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density

 
Dictionary: den·si·ty   (dĕn'sĭ-tē) pronunciation
 
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n., pl. -ties.
  1. The quality or condition of being dense.
    1. The quantity of something per unit measure, especially per unit length, area, or volume.
    2. The mass per unit volume of a substance under specified conditions of pressure and temperature.
  3. Computer Science. The number of units of useful information contained within a linear dimension.
  4. The number of individuals, such as inhabitants or housing units, per unit of area.
  5. The degree of optical opacity of a medium or material, as of a photographic negative.
  6. Thickness of consistency; impenetrability.
  7. Complexity of structure or content.
  8. Stupidity; dullness.

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Sci-Tech Encyclopedia: Density
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The mass per unit volume of a material. The term is applicable to mixtures and pure substances and to matter in the solid, liquid, gaseous, or plasma state. Density of all matter depends on temperature; the density of a mixture may depend on its composition, and the density of a gas on its pressure. Common units of density are grams per cubic centimeter, and slugs or pounds per cubic foot. The specific gravity of a material is defined as the ratio of its density to the density of some standard material, such as water at a specified temperature, for example, 60°F (15.6°C), or, for gases the basis may be air at standard temperature and pressure. Another related concept is weight density, which is defined as the weight of a unit volume of the material. See also Density measurement; Weight.

 
Marketing Dictionary: density
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In general: term used as a measure of population in proportion to a geographic area.

Direct marketing: measure of the proportion of customers or prospective customers in a geographic area compared to the total population of the area.

Photography: measure of the relative blackening of photographic images, which consist of silver particles spread in varying densities onto a surface. The particles create images of light, dark, and shadow. See also densitometer.

 
Business Dictionary: Density
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Computers: see Double Density.

Real estate: intensity of land use. For example, if a 10-acre Subdivision contains 30 single-family houses, the housing density is 3 dwelling units per acre. If the population density is 4 people per house, the population density per acre is 12.

 
Real Estate Dictionary: Density
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The intensity of a land use. See also Land Use Intensity.
Example: A 10-acre Subdivision contains 30 single-family houses. The density is 3 dwelling units per Acre.

 
Thesaurus: density
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noun

    The quality, condition, or degree of being thick: compactness, solidity, thickness. See thick/thin.

 
Antonyms: density
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n

Definition: bulk, mass
Antonyms: openness, sparseness, sparsity, thinness

 
Dental Dictionary: density
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n

The concentration of matter, measured by mass per unit volume.

 
Measures and Units: density
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The amount of some quantity per unit volume, notably of mass per unit volume, termed distinctively mass density but more properly volumic mass. Used similarly for amount of substance per unit volume (see amagat unit) and for electromagnetic quantities (see SI unit for examples). Used also per unit area in the form ‘surface density’, e.g. C·m-2 for surface density of charge.

 
Britannica Concise Encyclopedia: density
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Mass of a unit volume of a material substance. It is calculated by dividing an object's mass by its volume. In the International System of Units, and depending on the units of measurement used, density can be expressed in grams per cubic centimetre (g/cm3) or kilograms per cubic metre (kg/m3). The expression "particle density" refers to the number of particles per unit volume, not to the density of a single particle. See also specific gravity.

For more information on density, visit Britannica.com.

 
Architecture: density
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The degree of aggregation; the quantity of any entity distributed over an area per unit of areal measure, e.g., persons per acre, families per acre, or dwelling units per square mile.

 
Sports Science and Medicine: density
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The mass per unit volume of an object; density = mass/volume. A common unit of density is the kilogram per cubic metre.

 
Columbia Encyclopedia: density
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density, ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the substance. Most substances undergo expansion when heated and therefore have lower densities at higher temperatures. Many substances, especially gases, can be compressed into a smaller volume by increasing the pressure acting on them. For these reasons, the temperature and pressure at which the density of a substance is measured are usually specified. The density of a gas is often converted mathematically to what it would be at a standard temperature and pressure (see STP). Water is unusual in that it expands, and thus decreases in density, as it is cooled below 3.98°C (its temperature of maximum density). Density often is taken as an indication of how “heavy” a substance is. Iron is denser than cork, since a given volume of iron is more massive (and weighs more) than the same volume of cork. It is often said that iron is “heavier” than cork, although a large volume of cork obviously can be more massive and thus be heavier (i.e., weigh more) than a small volume of iron. See specific gravity.

 
Science Dictionary: density
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The relative heaviness of objects, measured in units of mass or weight per units of volume. (See specific gravity.)

