Density

The density ( rho ), also called bulk density, the mass of an object divided by its volume, ie. The density is thus a quotient size as defined in DIN 1313 It is often expressed in grams per cubic centimeter = kilograms per liter = tonnes per cubic meter.; rarely formed without the resolutions SI unit is kilograms per cubic meter used. The density is determined by the material of the body regardless of its shape and size.

In general, solids and liquids expand with increasing temperature, so that their density decreases; gaseous even stronger if they can expand as against constant external pressure and are not enclosed in a rigid vessel. An exception are substances with a so-called density anomaly such as water.

With density of compound words described, other variables, such as the energy density or charge density, which are based on the volume, but sometimes also to a surface, a length, a frequency interval or otherwise. Other examples from the fields of engineering and physics follow below.

• 6.1 Volume-related variables
• 6.2 sizes per unit area
• 6.3 sizes per unit length

Differentiation from other concepts

• The density (mass / volume) should not be confused with the specific gravity ( the gravity ). While when the density is the mass to volume ratio, is the specific gravity in place of the mass of the weight (force / volume).
• The relative density is the ratio of the density to the density of a standard, ie a dimensionless quantity.

Defines these differences in the density DIN 1306; Concepts, specifications.

Measuring the density of

Density determination by buoyancy

According to the principle of Archimedes undergoes a completely submerged in a fluid or in a body of gas, a lifting force equal to the weight of the volume of the displaced liquid. To determine the two unknown density and volume, two measurements are required.

If you dive completely an arbitrary body with the volume VK in two liquids or gases with known densities ρ1 and ρ2, as he learns the different resultant gravitational forces FG1 or FG2. Measurable are the resultant forces using a simple scale. The desired density ρK can be determined as follows.

Starting from the formulas

For the weight FG of the body and the buoyancy forces FAi measure the two scales in the liquid (or gas ) 1 or 2 submerged masses, the weight forces

Now you can convert both equations for i = 1,2 for the volume VK and equate the corresponding expressions. After some further transformations we obtain the solution:

If the density is much smaller than the other, such as air and water, the formula simplifies to:

If only one liquid, such as water with density one has, one can apply the above method as follows:

Weight of the body before immersion:

( Reduced ) weight of the body ( complete ) immersion, the volume is displaced (this is measured either by the overflow from the full vessel or in the measuring cylinder):

So after forming:

According to this method already Archimedes determined the density of the crown of a king who had doubts whether this really is pure gold there ( ρK = 19320 kg/m3).

On the buoyancy measurement for density determination of liquids based the hydrometer (spindle ) and the Mohr scale.

Other methods of measurement

• Pycnometer, density determination of solids or liquids by measuring the displaced liquid volumes
• Isotope method, density determination by absorption of radiation
• Bending vibrator, density determination by vibration measurement

A simple estimate of the density can be obtained with the Girolami method.

Density of solutions

The sum of the mass concentrations of the components of a solution yields the density of the solution by dividing the sum of the masses of the components by the volume of the solution.

The individual partial masses, the target volumes, and V are the total volume.

Location-dependent density

With the mass will, in a certain control volume. With constantly distributed mass can perform a border crossing, that is, letting the control volume are becoming smaller and, by the mass density

Define. The function is also referred to as a density patch.

For a homogeneous body whose mass density in its interior anywhere has the value, the total mass is the product of density and volume, that is, it applies:

In inhomogeneous bodies, the total mass is generally the volume integral

Of the mass density.

The density results from the masses of the atoms that make up the material and of their distances. In a homogeneous material, for example, in a crystal, the density is the same everywhere. It usually changes with the temperature and at compressible materials (such as gases ) and with the pressure. Thus, for example, the density of the atmosphere is location-dependent and increases with altitude from.

The inverse of the density and specific volume is known to play a role primarily in the thermodynamics of gases and vapors. The ratio of the density of a substance to the density under standard conditions is referred to as a relative density.

In the first edition of DIN 1306 density and specific gravity; Terms of August 1938, the density in the current sense standardized as the average density, and the location-dependent density was absolutely defined in a dot as a density " is the density ( without the addition, medium ') in a point of an object is the threshold value, the average density tends in a containing the dot volume, when you think of this so far reduced that it is small compared with the dimensions of the body, but still large remains against the structure units of its substance. " in the issue of August in 1958, the mean density in density renamed with the explanation: " mass, weight and volume are determined on a body whose dimensions are large compared to its structural constituents. "

Specific density in terms of technology

• True density ρ0, absolute density, true density, skeletal density (volume without voids), cf cubic meters, tapped density
• Density ρ, geometric density, bulk density, apparent density ( a porous body cavities included), cf cubic meter
• Body density ρ, apparent density (including pores, excl. Cavities ) Dry density, bulk density of wood, dry density of the rocks ( materials science )

• Grain density, test weight (Logistics )

As a dimensionless comparison size:

• Specific gravity d, specific gravity
• Storage density of cohesive and non-cohesive soils ( soil mechanics )

Other density terms

Volume-related variables

• Particle (parts)
• Space charge density (room )
• Volume density ( volume)
• Volumetric energy density ( energy)

Sizes per unit area

( Recommended designation to DIN 5485: surface density or land cover )

• Mass density / Basis Weight
• Surface charge density
• Current density
• Radiation power density
• Electrical and magnetic flux density
• Surface power density
• Outside of physics and engineering, for example, population density

Sizes per unit length

( Recommended designation to DIN 5485: length density, lining or behang )

• Line charge density
• Capacitance per unit length
• Inductance

Table values

Table values ​​for the density of various substances are found in the following articles:

• List of chemical elements