density

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physical quantity

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Density is a measure of the mass in a specific volume. The density is equal to the mass of the object divided by the volume, and can be expressed by the symbol ρ (read as [r 601ʊ]), International System of Units And Chinese legal measurement units, the unit of density is kg/m3, and the symbol is kg/m three

^[1]

Chinese name: density
Foreign name: density
Interpretation: Corporeal quality And volume Ratio of
Symbol: ρ

International unit: kg/m³
Calculation formula: ρ=m/V
Other units: g/cm³
See publications: Physics Terms (Second Edition), Science Press
Time of publication: 1996^[2]

catalog

seven Density and buoyancy
eight Common material density

Variant form

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Variant form

Original formula:

Deformation: 1

two

(ρ is the density, m is the mass of the object, and V is the volume of the object)

definition

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The density of any point P in the object is defined as:

，

Where

V is the volume element containing P point;

Densimeter

M is the mass of the volume element. In the centimeter gram second system, the unit of density is gram per cubic centimeter; stay International System of Units And Chinese legal measurement units, the unit of density is kg/m3.

Rubber densimeter

Density change rule

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Generally speaking, no matter what kind of substance or its state, its volume or density will change with the change of temperature and pressure. The relationship between temperature T, pressure F and density ρ (or volume) is called equation of state. The volume of a gas varies significantly with the pressure and temperature it is subjected to. For an ideal gas, the equation of state is

, where R is the gas constant, equal to 287.14m two (seconds two *On). If its temperature is constant, the density is proportional to the pressure; If its pressure is constant, its density is inversely proportional to its temperature. For ordinary gases, if the density is not large and the temperature is far from the liquefaction point, the volume changes with pressure close to the ideal gas; For high density gas, the above equation of state should also be properly modified.

The density of solid or liquid substances changes only slightly when the temperature and pressure change. For example, near 0 ℃, the temperature coefficient of various metals (the change rate of object volume when the temperature rises by 1 ℃) is mostly 10 -9 about. The pressure in deep water and underwater explosion can reach hundreds of atmospheres, or even higher (1 atmosphere=101325 Pa). At this time, the change of density with pressure must be considered. R. H. Cole recommends the following equation of state:

Where, p zero Is the density of water at one atmospheric pressure. if n and B It is taken as 7 and 3000 atmospheric pressure, then it is up to 10 five Atmospheric pressure, the error between the above formula and the measured data is within a few percent.

As far as the whole nature is concerned, the extreme pressure will make the density of substances in some celestial bodies differ greatly from the common density.

Density and life

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The density of human body is only 1.02 g/cm ³, which is only a little more than the density of water. The density of gasoline is smaller than that of water, so oil stains seen on the road will float on the water. The density of seawater is greater than that of water, so the human body is easier to float in seawater. (The density of the Dead Sea seawater reaches 1.3g/cm ³, which is higher than the density of human body, so people can float in the Dead Sea.)

application

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Scientifically

1. Identify the materials that make up the object.

Density is physical characteristic One is that every substance has a certain density, and the density of different substances is generally different. So we can use density to identify substances. The method is to measure the density of the substance to be measured, compare the measured density with the density of various substances in the density table, and then you can identify what the substance is made of.

2. Calculate the composition of various substances contained in the object.

3. Calculate the mass of objects that are difficult to weigh or the volume of objects with complex shapes.

According to the variable form of density formula: m=V ρ or V=m/ρ, the mass and volume of objects can be calculated, especially for some problems that are inconvenient to measure directly, such as calculating the volume of irregular shaped objects and the mass of monuments.

4. Determine whether the object is solid or hollow.

There are generally three methods to determine whether an object is hollow or solid:

(1) According to the formula, calculate its density, and then compare it with the density ρ of the material. If<ρ, it is hollow; if=ρ, it is solid.

(2) Known quality , by the formula,

Find V and then V matter Compare if V matter ＞ V, it is hollow, if V=V matter , the object is solid.

