Density is the intensity of the distribution of one quantity over another.
The term combines several different concepts, such as: the density of matter; optical density; population density; building density; density of fire and many others. Let us consider two concepts related to non-destructive testing.
1. Density of matter.
In physics, the density of a substance is the mass of this substance contained in a unit volume under normal conditions. Bodies of the same volume, made of different substances, have different masses, which characterizes their density. For example, two cubes of the same size, made of cast iron and aluminum, will differ in weight and density.
To calculate the density of any body, you need to accurately determine its mass and divide it by the exact volume of this body.
kg / m 3
— Units
density in international
system of units (SI)
g/cm 3
— Units
density in the cgs system
We derive a formula for calculating the density.
For example, let's determine the density of concrete. Let's take a concrete cube weighing 2.3 kg with a side of 10 cm. Let's calculate the volume of the cube.
We substitute the data in the formula.
We get a density of 2,300 kg / m 3.
What determines the density of a substance
The density of a substance depends on temperature. So in the vast majority of cases, as the temperature decreases, the density increases. The exception is water, cast iron, bronze and some other substances that behave differently in a certain temperature range. Water, for example, has a maximum density at 4 °C. As the temperature rises or falls, the density will decrease.
The density of a substance also changes when its state of aggregation changes. It grows abruptly as a substance changes from a gaseous state to a liquid state, and then to a solid state. There are also exceptions here: the density of water, bismuth, silicon and some other substances decreases during solidification.
How is the density of a substance measured?
Special devices and devices are used to measure the density of various substances. Thus, the density of liquids and the concentration of solutions is measured by various hydrometers. Several varieties of pycnometers are designed to measure the density of solids, liquids and gases.
2. Optical density.
In physics, optical density is the ability of transparent materials to absorb light, and opaque materials to reflect it. This concept in most cases characterizes the degree of attenuation of light radiation when it passes through layers and films of various substances.
Optical density is usually expressed as a decimal logarithm of the ratio of the radiation flux incident on the object to the flux that has passed through the object or reflected from it:
Optical density \u003d logarithm (radiation flux incident on an object where D is the optical density; F 0 is the radiation flux incident on the object; F is the radiation flux that has passed through the object or reflected from it).
To understand how and in what density is measured, first of all, it is necessary to define the word density. The density of a substance is a physical quantity determined for a homogeneous substance by the mass of its unit volume. In other words, density is the ratio of the mass of a substance to its volume.
There are two main methods for determining the density of a substance - this is a direct method and an indirect one. The indirect method includes the mathematical calculation of the density of a substance according to the formula, ρ = m / V, where ρ
- density, m- the mass of the substance, V is the volume of the substance.
The question arises, in what units is density measured? It depends on how much of the substance was taken as mass and for what unit volume. For example, if you fill a container with a volume of 1 liter with water, then weigh this container together with water and subtract the mass of the container from the resulting mass, we get the mass of water. Suppose the resulting value of the mass of water is 1 kg. After that, knowing the mass and volume of water, mathematically (by indirect method) it is possible to calculate the density of water by dividing the mass of water (1 kg) by the volume (1 liter). Received value 1 kg/l and is the density of water, where kg/l- something in which density is measured.
To directly measure the density of a liquid, measuring instruments such as hydrometers or electronic density meters , like a company - a manufacturer of density meters LEMIS Baltic. These measuring instruments will give the values of the density of the measured liquid in g/cm3 and in kg/m3 - this is the units in which the density is measured according to the standard in the SI system.
Those. There is no single answer in what density is measured. The most commonly used values have been listed previously. But others can also be used. For example, if a country uses a non-metric system of measurement, then the density units are completely different.
Everything around us is made up of different substances. Ships and baths are built of wood, irons and folding beds are made of iron, tires on wheels and erasers on pencils are made of rubber. And different items have different weights - any of us will easily bring a juicy ripe melon from the market, but you will have to sweat over a weight of the same size.
Everyone remembers the famous joke: “What is harder? A kilogram of nails or a kilogram of fluff? We will no longer fall for this childish trick, we know that the weight of both will be the same, but the volume will be significantly different. So why is this happening? Why do different bodies and substances have different weights for the same size? Or vice versa, the same weight for different sizes? Obviously, there is some characteristic that makes substances so different from each other. In physics, this characteristic is called the density of matter and is passed in the seventh grade.
