All physical quantities and their units. Physical quantities. Measurement of physical quantities – Knowledge Hypermarket
Physics, as a science that studies natural phenomena, uses standard research methods. The main stages can be called: observation, putting forward a hypothesis, conducting an experiment, substantiating the theory. During the observation, it is established distinctive features phenomena, the course of its course, possible reasons and consequences. A hypothesis allows us to explain the course of a phenomenon and establish its patterns. The experiment confirms (or does not confirm) the validity of the hypothesis. Allows you to establish a quantitative relationship between quantities during an experiment, which leads to an accurate establishment of dependencies. A hypothesis confirmed by experiment forms the basis of a scientific theory.
No theory can claim reliability if it has not received complete and unconditional confirmation during the experiment. Carrying out the latter involves measurements physical quantities, characterizing the process. - this is the basis of measurements.
What is it
Measurement concerns those quantities that confirm the validity of the hypothesis about patterns. A physical quantity is a scientific characteristic of a physical body, the qualitative relation of which is common to many similar bodies. For each body, this quantitative characteristic is purely individual.
If we turn to the specialized literature, then in the reference book by M. Yudin et al. (1989 edition) we read that a physical quantity is: “a characteristic of one of the properties of a physical object (physical system, phenomenon or process), common in qualitative terms for many physical objects, but quantitatively individual for each object.”
Ozhegov's dictionary (1990 edition) states that a physical quantity is “the size, volume, extension of an object.”
For example, length is a physical quantity. Mechanics interprets length as the distance traveled, electrodynamics uses the length of the wire, and in thermodynamics a similar value determines the thickness of the walls of blood vessels. The essence of the concept does not change: the units of quantities can be the same, but the meaning can be different.
A distinctive feature of a physical quantity, say, from a mathematical one, is the presence of a unit of measurement. Meter, foot, arshin are examples of units of length.
Units of measurement
To measure a physical quantity, it must be compared with the quantity taken as a unit. Remember the wonderful cartoon “Forty-Eight Parrots”. To determine the length of the boa constrictor, the heroes measured its length in parrots, baby elephants, and monkeys. In this case, the length of the boa constrictor was compared with the height of other cartoon characters. The result depended quantitatively on the standard.
Quantities are a measure of its measurement in a certain system of units. Confusion in these measures arises not only due to imperfection and heterogeneity of measures, but sometimes also due to the relativity of units.
Russian measure of length - arshin - the distance between the index and thumb hands. However, everyone's hands are different, and the arshin measured by the hand of an adult man is different from the arshin measured by the hand of a child or woman. The same discrepancy in length measures concerns fathoms (the distance between the fingertips of hands spread out to the sides) and elbows (the distance from the middle finger to the elbow of the hand).
It is interesting that small men were hired as clerks in the shops. Cunning merchants saved fabric using slightly smaller measures: arshin, cubit, fathom.
Systems of measures
Such a variety of measures existed not only in Russia, but also in other countries. The introduction of units of measurement was often arbitrary; sometimes these units were introduced only because of the convenience of their measurement. For example, to measure atmospheric pressure mmHg was administered. Known in which a tube filled with mercury was used, it was possible to introduce such an unusual value.
The engine power was compared with (which is still practiced in our time).
Various physical quantities made the measurement of physical quantities not only complex and unreliable, but also complicating the development of science.
Unified system of measures
A unified system of physical quantities, convenient and optimized in every industrialized country, has become an urgent need. The idea of choosing as few units as possible was adopted as a basis, with the help of which other quantities could be expressed in mathematical relationships. Such basic quantities should not be related to each other; their meaning is determined unambiguously and clearly in any economic system.
They tried to solve this problem in various countries. The creation of a unified SGS, ISS and others) was undertaken repeatedly, but these systems were inconvenient either with scientific point vision, or in household, industrial applications.
The task, posed at the end of the 19th century, was solved only in 1958. At a meeting of the International Committee for Legal Metrology, a unified system was presented.
