When they say that copper is a heavier metal than aluminum, their densities are compared. Similarly, when they say that copper is a better conductor than aluminum, their resistivity (ρ) is compared, the value of which does not depend on the size or shape of a particular sample - only on the material itself.
Theoretical background
Resistance is a measure of electrical conductivity resistance for a given material size. Its opposite is electrical conductivity. Metals are good electrical conductors (high conductivity and low ρ), while non-metals are mostly bad conductors (low conductivity and high ρ).
The more familiar thermal electrical resistance measures how difficult it is for a material to conduct electricity. It depends on the size of the part: resistance is higher for a longer or narrower section of the material. To eliminate the effect of size on resistance, the specific resistance of the wire is used - this is a material property that does not depend on size. For most materials, resistance increases with temperature. An exception is semiconductors (for example, silicon), in which it decreases with temperature.
The ease with which a material conducts heat is measured by thermal conductivity. As a first assessment, good electrical conductors are also good thermal conductors. Resistance is indicated by r, and its unit of measurement is an ohmmeter. The resistance of pure copper is 1.7 × 10 −8 Ω. This is a very small number - 0.000 000 017 Ohm suggests that a cubic meter of copper has virtually no resistance. The lower the resistivity (ohmmeter or Ωm), the better the material is used in electrical wiring. Resistance is the reverse side of conductivity.
Material classification
The material resistance value is often used for classification as a conductor, semiconductor or insulator. Solid elements are classified as insulators, semiconductors or conductors by their “static resistance” in the periodic table of elements. The resistivity in an insulator, semiconductor or conductive material is the main property that is considered for use in electrical engineering.
The table shows some data on ρ, σ and temperature coefficients. For metals, resistance increases with increasing temperature. For semiconductors and many insulators, the opposite is true.
Material | ρ (Ωm) at 20 ° C | σ (S / m) at 20 ° C | Temperature coefficient (1 / ° C) x10 ^ -3 |
Silver | 1.59 × 10 -8 | 6.30 × 10 7 | 3.8 |
Copper | 1.68 × 10 -8 | 5.96 × 10 7 | 3.9 |
Gold | 2.44 × 10 -8 | 4.10 × 10 7 | 3.4 |
Aluminum | 2.82 × 10 -8 | 3.5 × 10 7 | 3.9 |
Tungsten | 5.60 × 10 -8 | 1.79 × 10 7 | 4.5 |
Zinc | 5.90 × 10 -8 | 1.69 × 10 7 | 3,7 |
Nickel | 6.99 × 10 -8 | 1.43 × 10 7 | 6 |
Lithium | 9.28 × 10 -8 | 1.08 × 10 7 | 6 |
Iron | 1.0 × 10 -7 | 1.00 × 10 7 | 5 |
Platinum | 1.06 × 10 -7 | 9.43 × 10 6 | 3.9 |
Lead | 2.2 × 10 -7 | 4,55 × 10 6 | 3.9 |
Constantan | 4.9 × 10 -7 | 2.04 × 10 6 | 0.008 |
Mercury | 9.8 × 10 -7 | 1.02 × 10 6 | 0.9 |
Nichrome | 1.10 × 10 -6 | 9.09 × 10 5 | 0.4 |
Carbon (amorphous) | 5 × 10 -4 to 8 × 10 -4 | 1.25-2 × 10 3 | -0.5 |
Resistivity calculation
For any given temperature, we can calculate the electrical resistance of an object in ohms using the following formula.
In this formula:
- R is the resistance of the object, in ohms;
- ρ is the resistance (specific) of the material from which the object is made;
- L is the length of the object in meters;
- A — cross-sectional area of the object, in square meters.
The resistivity is equal to a certain number of ohmmeters. Despite the fact that the unit ρ in the SI system is usually an ohmmeter, sometimes the dimension ohm per centimeter is used.
The resistance of the material is determined by the magnitude of the electric field through it, which gives a certain current density.
ρ = E / J, where:
- ρ - in an ohmmeter;
- E is the magnitude of the electric field in volts per meter;
- J is the current density in amperes per square meter.
