In physics, the mass per unit volume of a substance is called the density of the substance. Beauty Instrument,Rf & Ems Beauty Instrument,Rf And Ems Beauty Instrument,Rf&Ems Beauty Instrument Foshan Liqia Hardware Products Co., Ltd. , https://www.liqiahardware.com
1. The ratio of the mass of a substance to its volume, that is, the mass of a substance per unit volume, is called the density of this substance. Symbol Ï. The unit is kg / m ^ 3.
The mathematical expression is Ï = m / V. In the International System of Units, the main unit of mass is kilograms, and the main unit of volume is cubic meters, so the mass of 1 cubic meter of the substance is taken as the density of the substance. For non-uniform materials it is called "average density".
2. The physical meaning of density. Using water as an example, the density of water at 10C is 10 ^ 3 kg / m ^ 3 or 1 g / cm ^ 3 (1.0 × 10 ^ 3kg / m ^ 3, physical). The meaning is: the mass of water per cubic meter is 1.0 × 10 ^ 3 kg.
The average density of the earth is 5.5 × 10 ^ 3 kg / m ^ 3.
The average density of dry air under standard conditions is 0.001293 × 10 ^ 3 kg / m ^ 3.
The density of common non-metallic solids, metals, liquids, and gases (omitted).
3. Refers to the mass of the substance contained in a unit volume at a specified temperature, expressed in kg / m ^ 3 (read as kilograms per cubic meter) or g / cm ^ 3 (read as grams per cubic centimeter). It is mainly used in the measurement of conversion quantity and delivery acceptance, the quality control of certain oil products, and the simple judgment of oil product performance.
4. In printing terms, reflection density refers to the shading ability of a surface; transmission density refers to the shading ability of a filter.
5. The density of the photosensitive material refers to the degree of image depth after exposure and development. Such as film, the more transparent the picture, the lower the density; conversely, the more opaque the place, the greater the density.
Density is a physical quantity that reflects the characteristics of a substance. The characteristics of a substance refer to a property that the substance itself has and that can be distinguished from each other. People often feel that a substance with a high density is "heavy" and a substance with a low density is "light". Here, " "Heavy" and "light" essentially refer to the size of the density.
: Mass is the amount of substance contained in the object. The content is reduced, so the quality is reduced. Density is a characteristic of a substance. It does not change with changes in mass and volume. The density of the same substance does not change.
Density is a characteristic of a substance. It is only related to the type of substance and has nothing to do with factors such as mass and volume. The density of different substances is generally different, and the density of the same substance is the same.
Density formula: (Ï indicates density, m indicates mass, V indicates volume)
When correctly understanding the density formula, we must pay attention to the conditions and the special meaning of each physical quantity. From a mathematical point of view, there are three situations:
(1) m is proportional to V at a certain time;
(2) When m is fixed, Ï is inversely proportional to V;
(3) When V is constant, Ï is proportional to m.
Combining the physical meaning, only the (1) statement is correct in the three cases, and (2) (3) are all wrong. Because the density of the same substance is fixed, it does not change with the change of volume and mass, so when understanding the physical formula, it is impossible to deviate from the physical facts, and it is impossible to understand the relationship between the quantities in the physical formula simply from a mathematical point of view
5. The unit of density in the International System of Units is: kg / m3. The correct reading is kilograms per cubic meter, the symbol is kg / m3, the commonly used unit is grams / cm3, the correct reading is grams per cubic centimeter, the symbol is g / cm3.
Conversion relationship between them:
l g / cm3 = 1000kg / m3
6. The density of water is 1000kg / m3
Its physical meaning is that the mass of water with a volume of 1m3 is 1000kg.
7. According to the variation of the density formula: or, the mass and volume of the object can be calculated, especially some problems that are inconvenient for direct measurement of mass and volume, such as calculating the volume of irregularly shaped objects, the mass of the monument, etc. Density is one of the characteristics of substances. Each substance has a certain density. 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, and compare the measured density with the density of various substances in the density table to identify what the object is made of.
8. According to the variation of the density formula: or, the mass and volume of the object can be calculated, especially some problems that are inconvenient for direct measurement of mass and volume, such as calculating the volume of irregularly shaped objects, the mass of the monument, etc. Density is one of the characteristics of substances. Each substance has a certain density. The density of different substances is generally different. So we can use density to identify substances. The method is to determine the density of the substance to be measured, and compare the measured density with the density of various substances in the density table to identify what substance is made of the object.
9. Use density knowledge to solve simple problems, such as judging whether an object is hollow, and use the "analysis method" to solve some more complex problems.
To determine whether an object is hollow or solid, there are generally three methods:
(1) According to the formula, find and compare with the density Ï of the substance, if <Ï, it is hollow, if = Ï, it is solid.
(2) Treat the object as a solid object, find it by the formula, and compare with it. If <, it is hollow, if =, the object is solid.
(3) Treat the object as a solid object, use to find the mass of the solid object with volume, and then compare m with the actual mass m of the object. If m> m, the object is hollow, if m = m object, then the object is solid.
10. The density of the human body is only 1.07 g / cm3, which is only a little more than the density of water, so it should not be too difficult to learn swimming! Gasoline has a lower density than water, so you know why the oil stains you see on the road will float on the water. The density of seawater is greater than that of water, and it is easier for the human body to float in seawater.
The density of water is even greater than ice. Now you go to the refrigerator to get some ice cubes and throw them in half a glass of water. See if the ice cubes are floating? Still sinking. The density of a substance will change under the influence of temperature. In general, the mass of a substance is not affected by temperature, but the volume will expand and contract with heat. Therefore, when the temperature rises, the volume expands, and the density becomes relatively smaller. Conversely, as the temperature decreases, the volume shrinks and the density increases. But water is an exception, because the density of water is the largest at 4 ℃, as long as the water temperature rises or falls from 4 ℃, the density will become smaller. That is to say, water at 4 ° C expands in volume when heated and expands when cooled. Therefore, water always starts to freeze from the surface, and the water with the highest density of 4 ℃ will sink into the bottom layer. This property is very important. In the cold winter, although the surface of the water has frozen, it is still maintained at about 4 ° C at the bottom of the lake, so that the creatures in the water can spend the winter in peace.