 
Veterinary Dictionary: density
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1. the ratio of the mass of a substance to its volume.
2. the quality of being compact.
3. the quantity of matter in a given space.
4. the quantity of electricity in a given area, volume or time.
5. the degree of film blackening in an area of a photograph or radiograph.

  • population d. — number of animals per unit of area; important in relation to the rate of spread of disease.
  • d. sampling — see sampling.
 
Cosmic Lexicon: Density
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Mass per unit volume; a measure of how much material is in a given space.

 
Wood Glossary: Density
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Weight per unit volume. Density of wood is influenced by rate of growth, percentage of late wood and in individual pieces, the proportion of the heartwood.

 
Word Tutor: density
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pronunciation

IN BRIEF: Closeness or thickness.

pronunciation The density of the insulation seemed meager because they were always cold.

 
Wikipedia: Density
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This article is about volumetric mass density. For other uses, see Density (disambiguation).

The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho).

Contents

Formula

Mathematically:

\rho = \frac{m}{V} \,

where:

ρ is the density,
m is the mass,
V is the volume.

Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging.

In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas.

History

In a well-known common story, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a wreath dedicated to the gods and replacing it with another, cheaper alloy.[1]

Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the weight; but the king did not approve of this.

Baffled, Archimedes took a relaxing immersion bath and observed from the rise of the warm water upon entering that he could calculate the volume of the gold crown through the displacement of the water. Allegedly, upon this discovery, he went running naked through the streets shouting, "Eureka! Eureka!" (Greek "I found it"). As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment.

This story first appeared in written form in Vitruvius' books of architecture, two centuries after it supposedly took place.[2] Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time. [3][4]

Measurement of density

For a homogeneous object, the mass divided by the volume gives the density. The mass is normally measured with an appropriate scale or balance; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. Hydrostatic weighing is a method that combines these two.

If the body is not homogeneous or heterogeneous, the density is a function of the coordinates \rho(\vec{r})=dm/dv, where dv is elementary volume with coordinates \vec{r}. The mass of the body then can be expressed as

m = \int_V \rho(\vec{r})dv,

where the integration is over the volume of the body V.

A very common instrument for the direct measurement of the density of a liquid is the hydrometer, which measures the volume displaced by an object of known mass. A common laboratory device for measuring fluid density is a pycnometer; a related device for measuring the absolute density of a solid is a gas pycnometer. Another instrument used to determine the density of a liquid or a gas is the digital density meter - based on the oscillating U-tube principle.

The density of a solid material can be ambiguous, depending on exactly how its volume is defined, and this may cause confusion in measurement. A common example is sand: if gently filled into a container, the density will be low; when the same sand is compacted into the same container, it will occupy less volume and consequently exhibit a greater density. This is because sand, like all powders and granular solids contains a lot of air space in between individual grains; this overall density is called the bulk density, which differs significantly from the density of an individual grain of sand.

Common units

The SI unit for density is:

Metric units outside the SI

  • kilograms per litre (kg/L). At 4 °C, water has a density of 1.000 kg/L, making this a convenient unit at about the room temperature,
  • kilograms per cubic decimeter (kg/dm³),
  • grams per millilitre (g/mL),
  • grams per cubic centimeter (g/cc or g/cm³).

These are all numerically equivalent to kg/L (1 kg/L = 1 kg/dm³ = 1 g/cm³ = 1 g/mL).

In U.S. customary units or Imperial units, the units of density include:

Changes of density

In general density can be changed by changing either the pressure or the temperature. Increasing the pressure will always increase the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions to this generalisation. For example, the density of water increases between its melting point at 0 °C and 4 °C and similar behaviour is observed in silicon at low temperatures.

The effect of pressure and temperature on the densities of liquids and solids is small so that a typical compressibility for a liquid or solid is 10–6 bar–1 (1 bar=0.1 MPa) and a typical thermal expansivity is 10–5 K–1.

In contrast, the density of gases is strongly affected by pressure. Boyle's law says that the density of an ideal gas is given by

\rho = \frac {MP}{RT} \,

where R is the universal gas constant, P is the pressure, M the molar mass, and T the absolute temperature.

This means that a gas at 300 K and 1 bar will have its density doubled by increasing the pressure to 2 bar or by reducing the temperature to 150 K.

Osmium is the densest known substance at standard conditions for temperature and pressure.

Density of water

See also: Water density
Temp (°C) Density (kg/m3)
100 958.4
80 971.8
60 983.2
40 992.2
30 995.6502
25 997.0479
22 997.7735
20 998.2071
15 999.1026
10 999.7026
4 999.9720
0 999.8395
−10 998.117
−20 993.547
−30 983.854
The density of water in kilograms per cubic meter (SI unit)
at various temperatures in degrees Celsius.
The values below 0 °C refer to supercooled water.