(3) Treat the object as a solid object, use, and calculate volume Is the mass of the solid object of v, and then m and the actual mass of the object m matter Comparison, if m ＞ m matter If m=m, the object is hollow matter , the object is solid.

5. Calculate the liquid interior pressure as well as buoyancy Etc. (The density can also calculate the cylinder pressure)

6. Identification of unknown substances

“ argon ”Is to calculate the unknown Gas The density of. After many experiments spectral analysis Confirm that the air contains a

If you don't know a new gas, name it argon.

In agriculture

It is an important basis for density. stay Agriculture It can be used to judge soil Fertility, including humus The soil is rich, and its density is generally 2.3 × 10 ³ kg/m ³. We can select seeds according to seed Seed selection shall be carried out according to the sinking and floating conditions in water: plump and healthy seeds sink due to high density; Collapsed shells and other weed seeds float on the water surface due to their low density. stay Industry In production, such as starch 's production to potato As raw material, potatoes with more starch have higher density, so the yield of starch can be estimated by measuring the density of potatoes.

Industrially

Factory in casting Before metal objects, estimate melt How many? Metal , according to Mold And the density of metal Amount of metal 。

measuring method

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[Experimental principle]

[Physical quantities to be measured]

Volume and mass

[Experimental equipment]

Balance, measuring cylinder water , metal block (small), string.

[Experiment steps]

1. Weigh the metal block with a balance quality m。

2. Inject proper amount of water into the measuring cylinder and read the volume as V 1。

3. Tie the metal block with string and put it into the measuring cylinder, immerse it, and read the volume as V 2。

Calculation Expression

Specific gravity cup method

[Experimental equipment]

Beaker, water, metal block (large), balance.

[Experiment steps]

1. Fill the beaker with water and put it on the balance to weigh the mass m one ；

2. Gently put the metal block into the water, overflow some water, and then put the beaker on the balance to weigh the mass of m two ;

3. Take out the metal block, put the beaker on the balance and weigh the mass m of the beaker and the remaining water three 。

Calculation Expression

ρ=ρ water （m2-m3）/（m1-m3）

Archimedes law method

[Experimental equipment]

Spring scale 、 Metal Block water , string.

[Experiment steps]

1. Tie the metal block with a string, and weigh the gravity G of the metal block with a spring scale;

2. Fully immerse the metal block in water, and weigh the apparent weight G 'of the metal block in water with a spring scale;

Calculation Expression

ρ=Gρ water /(G-G')

Buoyancy method 1

[Experimental equipment]

Wood block, water, fine needle, measuring cylinder.

[Experiment steps]

1. Inject proper amount of water into the measuring cylinder and read the volume as V one ；

2. Put the wood block into the water, float, and read the volume V after it is still two ；

3. Insert the wood block with a fine needle, completely immerse the wood block in water, and read the volume as V three 。

Calculation Expression ρ=ρ water (V two -V one )/(V three -V one )

Buoyancy method 2

[Experimental equipment]

Scale, large beaker, water, small beaker, small stone (ρ＞ water)

[Experiment steps]

1. Put an appropriate amount of water into the large beaker, and then gently put the small beaker with the mouth upward to float it, and measure the height of water in the large beaker with a scale h one

2. Gently put small stones into the small beaker (the small beaker should float at this time), and measure the height of water in the large beaker with a scale h two

3. Take out the small stone from the cup, put it into the water, and measure the height h of the water in the large beaker with a scale three

Calculation Expression

ρ=ρ water (h two -h one )/(h three -h one )

Densimeter method

[Experimental equipment] Egg, densimeter, water, salt, glass.

[Experiment steps]

1. Pour proper amount of water into the glass, gently put the eggs in, and the eggs sink;

2. Gradually add salt to the water, and mix with a densimeter while adding salt until the egg suspension , use a densimeter to measure the density of brine, that is, the density of eggs.