How to determine the resistivity? Many resistors and conductors have a uniform cross section with a uniform flow of electric current. Therefore, there is a more specific, but more widely used equation.
ρ = R * A / J, where:
- R is the resistance of a uniform sample of material measured in ohms;
- l is the length of the part of the material, measured in meters, m;
- A is the cross-sectional area of the sample, measured in square meters, m 2 .
Material Resistance Basics
The electrical resistance of a material is also known as electrical resistivity. This is an indicator of how strongly the material resists the flow of electric current. It can be determined by dividing the resistance per unit length and per unit cross-sectional area, for a particular material at a given temperature.
This means that low ρ indicates a material that can easily move electrons. Conversely, a material with a high ρ will have high resistance and impede the flow of electrons. Elements such as copper and aluminum are known for their low ρ. Silver and, in particular, gold have a very low ρ value, but for obvious reasons their use is limited.
Resistance area
Materials are placed in different categories depending on their indicator ρ. A summary is given in the table below.
The conductivity level of semiconductors depends on the doping level. Without doping, they look almost like insulators, which is similar for electrolytes. The ρ level of materials varies widely.
Equipment categories and type of materials | The resistance region of the most common materials depending on ρ |
Electrolytes | Variable |
Insulators | ~ 10 ^ 16 |
Metals | ~ 10 ^ -8 |
Semiconductors | Variable |
Superconductors | 0 |
Temperature coefficient of resistance
In most cases, resistance increases with temperature. As a result, it becomes necessary to understand the temperature dependence of the resistance. The reason for the temperature coefficient of resistance in a conductor can be justified intuitively. The resistance of a material is dependent on a number of phenomena. One of them is the number of collisions that occur between charge carriers and atoms in a material. The specific resistance of the conductor will increase with increasing temperature, as the number of collisions increases.
This may not always be due to the fact that with increasing temperature additional charge carriers are released, which will lead to a decrease in the resistivity of materials. This effect is often observed in semiconductor materials.
When considering the temperature dependence of resistance, it is usually assumed that the temperature coefficient of resistance follows a linear law. This applies to room temperature and for metals and many other materials. However, it was found that the resistance effects resulting from the number of collisions are not always constant, especially at very low temperatures (superconductivity phenomenon).
Resistance temperature graph
The resistance of the conductor at any given temperature can be calculated by the temperature value and its temperature coefficient of resistance.
R = Rref * (1+ α (T-Tref)), where:
- R is the resistance;
- Rref — resistance at a reference temperature;
- α is the temperature coefficient of resistance of the material;
- Tref is the reference temperature for which the temperature coefficient is indicated.
Temperature coefficient of resistance, usually standardized for a temperature of 20 ° C. Accordingly, the equation commonly used in a practical sense:
R = R20 * (1+ α20 (T- T20)), where:
- R20 = resistance at 20 ° C;
- α20 - temperature coefficient of resistance at 20 ° C;
- T20 - temperature equal to 20 ° C.
Resistance of materials at room temperature
The resistance table below contains many of the substances widely used in electrical engineering, including copper, aluminum, gold, and silver. These properties are especially important because they determine whether a substance can be used in the manufacture of a wide range of electrical and electronic components, from wires to more complex devices, such as resistors, potentiometers, and many others.
| Resistance table of various materials at an outdoor temperature of 20 ° C |
| Materials | Resistance OM at a temperature of 20 ° C |
| Aluminum | 2.8 x 10 -8 |
| Antimony | 3.9 × 10 -7 |
| Bismuth | 1.3 x 10 -6 |
| Brass | ~ 0.6 - 0.9 × 10 -7 |
| Cadmium | 6 x 10 -8 |
| Cobalt | 5.6 × 10 -8 |
| Copper | 1.7 × 10 -8 |
| Gold | 2.4 x 10 -8 |
| Carbon (graphite) | 1 x 10 -5 |
| Germanium | 4.6 x 10 -1 |
| Iron | 1.0 x 10 -7 |
| Lead | 1.9 × 10 -7 |
| Nichrome | 1.1 × 10 -6 |
| Nickel | 7 x 10 -8 |
| Palladium | 1.0 x 10 -7 |
| Platinum | 0.98 × 10 -7 |
| Quartz | 7 x 10 17 |
| Silicon | 6.4 × 10 2 |
| Silver | 1.6 × 10 -8 |
| Tantalum | 1.3 x 10 -7 |
| Tungsten | 4.9 x 10 -8 |
| Zinc | 5.5 x 10 -8 |
Comparison of copper and aluminum conductivity
Conductors are made of materials that conduct electrical current. Non-magnetic metals are generally considered ideal conductors of electricity. In the wire and cable industries, various metal conductors are used, but the most common are copper and aluminum. Conductors have different properties, such as conductivity, tensile strength, weight and environmental impact.
