Seals
| this article argues with the physical density, further meanings under density (term clarifying) |
the density, symbol: ρ (Greek: rho), is a physical characteristic of a material. It is defined over the relationship of the mass m of a body to its volume V:
- <math> \ rho = \ frac {m} {V}< /math>
in words:
- <math> {\ rm density} = \ frac {\ rm mass} {\ rm volumes}< /math>
The reciprocal value of the density is called specific volume and plays particularly in thermodynamics that gases and steams a role.
The density should not be confounded with the specific weight, because this is very similar to the density, differs however in one point: While with the density the volume stands in relation to the mass, this happens with the specific weight with the volume and the Gewichtskraft. The relationship of the density of a material to the density in the standard temperature and pressure is called relative one density. The density belongs with the tear tenacity, firmness, ductility, hardness, rigidity and the fusing temperature to the material properties of a material.
With porous materials becomes besides between the gross density (cavities including) and the pure density (volume without cavities) differentiated.
Table of contents |
unit
derived SI - unit of the density is kilogram per cubic meter, thus kg/m 3. Particularly with solids the indication is still common in g/cm 3 or is present printed. Further units existing in special cases are gram per litre (g/l) and/or. Gram per Kubikdezimeter (g/dm 3). Here applies:
1 ' 000 kg/m 3 = 1 kg/dm 3 = 1 kg/l or 1 g/cm 3 = 1 g/ml. All these sizes represent the reference density of water.
Water has its largest density ( anomaly of density ) with 999,975 kg/m 3, which corresponds to one thus approximately g/cm 3 as point of reference at a temperature of 3,98 °C. A litre was up to 12. General conference for measure and weight (1964) defines as the volume, which takes exactly one kilogram pure, air-free water with its highest density (with 3,98 °C ≈ 4 °C) with normal print. The deviation from 1 DM 3 is so small that the earlier litre definition with DM 3 than same can be regarded. Since 1964 1 litre = 1 is DM 3.
For solids the density is indicated to an air pressure of 1.013 , 25 hPa = 101,325 Pa frequently still in g/cm 3 for 20 °C and for gaseous materials in g/l for 0 °C and(Standard conditions).
example
the density of z. B. One can determine copper experimentally as follows: The material sample weighs z. B. 15.2 G. Now one partly fills a test tube with water; we take for example 16 ml. Now one lets the material dive in completely and reads off the new level of the water level: 17,7 ml. The difference of the two amount of filling amounts to thereby 1.7 ml which corresponds to the volume of the used piece of copper. From it the density of copper results:
- <math> \ rho = \ frac {15 {,} 2~ \ mathrm {g}} {1 {,} 7~ \ mathrm {ml}} = 8 {,} 9~ \ frac {\ mathrm {g}} {\ mathrm {cm} ^3} = 8.9 \ 10^3~ cdot \ frac {\ mathrm {kg}} {\ mathrm {m} ^3}< /math>
characteristics
the density of liquids depend clearly on the temperature , particularly with gases additionally on the pressure. An example for this is the temperature dependence of the atmospheric pressure in the lower section. The density of hygroscopic materials like for example wood depends besides on the humidity (effect on wood moisture). In order to be able to compare their results of measurement, one refers to a so-called normal climate.
Bodies in a liquid, which have a smaller density than these, rise according to the Archimedean principle upward (lift), until they reach an equilibrium sometime (swim). Bodies with larger density sink accordingly downward and/or. have a higher depth than bodies with smaller densities. In particular therefore fewer close ice on the water and displaced thereby exactly the volume at water, which has the same mass as the ice, can swim.
In gases appropriate applies. An airship filled with helium floats in air, since the helium has a smaller density than air at same pressure and same temperature.