Matter density: definition and formula
The definition of the density of a substance is as follows: density shows what the mass of a substance is equal to in a unit volume, for example, in one cubic meter. So, the density of water is 1000 kg / m3, and ice - 900 kg / m3, which is why ice is lighter and is located on top in winter on reservoirs. That is, what does the density of matter show us in this case? The density of ice equal to 900 kg/m3 means that a cube of ice with sides of 1 meter weighs 900 kg. And the formula for determining the density of a substance is as follows: density \u003d mass / volume. The quantities included in this expression are denoted as follows: mass - m, body volume - V, and density is denoted by the letter ρ (Greek letter "ro"). And the formula can be written as follows:
How to find the density of a substance
How to find or calculate the density of a substance? To do this, you need to know the volume of the body and body weight. That is, we measure the substance, weigh it, and then we simply substitute the obtained data into the formula and find the value we need. And how the density of a substance is measured is clear from the formula. It is measured in kilograms per cubic meter. Sometimes they also use such a value as a gram per cubic centimeter. Converting one value to another is very simple. 1 g = 0.001 kg, and 1 cm3 = 0.000001 m3. Accordingly, 1 g / (cm) ^ 3 \u003d 1000 kg / m ^ 3. It should also be remembered that the density of a substance is different in different states of aggregation. That is, solid, liquid, or gaseous. The density of solids, most often, is higher than the density of liquids and much higher than the density of gases. Perhaps a very useful exception for us is water, which, as we have already considered, weighs less in the solid state than in the liquid state. It is because of this strange feature of water that life is possible on Earth. Life on our planet, as you know, originated from the oceans. And if water behaved like all other substances, then the water in the seas and oceans would freeze through, ice, being heavier than water, would sink to the bottom and lie there without melting. And only at the equator in a small water column would life exist in the form of several types of bacteria. So we can say thank you to water for the fact that we exist.
Figure 1. Table of densities of some substances. Author24 - online exchange of student papers
All bodies in the world around us have different sizes and volumes. But even with the same volumetric data, the mass of substances will differ significantly. In physics, this phenomenon is called the density of matter.
Density is a basic physical concept that gives an idea of the characteristics of any known substance.
Definition 1
The density of a substance is a physical quantity that shows the mass of a certain substance per unit volume.
The units of volume in terms of the density of a substance are usually a cubic meter or a cubic centimeter. Determination of the density of a substance is carried out with special equipment and instruments.
To determine the density of a substance, it is necessary to divide its mass by its own volume. When calculating the density of a substance, the following quantities are used:
body weight ($m$); body volume ($V$); body density ($ρ$)
Remark 1
$ρ$ is the letter of the Greek alphabet "ro" and should not be confused with the similar symbol for pressure - $p$ ("pe").
Matter Density Formula
The calculation of the density of a substance occurs with the use of the SI measurement system. In it, density units are expressed in kilograms per cubic meter or grams per cubic centimeter. You can also use any system of measurement.
A substance has different degrees of density if it is in different states of aggregation. In other words, the density of a substance in a solid state will be different than the density of the same substance in a liquid or gaseous state. For example, water has a density in its normal liquid state of 1,000 kilograms per cubic meter. In the frozen state, water (ice) will already have a density of 900 kilograms per cubic meter. Water vapor at normal atmospheric pressure and a temperature close to zero degrees will have a density of 590 kilograms per cubic meter.
The standard formula for the density of a substance is as follows:
In addition to the standard formula, which is used only for solids, there is a formula for gas under normal conditions:
$ρ = M / Vm$, where:
- $M$ - molar mass of gas,
- $Vm$ - molar volume of gas.
There are two types of solids:
- porous;
- loose.
Remark 2
Their physical characteristics directly affect the density of the substance.
Density of homogeneous bodies
Definition 2
The density of homogeneous bodies is the ratio of the mass of a body to its volume.