Unified system of measures
The year 1960 was marked by the historic meeting of the General Conference on Weights and Measures. A unique system called “Systeme internationale d"unites” (abbreviated SI) was adopted by the decision of this honorable meeting. In the Russian version, this system is called the International System (abbreviation SI).
The basis is 7 main units and 2 additional ones. Their numerical value is determined in the form of a standard
Table of physical quantities SI
Name of main unit | Measured quantity | Designation |
|
International | Russian |
||
Basic units |
|||
kilogram | |||
Current strength | |||
Temperature | |||
Quantity of substance | |||
The power of light | |||
Additional units |
|||
Flat angle | |||
Steradian | Solid angle | ||
The system itself cannot consist of only seven units, since the variety of physical processes in nature requires the introduction of more and more new quantities. The structure itself provides not only for the introduction of new units, but also for their interrelation in the form of mathematical relationships (they are more often called dimensional formulas).
A unit of physical quantity is obtained using multiplication and division of the basic units in the dimensional formula. The absence of numerical coefficients in such equations makes the system not only convenient in all respects, but also coherent (consistent).
Derived units
The units of measurement that are formed from the seven basic ones are called derivatives. In addition to the basic and derivative units, there was a need to introduce additional ones (radians and steradians). Their dimension is considered to be zero. The lack of measuring instruments to determine them makes it impossible to measure them. Their introduction is due to their use in theoretical research. For example, the physical quantity “force” in this system is measured in newtons. Since force is a measure of the mutual action of bodies on each other, which is the reason for the variation in the speed of a body of a certain mass, it can be defined as the product of a unit of mass by a unit of speed divided by a unit of time:
F = k٠M٠v/T, where k is the proportionality coefficient, M is the unit of mass, v is the unit of speed, T is the unit of time.
SI gives the following formula for dimensions: H = kg٠m/s 2, where three units are used. And the kilogram, and the meter, and the second are classified as basic. The proportionality factor is 1.
It is possible to introduce dimensionless quantities, which are defined as a ratio of homogeneous quantities. These include, as is known, equal to the ratio of the friction force to the normal pressure force.
Table of physical quantities derived from basic ones
Unit name | Measured quantity | Dimensional formula |
kg٠m 2 ٠s -2 |
||
pressure | kg٠ m -1 ٠s -2 |
|
magnetic induction | kg ٠А -1 ٠с -2 |
|
electrical voltage | kg ٠m 2 ٠s -3 ٠A -1 |
|
Electrical resistance | kg ٠m 2 ٠s -3 ٠A -2 |
|
Electric charge | ||
power | kg ٠m 2 ٠s -3 |
|
Electrical capacity | m -2 ٠kg -1 ٠c 4 ٠A 2 |
|
Joule to Kelvin | Heat capacity | kg ٠m 2 ٠s -2 ٠К -1 |
Becquerel | Activity of a radioactive substance | |
Magnetic flux | m 2 ٠kg ٠s -2 ٠A -1 |
|
Inductance | m 2 ٠kg ٠s -2 ٠A -2 |
|
Absorbed dose | ||
Equivalent radiation dose | ||
Illumination | m -2 ٠kd ٠av -2 |
|
Luminous flux | ||
Strength, weight | m ٠kg ٠s -2 |
|
Electrical conductivity | m -2 ٠kg -1 ٠s 3 ٠A 2 |
|
Electrical capacity | m -2 ٠kg -1 ٠c 4 ٠A 2 |
Non-system units
The use of historically established quantities that are not included in the SI or differ only by a numerical coefficient is allowed when measuring quantities. These are non-systemic units. For example, mm of mercury, x-ray and others.
Numerical coefficients are used to introduce submultiples and multiples. Prefixes correspond to a specific number. Examples include centi-, kilo-, deca-, mega- and many others.
1 kilometer = 1000 meters,
1 centimeter = 0.01 meters.