A certain iridium isotope is the closest of all pure elements with 22,65 kg/dm ³. All occurring isotopes is on the average iridium not the closest element, but osmium. Whether iridium is the closest element, or whether it is osmium, is thus pure definition thing. In the anglo-saxon literature predominantly osmium applies as the closest element.
table codes
of table codes for the density of different materials are to be found in the following articles:
- List of the density of solid materials
- list of the density of liquid substances
- list of the density of gaseous materials
temperature dependence of the atmospheric pressure
| the effect of the temperature with air | |||
| <math> \ vartheta< /math> in °C | ρ in kg/m 3 | ||
| - 10 | 1,341 | ||
| - 5 | 1,316 | ||
| 0 | 1.293 | ||
| + 5 | 1.269 | ||
| + 10 | 1.247 | ||
| + 15 | 1.225 | ||
| + 20 | 1.204 | ||
| + 25 | 1.184 | ||
| + 30 | 1.164 | ||
the effect of the temperature on the atmospheric pressure is in the following table represented.
Sizes:
- <math> \ vartheta< /math> (theta) = temperature in °C
- ρ (rho) = atmospheric pressure or density of air in kg/m 3
measuring methods
of a body with accurately well-known geometry can the density by means of mass and computed volume be determined.
According to the principle of Archimedes a body in the environment of a liquid experiences exactly the same much lift strength, as the liquid at Gewichtskraft, displaced by its volume, would exercise. All direct density measuring procedures are based this very day on this principle and can be transferred also to the density determination by gases. With well-known density the liquid, also the volume of the immersed solid body can be determined and to be finally determined also its density.
Example of the determination of the density of a solid body:
The weight of the solid body is based on air. Actually one would have to accomplish the measurement in the vacuum, since the solid body experiences a certain lift also in air. One receives <math> m_ {\ rm} /math< to air>. Subsequently, the solid body is immersed and weighed in water. It seems to be easier than at air. One receives <math> m_ {\ rm} /math< to water>. According to the principle of Archimedes the mass of the displaced water is <math> m_ {\ rm water} = m_ {\ rm air} - m_ {\ rm water}< /math>. The volume of the displaced water <math> V_ {\ rm water}< /math> V_ is {\ rm Festk <\> ddot {o} more rper} /math equal the volume of the solid body< math>. It is well-known that <math> \ rho_ {\ rm water}< /math> to the density of the water applies. By using and transforming one receives therefore: <math> \ frac {m_ {\ rm air} - m_ {\ rm water}} {\ rho_ {\ rm water}} = V_ {\ rm Festk \ ddot {o} more rper}< /math>.
In the last step one receives thus for the density of the solid body: <math> \ rho_ {\ rm Festk \ ddot {o} more rper} = \ frac {m_ {\ rm air}} {\ frac {m_ {\ rm air} - m_ {\ rm water}} {\ rho_ {\ rm water}}}< /math>
Densities of liquids are measured with an araeometer. Densities of solids become z. B. with a density bottle measured or over indirect regulation procedures, how determines the isotope method. The resonance frequency transducer makes it possible with the help of a U-Rohres filled with measuring liquid to determine the density of liquid pure materials and binary mixtures accurately.
One can determine the density of wood with a Resistographen.
a very
rare characteristic has examples water water, by possessing the largest density with 3,98 °C (anomaly of the water). Expands with the further cooling, the removing density causes a volumetric expansion. Thereby frost damages arise for example under the frost decomposition . With frozen over lakes is so also 3.98 °C the warm water at the sea-soil, while colder water with smaller density rises upward. This prevents the Zufrieren of waters up to the reason and makes it only for organisms possible to be able to survive in lakes and seas.
Atmosphere
in the atmosphere warmed up and thus less close air layers of the soil rise to (convection). They cooling thereby however whereby water vapour can condense and itself thereupon clouds to train. Accordingly cooler air layers drop again.
derived designations
in analogy are designated also different sizes per volume unit than densities, for example the number density, the charge density or the Wahrscheinlichkeitsdichte.
The term density is partly used also for sizes per unit area (current density, radiation current density, electrical and magnetic flow density).
A specific density is API degrees for crude oil.
Further analogies (beside specified the already):
- Darrdichte
- filling density
- knocking density
- length density
- press density
- relative one density
- bulk density
- sinter density
- Stopfdichte