The definition of the density of a homogeneous and uniformly distributed body with an inhomogeneous structure, which consists of this substance, is included in the concept of the density of a substance. This is a constant value and for a better understanding of the information, special tables are formed, where all common substances are collected. The values for each substance are divided into three components:
- solid state density;
- the density of the body in the liquid state;
- the density of a body in a gaseous state.
Water is a fairly homogeneous substance. Some substances are not so homogeneous, therefore, the average density of the body is determined for them. To derive this value, it is necessary to know the result of ρ of the substance for each component separately. Loose and porous bodies have true density. It is determined without taking into account voids in its structure. Specific gravity can be calculated by dividing the mass of a substance by the total volume it occupies.
Similar values are interconnected by the porosity coefficient. It represents the ratio of the volume of voids to the total volume of the body that is currently being examined.
The density of substances depends on many additional factors. A number of them simultaneously increase this value for some substances, and lower it for the rest. At low temperatures, the density of a substance increases. Some substances are able to respond to temperature changes in different ways. In this case, it is customary to say that the density at a certain temperature range behaves anomalously. Such substances often include bronze, water, cast iron and some other alloys. The density of water is highest at 4 degrees Celsius. With further heating or cooling, this indicator can also change significantly.
Metamorphoses with the density of water occur during the transition from one state of aggregation to another. The index ρ in these cases changes its values abruptly. It progressively increases upon transition to a liquid from a gaseous state, as well as at the moment of liquid crystallization.
There are, and many, exceptional cases. For example, silicon has small density values during solidification.
Measuring the density of a substance
In order to effectively measure the density of a substance, special equipment is usually used. It consists of:
- scales;
- measuring device in the form of a ruler;
- measuring flask.
If the test substance is in a solid state, then a measuring device in the form of a centimeter is used as a measuring device. If the test substance is in a liquid state of aggregation, then a volumetric flask is used for measurements.
First you have to measure the volume of the body with a centimeter or volumetric flask. The researcher observes the measurement scale and records the result. If a wooden beam of a cubic shape is being examined, then the density will be equal to the value of the side raised to the third power. When examining a liquid, it is necessary to additionally take into account the mass of the vessel with which measurements are taken. The obtained values \u200b\u200bmust be substituted into the universal formula for the density of the substance and the indicator calculated.
For gases, the calculation of the indicator is very difficult, since it is necessary to use various measuring instruments.
Usually, a hydrometer is used to calculate the density of substances. It is designed to get results on liquids. The true density is studied using a pycnometer. Soils are examined with the help of Kaczynski and Seidelman drills.
The bodies around us consist of various substances: iron, wood, rubber, etc. The mass of any body depends not only on its size, but also on the substance of which it consists. Bodies of the same volume, consisting of different substances, have different masses. For example, weighing two cylinders of different substances - aluminum and lead, we will see that the mass of aluminum is less than the mass of a lead cylinder.
At the same time, bodies with the same masses, consisting of different substances, have different volumes. So, an iron bar with a mass of 1 t occupies a volume of 0.13 m 3, and ice with a mass of 1 t - a volume of 1.1 m 3. The volume of ice is almost 9 times greater than the volume of an iron bar. That is, different substances can have different densities.
It follows that bodies with the same volume, consisting of different substances, have different masses.
Density shows what is the mass of a substance taken in a certain volume. That is, if the mass of the body and its volume are known, the density can be determined. To find the density of a substance, it is necessary to divide the mass of the body by its volume.
The density of the same substance in solid, liquid and gaseous states is different.
The density of some solids, liquids and gases is given in the tables.
Densities of some solids (at normal atm. pressure, t = 20 ° C).
|
Solid |
ρ , kg / m 3 |
ρ , g/cm 3 |
Solid |
ρ , kg / m 3 |
ρ , g/cm 3 |
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Window glass |
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Pine (dry) |
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plexiglass |
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Rafinated sugar |
Polyethylene |
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Oak (dry) |
Densities of some liquids (at norm. atm. pressure t =20 ° C).
|
Liquid |
ρ , kg / m 3 |
ρ , g/cm 3 |
Liquid |
ρ , kg / m 3 |
ρ , g/cm 3 |
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The water is clean |
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Whole milk |
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Sunflower oil |
Liquid tin (at t= 400°C) |
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Machine oil |
Liquid air (at t= -194°C) |