Typology of quantities
We will try to indicate several basic features that allow us to establish the type of value.
1. Direction. If the action of a physical quantity is directly related to the direction, it is called vector, others - scalar.
2. Availability of dimension. The existence of a formula for physical quantities makes it possible to call them dimensional. If all units in a formula have a zero degree, then they are called dimensionless. It would be more correct to call them quantities with a dimension equal to 1. After all, the concept of a dimensionless quantity is illogical. The main property - dimension - has not been canceled!
3. If possible, addition. An additive quantity, the value of which can be added, subtracted, multiplied by a coefficient, etc. (for example, mass) is a physical quantity that is summable.
4. In relation to the physical system. Extensive - if its value can be compiled from the values of the subsystem. An example would be area measured in square meters. Intensive - a quantity whose value does not depend on the system. These include temperature.
Physical quantity
Physical quantity - physical property material object, physical phenomenon, process that can be characterized quantitatively.
Physical quantity value- one or more (in the case of a tensor physical quantity) numbers characterizing this physical quantity, indicating the unit of measurement on the basis of which they were obtained.
Size of physical quantity- the meanings of the numbers appearing in physical quantity value.
For example, a car can be characterized by physical quantity, like a mass. At the same time, meaning of this physical quantity will be, for example, 1 ton, and size- number 1, or meaning will be 1000 kilograms, and size- number 1000. The same car can be characterized using another physical quantity- speed. At the same time, meaning of this physical quantity will be, for example, a vector of a certain direction of 100 km/h, and size- number 100.
Dimension of a physical quantity- unit of measurement appearing in physical quantity value. As a rule, a physical quantity has many different dimensions: for example, length - nanometer, millimeter, centimeter, meter, kilometer, mile, inch, parsec, light year, etc. Some of these units of measurement (without taking into account their decimal factors) can enter into various systems physical units - SI, GHS, etc.
Often a physical quantity can be expressed in terms of other, more fundamental physical quantities. (For example, force can be expressed in terms of the mass of a body and its acceleration.) Which means accordingly, the dimension such a physical quantity can be expressed through the dimensions of these more general quantities. (The dimension of force can be expressed in terms of the dimensions of mass and acceleration.) (Often, such a representation of the dimension of a certain physical quantity through the dimensions of other physical quantities is an independent task, which in some cases has its own meaning and purpose.) The dimensions of such more general quantities are often already basic units one or another system of physical units, that is, those that themselves are no longer expressed through others, even more general quantities.
Example.
If the physical quantity power is written as
W- this is an abbreviation one of units of measurement of this physical quantity (watt). Litera To is the International System of Units (SI) designation for the decimal factor "kilo".
Dimensional and dimensionless physical quantities
- Dimensional physical quantity- a physical quantity, to determine the value of which it is necessary to apply some unit of measurement of this physical quantity. The vast majority of physical quantities are dimensional.
- Dimensionless physical quantity- a physical quantity, to determine the value of which it is enough to indicate its size. For example, relative dielectric constant is a dimensionless physical quantity.
Additive and non-additive physical quantities
- Additive physical quantity- physical quantity, different meanings which can be summed, multiplied by a numerical coefficient, divided by each other. For example, the physical quantity mass is an additive physical quantity.
- Non-additive physical quantity- a physical quantity for which summing, multiplying by a numerical coefficient or dividing its values by each other has no physical meaning. For example, the physical quantity temperature is a non-additive physical quantity.
Extensive and intensive physical quantities
The physical quantity is called
- extensive, if the magnitude of its value is the sum of the values of this physical quantity for the subsystems that make up the system (for example, volume, weight);
- intensive, if the magnitude of its value does not depend on the size of the system (for example, temperature, pressure).
Some physical quantities, such as angular momentum, area, force, length, time, are neither extensive nor intensive.
Derived quantities are formed from some extensive quantities:
- specific quantity is a quantity divided by mass (for example, specific volume);
- molar quantity is a quantity divided by the amount of substance (for example, molar volume).