Physical particles

quantum mechanics It is clearly pointed out that for physical particles, the density ρ means that the particle space The probability of occurrence in any small area (the mathematical term is "volume element") is the probability density.

Density and buoyancy

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Object in water

ρ object ＜ρ water , Object float (floating upward)

ρ object = ρ water , Object suspension

ρ object ＞ρ water , object sinking（ sink ）

For any liquid

ρ object ＜ρ liquid , objects floating (or floating up)

ρ object = ρ liquid , object suspension

ρ object ＞ρ liquid , objects sink to the bottom (or sink)

When ρ object ≤ ρ liquid (object floats or suspends)

Volume of object in water: volume of object=ρ object ：ρ liquid

When ρ object = ρ water When (object) is suspended, the volume of the object in water: the volume of the object=1:1

Common material density

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(Under normal temperature and pressure, unit: g/cm three ）

Liquid density meter
material	Density (kg/m three )
Mercury	13.6×10 three
vegetable oil	0.9×10 three
Alcohol, kerosene	0.8×10 three
Water (4 ℃)	1.0×10 three
ethanol	0.79×10 three
gasoline	0.75×10 three
liquid helium	zero point one two five two
bromine	zero point zero zero seven one four

solid

Foam plastic 0.01～0.03

copper eight point nine

iron seven point eight

White cast iron 7.40～7.70

Grey cast iron 6.60～7.40

Malleable cast iron 7.20 ～ 7.40

casting steel seven point eight zero

Glass 2.4～2.8

paraffin wax zero point nine

Dry pine 0.5

Wood 0.4 ～ 0.9

ice zero point nine

lead eleven point three

silver ten point five

Chromium molybdenum aluminum steel 7.65

zinc seven point one zero

copper eight point nine

High strength alloy steel 7.82

Industrial pure iron 7.87

lithium zero point five three four

potassium zero point eight seven

sodium zero point nine seven

Carbon tool steel seven point eight five

rubidium one point five three

calcium one point five five

magnesium one point seven four

White phosphorus one point eight three

beryllium one point eight five

cesium one point eight seven

yellow arsenic two

pure aluminum two point seven zero

sulfur two point zero seven

phosphorus two point two zero

Osmium twenty-two point six

Chromium nickel tungsten steel 7.80

Nickel chromium alloy eight point seven two

ordinary carbon steel seven point eight five

Free cutting steel 7.85

Cast magnesium 1.80

Beryllium bronze eight point three zero

High quality carbon steel 7.85

manganese steel seven point eight one

Bearing steel seven point eight one

Cast zinc 6.86

Jin 19.32

Gas

hydrogen zero point zero zero zero zero nine

helium zero point zero zero zero one eight

neon zero point zero zero zero nine zero

nitrogen zero point zero zero one two five

oxygen zero point zero zero one four three

Fluorine gas zero point zero zero one six nine six

argon zero point zero zero one seven eight

ozone zero point zero zero two one four

ammonia zero point zero zero zero seven seven

Krypton zero point zero zero three seven four

Xenon zero point zero zero five eight nine

Radon gas zero point zero zero nine seven three

coal gas zero point zero zero zero six zero

carbon monoxide zero point zero zero one two five

chlorine zero point zero zero three two one

hydrogen sulfide zero point zero zero one five four

Hydrogen chloride zero point zero zero one six four

methane zero point zero zero zero seven eight

Nitrogen oxide zero point zero zero one three four

hydrogen sulfide zero point zero zero one five four

acetylene zero point zero zero one one seven

ethane zero point zero zero one three six

propane zero point zero zero two zero one

propylene zero point zero zero one nine two

carbon dioxide zero point zero zero one nine eight

sulfur dioxide zero point zero zero two nine three

Hydrogen iodide zero point zero zero five six six

Dichlorous oxide zero point zero zero three eight nine

Nitrous oxide 0.00198

Hydrogen bromide zero point zero three five zero

atmosphere zero point zero zero one two nine

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