Density of air

T in °C ρ in kg/m3 (at 1 atm)
–25 1.423
–20 1.395
–15 1.368
–10 1.342
–5 1.316
  0 1.293
  5 1.269
10 1.247
15 1.225
20 1.204
25 1.184
30 1.164
35 1.146


Density of solutions

The density of a solution is the sum of the mass (massic) concentrations of the components of that solution.
Mass (massic) concentration of a given component ρi in a solution can be called partial density of that component.

Density of composite material

ASTM specification D792-00[5] describes the steps to measure the density of a composite material. \rho = \frac{W_a}{W_a + W_w - W_b} \left (0.9975 \right ) \,

where:

ρ is the density of the composite material, in g/cm3

and

Wa is the weight of the specimen when hung in the air
Ww is the weight of the partly immersed wire holding the specimen
Wb is the weight of the specimen when immersed fully in distilled water, along with the partly immersed wire holding the specimen
0.9975 is the density in g/cm3 of the distilled water at 23°C

Densities of various materials

Material ρ in kg/m3 Notes
Interstellar medium 10-25 − 10-15 Assuming 90% H, 10% He; variable T
Earth's atmosphere 1.2 At sea level
Aerogel 1 − 2
Styrofoam 30 − 120 From
Cork 220 − 260 From
Water 1000 At STP
Plastics 850 − 1400 For polypropylene and PETE/PVC
The Earth 5515.3 Mean density
Copper 8920 − 8960 Near room temperature
Lead 11340 Near room temperature
Tungsten 19250 Near room temperature
Gold 19300 Near room temperature
The Inner Core of the Earth ~13000 As listed in Earth
Uranium 19100 Near room temperature
Iridium 22500 Near room temperature
Osmium 22610 Near room temperature
The core of the Sun ~150000
White dwarf star 1 × 109[6]
Atomic nuclei 2.3 × 1017 [7] Does not depend strongly on size of nucleus
Neutron star 8.4 × 1016 — 1 × 1018
Black hole 4 × 1017 Mean density inside the Schwarzschild radius of an earth-mass black hole (theoretical)


References

  1. ^ Archimedes, A Gold Thief and Buoyancy - by Larry "Harris" Taylor, Ph.D.
  2. ^ Vitruvius on Architecture, Book IX, paragraphs 9-12, translated into English and in the original Latin.
  3. ^ The first Eureka moment, Science 305: 1219, August 2004.
  4. ^ Fact or Fiction?: Archimedes Coined the Term "Eureka!" in the Bath, Scientific American, December 2006.
  5. ^ (2004). Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement. ASTM Standard D792-00. Vol 81.01. American Society for Testing and Materials. West Conshohocken. PA.
  6. ^ Extreme Stars: White Dwarfs & Neutron Stars, Jennifer Johnson, lecture notes, Astronomy 162, Ohio State University. Accessed on line May 3, 2007.
  7. ^ Nuclear Size and Density, HyperPhysics, Georgia State University. Accessed on line June 26, 2009.

See also

External links


This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
Translations: Density
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Dansk (Danish)
n. - kompakthed, tæthed, dumhed, vægtfylde, massefylde, optisk tæthed, lydabsorption

Nederlands (Dutch)
dichtheid, bevolkingsdichtheid, compactheid

Français (French)
n. - (Comput) densité, épaisseur (du brouillard), densité (de la population)

Deutsch (German)
n. - Dichte

Ελληνική (Greek)
n. - πυκνότητα, (μτφ.) βλακεία, χοντροκεφαλιά

Italiano (Italian)
densità

Português (Portuguese)
n. - densidade (f)

Русский (Russian)
плотность, густота

Español (Spanish)
n. - densidad, estupidez, densidad de población, grado de opacidad de una substancia, cantidad de información almacenada

Svenska (Swedish)
n. - densitet, täthet

中文（简体）(Chinese (Simplified))
密度

中文（繁體）(Chinese (Traditional))
n. - 密度

한국어 (Korean)
n. - 농도, 어리석음

日本語 (Japanese)
n. - 密集, 密度, 比重, 頭の悪さ, 濃度

العربيه (Arabic)
‏(الاسم) كثافه, سعه, العلاقه بين الوزن والحجم ( في الفيزياء)‏

עברית (Hebrew)
n. - ‮צפיפות, דחיסות, אטימות של צילום, טיפשות (מדוברת)‬

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