Scalar, vector, tensor quantities
In the most general case we can say that a physical quantity can be represented by a tensor of a certain rank (valence).
System of units of physical quantities
A system of units of physical quantities is a set of units of measurement of physical quantities, in which there is a certain number of so-called basic units of measurement, and the remaining units of measurement can be expressed through these basic units. Examples of systems of physical units are the International System of Units (SI), GHS.
Symbols of physical quantities
Literature
- RMG 29-99 Metrology. Basic terms and definitions.
- Burdun G. D., Bazakutsa V. A. Units of physical quantities. - Kharkov: Vishcha school, .
Measurements are based on comparison of identical properties of material objects. For properties for which physical methods are used for quantitative comparison, metrology has established a single generalized concept - a physical quantity. Physical quantity- a property that is qualitatively common to many physical objects, but quantitatively individual for each object, for example, length, mass, electrical conductivity and heat capacity of bodies, gas pressure in a vessel, etc. But smell is not a physical quantity, since it is established using subjective sensations.
A measure for quantitative comparison of identical properties of objects is unit of physical quantity - physical quantity to which by agreement is assigned numeric value, equal to 1. Units of physical quantities are assigned a full and abbreviated symbolic designation - dimension. For example, mass - kilogram (kg), time - second (s), length - meter (m), force - Newton (N).
The value of a physical quantity is assessment of a physical quantity in the form of a certain number of units accepted for it characterizes the quantitative individuality of objects. For example, the diameter of the hole is 0.5 mm, the radius of the globe is 6378 km, the speed of the runner is 8 m/s, the speed of light is 3 10 5 m/s.
By measuring is called finding the value of a physical quantity using special technical means. For example, measuring the shaft diameter with a caliper or micrometer, liquid temperature with a thermometer, gas pressure with a pressure gauge or vacuum gauge. Physical quantity value x^, obtained during measurement is determined by the formula x^ = ai, Where A- numerical value (size) of a physical quantity; and is a unit of physical quantity.
Since the values of physical quantities are found experimentally, they contain measurement error. In this regard, a distinction is made between true and actual values of physical quantities. True meaning - the value of a physical quantity that ideally reflects the corresponding property of an object in qualitative and quantitative terms. It is the limit to which the value of a physical quantity approaches with increasing measurement accuracy.
Real value - a value of a physical quantity found experimentally that is so close to the true value that it can be used instead for a certain purpose. This value varies depending on the required measurement accuracy. In technical measurements, the value of a physical quantity found with an acceptable error is accepted as the actual value.
Measurement error is the deviation of the measurement result from the true value of the measured value. Absolute error called the measurement error expressed in units of the measured value: Oh = x^- x, Where X- the true value of the measured quantity. Relative error - the ratio of the absolute measurement error to the true value of a physical quantity: 6=Ax/x. The relative error can also be expressed as a percentage.
Since the true value of the measurement remains unknown, in practice only an approximate estimate of the measurement error can be found. In this case, instead of the true value, the actual value of a physical quantity is taken, obtained by measuring the same quantity with a higher accuracy. For example, the error in measuring linear dimensions with a caliper is ±0.1 mm, and with a micrometer - ± 0.004 mm.
The measurement accuracy can be expressed quantitatively as the reciprocal of the relative error modulus. For example, if the measurement error is ±0.01, then the measurement accuracy is 100.
Physical quantity - a property of physical objects that is qualitatively common to many objects, but quantitatively individual for each of them. The qualitative side of the concept of “physical quantity” determines its type (for example, electrical resistance as a general property of electrical conductors), and the quantitative side determines its “size” (value electrical resistance specific conductor, for example R = 100 Ohm). The numerical value of the measurement result depends on the choice of unit of physical quantity.
Physical quantities are assigned alphabetic symbols used in physical equations expressing relationships between physical quantities that exist in physical objects.
