5. Estado de la Deuda Pública al 31 de Diciembre de 2014
5.3. Operaciones de financiamiento intra sector público
Solids
1 Electric current is the movement of valence electrons. Conduction is the name of this process.
U It is more fully described in Chapter 1 of this Module. Generally, only metals conduct electricity.
Some conduct better than others.
IU The exception to this is graphite (one of the forms of the element Carbon). Carbon is a non-metal which exhibits some electrical conductivity.
U Liquids
The only liquid elements which conduct are the liquid metals. At room temperature liquid L i mercury is a conductor. Other metals continue to conduct electricity when they are melted.
Non-metals such as water, alcohol, ethanoic acid, propanone, hexane and so on, are all non conductors of electricity.
However, it is possible to make some non-conducting liquids conduct electricity, by a process called ionization. Ionized substances are called ionic substances.
Ionic substances are made of charged particles - positive and negative ions. In the solid state they are held very firmly in place in a lattice structure. In the solid state the ions cannot move about at all. When the ionic solid is melted, the bonds holding the ions in place in the lattice are broken.
The ions can then move around freely.
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When an electric current is applied to an ionic melt the electricity is carried by the ions that are now able to move. In an ionic melt the electric current is a flow of ions.is 2-9
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Taking water as an example. Remember firstly, that water is considered to be a non-conductor of electricity. It can allow some electricity through it if a high voltage is applied to it. This is due to the presence of a minute concentration of Hfi and OH- ions in the water. However, electrons cannot flow through water.
Covalent substances do not conduct at all in solution.
Ionic substances are able to conduct electricity when they are dissolved in water.
The reason lies again in the fact that ionic substances are made of charged particles - ions.
When the ionic solid is dissolved in water the ionic lattice breaks up and the ions become free to move around in the water. When you pass electricity through the ionic solution, the ions are
able to carry the electric current because of their ability to move freely. A solution conducts by means of freely moving ions.
An electrolyte is a liquid which can carry an electric current through it. Ionic solutions and ionic melts are all electrolytes.
Electrolysis describes the process which takes place when an ionic solution or melt has electricity passed through it.
Gases
A gas in its normal state is one of the best insulators known. However, in a similar way as
liquid, it can be forced to conduct electricity by ionisation of the gas molecules. Ionisation of the gas molecules can be effected by extremely high voltages. For example, lightning, is electric current flowing through an ionised path through air due to the huge electrical potential difference between the storm cloud and the ground.
In air, and other ordinary gases, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases,
ultraviolet light, or cosmic rays. Since the electrical
conductivity is extremely low, gases are dielectrics or L_I
insulators. However, once the applied electric field approaches the breakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and ionizing, neutral gas atoms or molecules in a process called avalanche breakdown. The breakdown process forms a plasma that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. In the process, it forms a light emitting conductive path, such as a spark, arc or lightning.
Figure 2.4 - Lightning is electric current flowing through an ionized plasma of its own making
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from their molecules or atoms. A plasma can be formed by high temperature, or 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.
Vacuum
It is a common belief that electricity cannot flow through a vacuum. This is however incorrect.
Remember that a conductor is "something through which electricity can flow," rather than
"something which contains movable electricity." A vacuum offers no blockage to moving
charges. Should electrons be injected into a vacuum, the electrons will flow uninhibited and unretarded. As such, a vacuum is an ideal conductor.
This fact is taken advantage of in many situations, from televisions to vacuum valves. A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum
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begin to emit electrons either through heating (thermionic emission) or via an electric field that is sufficient to cause field emission. Once initiated, a vacuum arc can persist since the freed particles gain kinetic energy from the electric field, heating the metal surfaces through high speed particle collisions. This process can create an incandescent cathode spot which frees more particles, thereby sustaining the arc. At sufficiently high currents an incandescent anode spot may also be formed.
Electrical discharge in vacuum is important for certain types of vacuum tubes and for high voltage vacuum switches.
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