The resistivity of a copper conductor is much more commonly used in cable manufacturing than aluminum. Almost all electronic cables are made of copper, like other devices and equipment that use high conductivity of copper. Copper conductors are also widely used in systems for the distribution and production of electricity, the automotive industry. To save weight and costs, power transmission companies use aluminum in overhead power lines.
Aluminum is used in industries where its lightness, such as aircraft construction, is important, in the future it is expected to increase its use in the automotive industry. For more powerful cables, copper-coated aluminum wire is used to use copper resistivity, resulting in significant structural weight savings from lightweight aluminum.
Copper conductors
Copper is one of the oldest known materials. Its ductility and electrical conductivity were used by early electricity experimenters such as Ben Franklin and Michael Faraday. The low ρ of copper materials has led to its acceptance as the main conductors used in inventions such as telegraph, telephone and electric motor. Copper is the most common conductive metal. In 1913, the International Standard for Copper Calcination (MACO) was adopted to compare the conductivity of other metals with copper.
According to this standard, commercially pure annealed copper exhibits 100% IACS conductivity. The resistivity of the materials is compared with the standard. The commercially pure copper produced today may have higher IACS conductivity values, as processing technology has stepped forward significantly over time. In addition to excellent copper conductivity, the metal has high tensile strength, thermal conductivity and thermal expansion. Annealed copper wire used for electrical purposes meets all the requirements of the standard.
Aluminum conductors
Despite the fact that copper has a long history as a material for the production of electricity, aluminum has certain advantages that make it attractive for a particular application, and its specific current resistance allows you to expand its area of use many times. Aluminum has 61% copper conductivity and only 30% copper weight. This means that an aluminum wire weighs half as much as a copper wire with the same electrical resistance.
Aluminum is generally cheaper compared to copper core. Aluminum conductors consist of various alloys and have a minimum aluminum content of 99.5%. In the 1960s and 1970s, due to the high price of copper, this class of aluminum became widely used for household wiring.
Due to the poor workmanship of the joints and the physical differences between aluminum and copper, devices and wires made on the basis of their joints at the points of contact between copper and aluminum have become flammable. To counter the negative process, aluminum alloys have been developed with creep and elongation properties more similar to copper. These alloys are used for the manufacture of stranded aluminum wires, the current resistivity of which is acceptable for mass use, meeting safety requirements for electrical networks.
If aluminum is used in places where copper was previously used to maintain equal network performance, it is necessary to use aluminum wire twice the size of the copper wire.
The use of electrical conductivity of materials
Many of the materials found in the resistivity table are widely used in electronics. Aluminum and especially copper are used because of their low resistance level. Most of the wires and cables used today for electrical connections are made of copper because it provides a low level of ρ and is reasonably priced. The good conductivity of gold, despite the price, is also used in some highly accurate instruments.
Often gold plating is found on high-quality low-voltage compounds, where the task is to provide the least contact resistance. Silver is not so widely used in industrial electrical engineering, since it oxidizes quickly, and this leads to a large contact resistance. In some cases, the oxide may act as a rectifier. Tantalum resistance is used in capacitors; nickel and palladium are used in terminations for many surface mount components. Quartz finds its main application as a piezoelectric resonant element. Quartz crystals are used as frequency elements in many generators, where its high value allows you to create reliable frequency circuits.