Size of physical quantity - quantitative determination of a value inherent in a specific object, system, phenomenon or process.
Physical quantity value- assessment of the size of a physical quantity in the form of a certain number of units of measurement accepted for it. Numerical value of a physical quantity- an abstract number expressing the ratio of the value of a physical quantity to the corresponding unit of a given physical quantity (for example, 220 V is the value of the voltage amplitude, and the number 220 itself is a numerical value). It is the term “value” that should be used to express the quantitative side of the property under consideration. It is incorrect to say and write “current value”, “voltage value”, etc., since current and voltage are themselves quantities (the correct use of the terms “current value”, “voltage value”).
With a selected assessment of a physical quantity, it is characterized by true, actual and measured values.
The true value of a physical quantity They call the value of a physical quantity that would ideally reflect the corresponding property of an object in qualitative and quantitative terms. It is impossible to determine it experimentally due to inevitable measurement errors.
This concept is based on two main postulates of metrology:
§ the true value of the quantity being determined exists and is constant;
§ the true value of the measured quantity cannot be found.
In practice, they operate with the concept of a real value, the degree of approximation of which to the true value depends on the accuracy of the measuring instrument and the error of the measurements themselves.
The actual value of a physical quantity they call it a value found experimentally and so close to the true value that for a certain purpose it can be used instead.
Under measured value understand the value of the quantity measured by the indicator device of the measuring instrument.
Unit of physical quantity - a fixed-size value, which is conventionally assigned a standard numerical value equal to one.
Units of physical quantities are divided into basic and derivative and combined into systems of units of physical quantities. The unit of measurement is established for each of the physical quantities, taking into account the fact that many quantities are interconnected by certain dependencies. Therefore, only some of the physical quantities and their units are determined independently of the others. Such quantities are called main. Other physical quantities - derivatives and they are found using physical laws and dependencies through the basic ones. A set of basic and derived units of physical quantities, formed in accordance with accepted principles, is called system of units of physical quantities. The unit of a basic physical quantity is basic unit systems.
International system units (SI system; SI - French. Systeme International) was adopted by the XI General Conference on Weights and Measures in 1960.
The SI system is based on seven basic and two additional physical units. Basic units: meter, kilogram, second, ampere, kelvin, mole and candela (Table 1).
Table 1. International SI units
|
Name |
Dimension |
Name |
Designation |
|
|
international |
||||
|
Basic |
||||
|
kilogram |
||||
|
Electric current strength |
||||
|
Temperature |
||||
|
Quantity of substance |
||||
|
The power of light |
||||
|
Additional |
||||
|
Flat angle |
||||
|
Solid angle |
steradian |
Meter equal to the distance traveled by light in a vacuum in 1/299792458 of a second.
Kilogram- a unit of mass defined as the mass of the international prototype kilogram, representing a cylinder made of an alloy of platinum and iridium.
Second is equal to 9192631770 periods of radiation corresponding to the energy transition between two levels of the hyperfine structure of the ground state of the cesium-133 atom.
Ampere- the strength of a constant current, which, passing through two parallel straight conductors of infinite length and negligibly small circular cross-sectional area, located at a distance of 1 m from each other in a vacuum, would cause an interaction force equal to 210 -7 N (newton) on each section of the conductor 1 m long.
Kelvin- a unit of thermodynamic temperature equal to 1/273.16 of the thermodynamic temperature of the triple point of water, i.e., the temperature at which the three phases of water - vapor, liquid and solid - are in dynamic equilibrium.
Mole- the amount of substance containing as many structural elements as are contained in carbon-12 weighing 0.012 kg.
Candela- the intensity of light in a given direction of a source emitting monochromatic radiation with a frequency of 54010 12 Hz (wavelength about 0.555 microns), whose energy radiation intensity in this direction is 1/683 W/sr (sr - steradian).
Additional units SI systems are intended only to form units of angular velocity and angular acceleration. Additional physical quantities of the SI system include plane and solid angles.
