7 junio, 2024

Electrical properties of materials: what are they, which ones, examples

What are the electrical properties of materials?

The electrical properties of materials are those that determine their response to the passage of electric current, that is, their conductivity and resistivity capacity (property of transmitting electricity and resistance to its passage, respectively). According to this criterion, materials are classified into three categories: conductors, insulators, and semiconductors.

The arrangement of the particles that make up the atom is responsible for this response. Two of the most important, protons and electrons, are characterized by having an electric charge, a property of matter, just like mass.

In the case of conductive materials, it is easy to establish an electric current inside, since some have free electrons, which are not linked to a particular atom. Normally, the movement of these electrons is random, but if some external agent takes care of moving them in an orderly fashion, a current is generated.

On the contrary, the atomic nucleus in insulating materials is capable of holding electrons more firmly, so it is not so easy for electrical charges to flow through them.

As for semiconductor materials, they have intermediate characteristics, that is, they can conduct electricity under certain circumstances. This makes them especially useful in electronic devices, since they serve as amplifiers and as regulators of the intensity and flow of current, among other functions.

What are the electrical properties of materials?

Electric conductivity

The English physicist Stephen Gray (1666-1736) was one of the first to classify materials into conductors and insulators, based on their ability to conduct electricity. Naturally, the best way to find out is by passing an electric current through different materials and studying the response of each one.

Now, when an electric current is circulated through an object, a current density (intensity per unit area) is created inside it, which, for many substances, is proportional to the electric field produced.

Both the electric field and the current density are vector quantities, so they are denoted in bold, to differentiate them from those that are not. If the electric field is called AND and the current density is Jthen it can be written:

J AND

Where the symbol “∝” reads “… is proportional to…”. To establish equality, a constant of proportionality is required, called σ (read “sigma”), which is known as electric conductivity of the material. Thus:

J = σ AND

units

Electrical conductivity is expressed in amperes/volt-meters, or A/V∙m for short, since the current density is given in A/m2 and the electric field in V/m. The quotient between the current that passes through a material and the voltage applied to it is the conductance G and its unit of measurement is the siemens and is abbreviated S, therefore the conductivity σ can also be expressed as S/m or S∙m−1.

The materials in which it is fulfilled J = σ AND are known as ohmic materialssince this is the microscopic form of the well-known Ohm’s law for resistive electrical circuits V = I∙R, where V is the voltage, I the current and R an electrical resistance.

Conductive substances and materials

Ohm’s law states that the greater the electric field inside the conductor, the greater the current density, a fact that is favored when σ is large. Hence, good conductors are those with high σ conductivity.

Materials with ease to carry current can be electronic conductors or electrolytic conductors. The first ones have so-called free electrons, which are electrons that are little or not at all linked to any particular atom, and that is why they can circulate through the material. Among them metals stand out: silver, copper and gold, for example.

When a voltage is established in a piece of copper, an electric field is created within which free electrons move, generating an electric current in the opposite direction of the field.

The second type of conductors, electrolytics, are solutions in aqueous medium of different acids, bases or salts. In these, conduction is carried out thanks to positive and negative ions (cations and anions respectively), capable of moving in the medium, guided by electrodes with charges of the opposite sign.

Except for high voltages, electrolytic conductors also follow Ohm’s law.

conductivity table

The following table shows the conductivities of various materials, conductors, semiconductors, and insulators, at a temperature of 20°C.

Temperature is an important factor for electrical conductivity, since at higher temperatures the conductivity decreases, due to thermal agitation. In this way, the atoms vibrate faster, increasing the number of collisions between them and the free electrons, whose movement is more disordered.

On the contrary, when the temperature drops, the materials tend to increase their conductivity. Some can become superconducting at very low temperatures, which means that their conductivity is practically infinite.

Although metals are the conductive materials par excellence, graphene is the one with the highest conductivity, as we can see in the table.

He graphene it is not a metal, but a substance made of pure carbon, whose atoms are arranged in a highly regular structure. Being also an excellent conductor of heat, graphene can withstand the passage of high electrical currents without being damaged by heat.

conductivity and resistivity

When it comes to electronic conductors, you work a lot with resistivity, rather than conductivity.

Resistivity is the reciprocal or inverse of conductivity. This means that the higher the conductivity of a material, the lower its resistivity.

The resistivity is denoted by the Greek letter ρ (it is read “rho”) and according to what was said before, it can be expressed by:

ρ = 1 / σ

Contrary to conductivity, resistivity increases with temperature, so the higher the temperature, the higher the resistivity.

References

Bauer, W. 2011. Physics for Engineering and Science. Volume 2. Mc Graw Hill.
Callister, W. Materials Science and Engineering. I reversed.
Open Stax. College Physics. Retrieved from: openstax.org.

Deja una respuesta

Tu dirección de correo electrónico no será publicada. Los campos obligatorios están marcados con *