Density

Density
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For other uses, see Density (disambiguation).
In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as:


where, in SI units:

ρ (rho) is the density of the substance, measured in kg·m-3
m is the mass of the substance, measured in kg
V is the volume of the substance, measured in m3
Contents [hide]
1 History
2 Measurement of density
3 Common units
4 Changes of density
5 Density of water
6 Density of air
7 References
8 Books
9 See also
10 External links



[edit] History
In a famous problem, Archimedes was given the task of determining if King Hiero's goldsmith was embezzling gold during the manufacture of the king's crown and replacing it with another, cheaper alloy.[1]

Archimedes knew that the crown could be smashed into a cube or sphere, where the volume could be calculated more easily when compared with the weight; the king did not approve of this.

Baffled, Archimedes went to take a bath and observed from the rise of the water upon entering that he could calculate the volume of the crown by the displacement of the water. Allegedly, Archimedes went running though the streets naked shouting, "Eureka! Eureka!"

It is not known what happened to the goldsmith.


[edit] Measurement of density
For a homogeneous object, the formula mass/volume may be used. The mass is normally measured with an appropriate scale; the volume may be measured directly (from the geometry of the object) or by the displacement of a liquid. A very common instrument for the direct measurement of the density of a liquid is the hydrometer. A less common device for measuring fluid density is a pycnometer, a similar device for measuring the absolute density of a solid is a gas pycnometer.

The density of a solid material can be ambiguous, depending on exactly how it is defined, and this may cause confusion in measurement. A common example is sand: if gently filled into a container, the density will be small; when the same sand is compacted into the same container, it will occupy less volume and consequently carry a greater density. This is because "sand" 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.





[edit] Common units
In U.S. customary units or Imperial units, the units of density include;

ounces per cubic inch (oz/in³)
pounds per cubic inch (lb/in³)
pounds per cubic foot (lb/ft³)
pounds per cubic yard (lb/yd³)
pounds per gallon (for U.S. or imperial gallons) (lb/gal)
pounds per U.S. bushel (lb/bu)
slugs per cubic foot.

[edit] 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


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.


[edit] Density of water
Temperature Density[2] (at 1 atm)
°C °F kg/m³
0.0 32.0 999.8425
4.0 39.2 999.9750
15.0 59.0 999.1026
20.0 68.0 998.2071
25.0 77.0 997.0479
37.0 98.6 993.3316
50.0 122.0 988.04
100.0 212.0 958.3665




[edit] Density of air
T in °C ρ in kg/m³ (at 1 atm)
−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




[edit] References
^ Archimedes, A Gold Thief and Buoyancy by Larry "Harris" Taylor, Ph.D.[1]
^ Density of water, as reported by Daniel Harris in Quantitative Chemical Analysis, 4th ed., p. 36, W. H. Freeman and Company, New York, 1995.

[edit] Books
Fundamentals of Aerodynamics Second Edition, McGraw-Hill, John D. Anderson, Jr.
Fundamentals of Fluid Mechanics Wiley, B.R. Munson, D.F. Young & T.H. Okishi
Introduction to Fluid Mechanics Fourth Edition, Wiley, SI Version, R.W. Fox & A.T. McDonald
Thermodynamics: An Engineering Approach Second Edition, McGraw-Hill, International Edition, Y.A. Cengel & M.A. Boles

[edit] See also
Charge density
Buoyancy
Bulk density
Dord
Energy density
Lighter than air
Number density
Population density
Specific weight
Standard temperature and pressure

[edit] External links
Glass Density Calculation Calculation of the density of glass at room temperature and of glass melts at 1000-1400°C
List of Elements of the Periodic Table - Sorted by Density
Retrieved from "http://en.wikipedia.org/wiki/Density"
Categories: Continuum mechanics | Introductory physics | Fundamental physics concepts | Physical quantity | Physical chemistry

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