Radian (glad) - the angle between two radii of a circle whose arc length is equal to this radius. In practical cases, the following units of measurement of angular quantities are often used:
degree - 1 _ = 2p/360 rad = 1.745310 -2 rad;
minute - 1" = 1 _ /60 = 2.9088 10 -4 rad;
second - 1"= 1"/60= 1 _ /3600 = 4.848110 -6 rad;
radian - 1 rad = 57 _ 17 "45" = 57.2961 _ = (3.4378 10 3)" = (2.062710 5)".
Steradian (Wed) - a solid angle with a vertex at the center of the sphere, cutting out an area on its surface equal to the area of a square with a side equal to the radius of the sphere.
Measure solid angles using plane angles and calculation
Where b- solid angle; ts- a plane angle at the vertex of a cone formed inside a sphere by a given solid angle.
Derived units of the SI system are formed from basic and supplementary units.
In the field of measuring electrical and magnetic quantities, there is one basic unit - ampere (A). Through the ampere and the unit of power - watt (W), common for electrical, magnetic, mechanical and thermal quantities, all other electrical and magnetic units can be determined. However, today there are no sufficiently accurate means of reproducing watts using absolute methods. Therefore, electrical and magnetic units are based on units of current and the ampere-derived unit of capacitance, the farad.
Physical quantities derived from ampere also include:
§ unit of electromotive force (EMF) and electrical voltage- volt (V);
§ unit of frequency - hertz (Hz);
§ unit of electrical resistance - ohm (Ohm);
§ unit of inductance and mutual inductance of two coils - henry (H).
In table 2 and 3 show the derived units most used in telecommunication systems and radio engineering.
Table 2. Derived SI units
|
Magnitude |
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|
Name |
Dimension |
Name |
Designation |
|
|
international |
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|
Energy, work, amount of heat |
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Strength, weight |
||||
|
Power, energy flow |
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|
Amount of electricity |
||||
|
Electrical voltage, electromotive force (EMF), potential |
||||
|
Electrical capacity |
L -2 M -1 T 4 I 2 |
|||
|
Electrical resistance |
||||
|
Electrical conductivity |
L -2 M -1 T 3 I 2 |
|||
|
Magnetic induction |
||||
|
Magnetic induction flux |
||||
|
Inductance, mutual inductance |
Table 3. SI units used in measurement practice
|
Magnitude |
||||
|
Name |
Dimension |
Unit of measurement |
Designation |
|
|
international |
||||
|
Electric current density |
ampere per square meter |
|||
|
Electric field strength |
volt per meter |
|||
|
Absolute dielectric constant |
L 3 M -1 T 4 I 2 |
farad per meter |
||
|
Electrical resistivity |
ohm per meter |
|||
|
Total power of the electrical circuit |
volt-ampere |
|||
|
Reactive power of an electrical circuit |
||||
|
Magnetic field strength |
ampere per meter |
Abbreviations for units, both international and Russian, named after great scientists, are written with capital letters, for example ampere - A; om - Om; volt - V; farad - F. For comparison: meter - m, second - s, kilogram - kg.
In practice, the use of whole units is not always convenient, since very large or very small values are obtained as a result of measurements. Therefore, the SI system has its decimal multiples and submultiples, which are formed using multipliers. Multiple and submultiple units of quantities are written together with the name of the main or derived unit: kilometer (km), millivolt (mV); megaohm (MΩ).
Multiple unit of physical quantity- a unit greater than an integer number of times the system one, for example kilohertz (10 3 Hz). Submultiple unit of physical quantity- a unit that is an integer times smaller than the system one, for example a microhenry (10 -6 H).
The names of multiple and submultiple units of the SI system contain a number of prefixes corresponding to the factors (Table 4).
Table 4. Factors and prefixes for the formation of decimal multiples and submultiples of SI units
|
Factor |
Prefix |
Prefix designation |
|
|
international |
|||
What does it mean to measure a physical quantity? What is a unit of physical quantity called? Here you will find answers to these very important questions.
