at the electrodes through the transfer of electrons . This can be used, for example, for the electrodeposition of metal , for the release of chlorine (from sodium chloride ) or for the electrolysis of water to form hydrogen and oxygen .

At higher temperatures (above approx. 600 ° C), glass (also) becomes electrically conductive as an ion conductor. This is z. B. used in appropriate melting furnaces , in that after conventional heating the glass melt is then directly heated by electrodes that are immersed - i.e. by the flow of current .

Heat conductor

The heat conduction is one of three mechanisms in which thermal energy can be transported. (The other two options are radiation and convection ( flow ).)

In solids , heat is transported through the propagation of lattice vibrations. Conduction electrons offer a good way of spreading these stimulating vibrations, so electrical conductors, especially metals, are also good heat conductors ( Wiedemann-Franz's law ). This phenomenon is usually expediently treated in the model of a free or quasi-free electron gas (i.e. electrons which, to a good approximation, can move almost freely in the solid, comparable to the mobility of a gas ( Drude theory , Sommerfeld theory )). Since the electrons are moved in this line, there is also a flow of current ( Seebeck effect ).

In electrical insulators, the heat is mainly transmitted by lattice vibrations ( phonons ). The thermal conductivity therefore depends on the speed of sound .

Both effects occur in semiconductors.

Good conductors of heat are: Metals.
Bad heat conductors are: wood, plastics, salts.

Contrary to popular belief, water is a poor conductor of heat. In contrast to solid bodies, convection makes a significant contribution to heat transport .

Other models: Einstein model of the solid

Electromagnetic waveguide

High frequency and microwave conductors

A well-known waveguide for high-frequency electromagnetic waves is the coaxial cable .

The waveguide for microwaves makes use of the fact that the waves induce currents. They usually consist of a metallic tube (round or rectangular), the diameter of which is slightly larger than half the wavelength of the wave to be transported.

Waveguide

→ Main article: waveguide

A waveguide is a waveguide for electromagnetic waves primarily in the centimeter wave range (3 to 30 GHz). Waveguides are round or rectangular metal pipes in which such high frequencies can be transmitted with very little loss, in contrast to cables.

light

Optical conductors, or more precisely: optical waveguides are available in two designs:

one-dimensional:

An example of such are glass fibers serving as optical waveguides . With conventional glass fibers, the light is guided with the help of total reflection ; in some modern variants, the light is instead guided with the help of photonic crystals .

two-dimensional:

An example here are planar waveguides. These are z. B. used in semiconductor lasers .

Magnetic conductor

Permeability numbers for selected materials
medium	µ _r	Classification
Superconductors of the first kind	0	ideally diamagnetic
Lead , tin , copper	<1	diamagnetic
vacuum	1	(neutral by definition)
Air , aluminum , platinum	> 1	paramagnetic
cobalt	80 ... 200	ferromagnetic
iron	300 ... 10,000	ferromagnetic
Ferrites	4… 15,000	ferromagnetic
Mumetal (NiFe), annealed in hydrogen	50,000 ... 140,000	ferromagnetic

The magnetic conductivity, also called magnetic permeability ( μ ), is a measure of the permeability for magnetic fields . It is closely related to magnetic susceptibility . Permeability is the ratio of the magnetic flux density B to the magnetic field strength H .

{\ displaystyle \ mu = {\ frac {\ left | {\ vec {B}} \ right |} {\ left | {\ vec {H}} \ right |}}}

.

The magnetic field constant μ ₀ is a physical constant for the magnetic conductivity of the vacuum. The permeability number μ _r , formerly also referred to as relative permeability, is the ratio of μ to the magnetic field constant μ ₀ .

{\ displaystyle \ mu _ {\ mathrm {r}} = {\ frac {\ mu} {\ mu _ {0}}}}

The result for the vacuum is a permeability number of 1. The size of the dimension number μ _r is related to the magnetic susceptibility χ via the formula . ${\ displaystyle \ mu _ {\ mathrm {r}} = 1 + \ chi}$

The complete impermeability of superconductors to magnetic fields is called the Meissner-Ochsenfeld effect .

literature

Horst Stöcker : Pocket book of physics. 4th edition Harry Deutsch, Frankfurt am Main 2000. ISBN 3-8171-1628-4
Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics. 1 edition. Westermann Schulbuchverlag, Braunschweig 1998. ISBN 3-14-221730-4
Günter Springer: Expertise in electrical engineering. 18th edition Europa - Lehrmittel, Wuppertal 1989. ISBN 3-8085-3018-9

Web links

Commons : Electrical conductors - collection of images, videos and audio files

Individual evidence

^ Rainer Ose: Electrical engineering for engineers: Fundamentals . Carl Hanser Verlag GmbH & Company KG, 2013, ISBN 978-3-446-43955-9 ( limited preview in Google Book Search [accessed December 8, 2016]).
↑ Ludwig Bergmann, Clemens Schaefer, Rainer Kassing, Stefan Blügel: Textbook of Experimental Physics 6. Solid . Walter de Gruyter, 2005, ISBN 978-3-11-017485-4 ( limited preview in Google Book Search [accessed December 8, 2016]).
↑ Moeller basics of electrical engineering . Springer-Verlag, 2008, ISBN 978-3-8351-0109-8 ( limited preview in Google Book Search [accessed November 27, 2016]).

[1] Rainer Ose: Electrical engineering for engineers: Fundamentals . Carl Hanser Verlag GmbH & Company KG, 2013, ISBN 978-3-446-43955-9 ( limited preview in Google Book Search [accessed December 8, 2016]).

[2] Ludwig Bergmann, Clemens Schaefer, Rainer Kassing, Stefan Blügel: Textbook of Experimental Physics 6. Solid . Walter de Gruyter, 2005, ISBN 978-3-11-017485-4 ( limited preview in Google Book Search [accessed December 8, 2016]).

[3] Moeller basics of electrical engineering . Springer-Verlag, 2008, ISBN 978-3-8351-0109-8 ( limited preview in Google Book Search [accessed November 27, 2016]).