1. Let's find out what is called a physical quantity
For a long time, people have used their characteristics to more accurately describe certain events, phenomena, properties of bodies and substances. For example, when comparing the bodies that surround us, we say that the book is smaller than bookshelf, and the horse is larger than the cat. This means that the volume of the horse is greater than the volume of the cat, and the volume of the book is less than the volume of the cabinet.
Volume is an example of a physical quantity that characterizes the general property of bodies to occupy one or another part of space (Fig. 1.15, a). In this case, the numerical value of the volume of each of the bodies is individual.
Rice. 1.15 To characterize the property of bodies to occupy one or another part of space, we use the physical quantity volume (o, b), to characterize movement - speed (b, c)
A general characteristic of many material objects or phenomena, which can acquire individual meaning for each of them, is called physical quantity.
Another example of a physical quantity is the familiar concept of “speed”. All moving bodies change their position in space over time, but the speed of this change is different for each body (Fig. 1.15, b, c). Thus, in one flight, an airplane manages to change its position in space by 250 m, a car by 25 m, a person by I m, and a turtle by only a few centimeters. That's why physicists say that speed is a physical quantity that characterizes the speed of movement.
It is not difficult to guess that volume and speed are not all the physical quantities that physics operates with. Mass, density, force, temperature, pressure, voltage, illumination - this is only a small part of the physical quantities that you will become familiar with while studying physics.
2. Find out what it means to measure a physical quantity
In order to quantitatively describe the properties of any material object or physical phenomenon, it is necessary to establish the value of the physical quantity that characterizes this object or phenomenon.
The value of physical quantities is obtained by measurements (Fig. 1.16-1.19) or calculations.
Rice. 1.16. “There are 5 minutes left before the train departs,” you measure the time with excitement.
Rice. 1.17 “I bought a kilogram of apples,” says mom about her mass measurements
Rice. 1.18. “Dress warmly, it’s cooler outside today,” your grandmother says after measuring the air temperature outside.
Rice. 1.19. “My blood pressure has risen again,” a woman complains after measuring her blood pressure.
To measure a physical quantity means to compare it with a homogeneous quantity taken as a unit. 
Rice. 1.20 If a grandmother and grandson measure distance in steps, they will always get different results
Let's give an example from fiction: “After walking three hundred paces along the river bank, the small detachment entered the arches of a dense forest, along the winding paths of which they had to wander for ten days.” (J. Verne “The Fifteen-Year-Old Captain”)
Rice. 1.21.
The heroes of the novel by J. Verne measured the distance traveled, comparing it with the step, that is, the unit of measurement was the step. There were three hundred such steps. As a result of the measurement, a numerical value (three hundred) of a physical quantity (path) in selected units (steps) was obtained.
Obviously, the choice of such a unit does not allow comparing the measurement results obtained different people, since everyone’s step length is different (Fig. 1.20). Therefore, for the sake of convenience and accuracy, people long ago began to agree to measure the same physical quantity with the same units. Nowadays, in most countries of the world, the International System of Units of Measurement, adopted in 1960, is in force, which is called the “System International” (SI) (Fig. 1.21).
In this system, the unit of length is the meter (m), time - the second (s); Volume is measured in cubic meters (m3), and speed is measured in meters per second (m/s). You will learn about other SI units later.
3. Remember multiples and submultiples
From your mathematics course, you know that to shorten the notation of large and small values of different quantities, multiple and submultiple units are used.
Multiples are units that are 10, 100, 1000 or more times larger than the base units. Sub-multiple units are units that are 10, 100, 1000 or more times smaller than the main ones.
Prefixes are used to write multiples and submultiples. For example, units of length that are multiples of one meter are a kilometer (1000 m), a decameter (10 m).
Units of length subordinate to one meter are decimeter (0.1 m), centimeter (0.01 m), micrometer (0.000001 m) and so on.
The table shows the most commonly used prefixes.
4. Getting to know the measuring instruments
Scientists measure physical quantities using measuring instruments. The simplest of them - a ruler, a tape measure - are used to measure distance and linear dimensions of the body. You are also well aware of these measuring instruments, like a watch - a device for measuring time, a protractor - a device for measuring angles on a plane, a thermometer - a device for measuring temperature and some others (Fig. 1.22, p. 20). You still have to get acquainted with many measuring instruments.
Most measuring instruments have a scale that allows for measurement. In addition to the scale, the device indicates the units in which the value measured by this device is expressed*.
Using the scale, you can set the two most important characteristics of the device: measurement limits and division value.
Measurement limits- these are the largest and smallest values of a physical quantity that can be measured by this device.
Nowadays, electronic measuring instruments are widely used, in which the value of the measured quantities is displayed on the screen in the form of numbers. Measurement limits and units are determined from the device passport or are set with a special switch on the device panel.
Rice. 1.22. Measuring instruments
Division price- this is the value of the smallest scale division of the measuring device.
For example, the upper measurement limit of a medical thermometer (Fig. 1.23) is 42 °C, the lower one is 34 °C, and the scale division of this thermometer is 0.1 °C.
We remind you: to determine the price of a scale division of any device, you need to divide the difference between any two values indicated on the scale by the number of divisions between them.
Rice. 1.23. Medical thermometer
- Let's sum it up
A general characteristic of material objects or phenomena, which can acquire individual meaning for each of them, is called a physical quantity.
To measure a physical quantity means to compare it with a homogeneous quantity taken as a unit.
As a result of measurements, we obtain the value of physical quantities.
When talking about the value of a physical quantity, you should indicate its numerical value and unit.
Measuring instruments are used to measure physical quantities.
To reduce the recording of numerical values of large and small physical quantities, multiple and submultiple units are used. They are formed using prefixes.
- Security questions
1. Define a physical quantity. How do you understand it?
2. What does it mean to measure a physical quantity?
3. What is meant by the value of a physical quantity?
4. Name all the physical quantities mentioned in the excerpt from J. Verne’s novel given in the text of the paragraph. What is their numerical value? units of measurement?
5. What prefixes are used to form submultiple units? multiple units?
6. What characteristics of the device can be set using the scale?
7. What is the division price called?
- Exercises
1. Name the physical quantities known to you. Specify the units of these quantities. What instruments are used to measure them?
2. In Fig. Figure 1.22 shows some measuring instruments. Is it possible, using only a drawing, to determine the price of division of the scales of these instruments? Justify your answer.
3. Express the following physical quantities in meters: 145 mm; 1.5 km; 2 km 32 m.
4. Write down the following values of physical quantities using multiples or submultiples: 0.0000075 m - diameter of red blood cells; 5,900,000,000,000 m - the radius of the orbit of the planet Pluto; 6,400,000 m is the radius of planet Earth.
5 Determine the measurement limits and the price of division of the scales of the instruments that you have at home.
6. Remember the definition of a physical quantity and prove that length is a physical quantity.
- Physics and technology in Ukraine
One of the outstanding physicists of our time - Lev Davidovich Landau (1908-1968) - demonstrated his abilities while still studying at high school. After graduating from university, he interned with one of the creators quantum physics Niels Bohr. Already at the age of 25, he headed the theoretical department of the Ukrainian Institute of Physics and Technology and the department of theoretical physics at Kharkov University. Like most outstanding theoretical physicists, Landau had an extraordinary breadth of scientific interests. Nuclear physics, plasma physics, the theory of superfluidity of liquid helium, the theory of superconductivity - Landau made significant contributions to all these areas of physics. For work in physics low temperatures he was awarded the Nobel Prize.
Physics. 7th grade: Textbook / F. Ya. Bozhinova, N. M. Kiryukhin, E. A. Kiryukhina. - X.: Publishing house "Ranok", 2007. - 192 p.: ill.
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