Physical quantities and designations. Characteristics of electric current. How is power defined in current physics. Physical quantities of electric current

The study of physics at school lasts several years. At the same time, students are faced with the problem that the same letters denote completely different quantities. Most often this fact concerns Latin letters. How then to solve problems?

There is no need to be afraid of such a repetition. Scientists tried to introduce them into the designation so that the same letters did not meet in one formula. Most often, students come across the Latin n. It can be lowercase or uppercase. Therefore, the question logically arises as to what n is in physics, that is, in a certain formula that the student encountered.

What does the capital letter N stand for in physics?

Most often in school course it occurs in the study of mechanics. After all, there it can be immediately in spirit values - the power and strength of the normal reaction of the support. Naturally, these concepts do not intersect, because they are used in different sections of mechanics and are measured in different units. Therefore, it is always necessary to define exactly what n is in physics.

Power is the rate of change in the energy of a system. It is a scalar value, that is, just a number. Its unit of measurement is the watt (W).

The force of the normal reaction of the support is the force that acts on the body from the side of the support or suspension. In addition to a numerical value, it has a direction, that is, it is a vector quantity. Moreover, it is always perpendicular to the surface on which the external action is performed. The unit of this N is the newton (N).

What is N in physics, in addition to the quantities already indicated? It could be:

the Avogadro constant;

magnification of the optical device;

substance concentration;

Debye number;

total radiation power.

What can a lowercase n stand for in physics?

The list of names that can be hidden behind it is quite extensive. The designation n in physics is used for such concepts:

refractive index, and it can be absolute or relative;

neutron - a neutral elementary particle with a mass slightly greater than that of a proton;

frequency of rotation (used to replace the Greek letter "nu", as it is very similar to the Latin "ve") - the number of repetitions of revolutions per unit of time, measured in hertz (Hz).

What does n mean in physics, besides the already indicated values? It turns out that the main quantum number is hidden behind it ( the quantum physics), concentration and Loschmidt constant (molecular physics). By the way, when calculating the concentration of a substance, you need to know the value, which is also written in the Latin "en". It will be discussed below.

What physical quantity can be denoted by n and N?

Its name comes from Latin word numerus, in translation it sounds like "number", "quantity". Therefore, the answer to the question of what n means in physics is quite simple. This is the number of any objects, bodies, particles - everything that is discussed in a particular task.

Moreover, “quantity” is one of the few physical quantities that do not have a unit of measure. It's just a number, no name. For example, if the problem is about 10 particles, then n will be equal to just 10. But if it turns out that the lowercase “en” is already taken, then you have to use an uppercase letter.

Formulas that use an uppercase N

The first of them defines the power, which is equal to the ratio of work to time:

IN molecular physics There is such a thing as the chemical quantity of a substance. Denoted by the Greek letter "nu". To calculate it, you should divide the number of particles by the Avogadro number:

By the way, the last value is also denoted by the so popular letter N. Only it always has a subscript - A.

To determine the electric charge, you need the formula:

Another formula with N in physics - oscillation frequency. To calculate it, you need to divide their number by the time:

The letter "en" appears in the formula for the circulation period:

Formulas that use a lowercase n

In a school physics course, this letter is most often associated with the refractive index of matter. Therefore, it is important to know the formulas with its application.

So, for the absolute refractive index, the formula is written as follows:

Here c is the speed of light in vacuum, v is its speed in a refracting medium.

The formula for the relative refractive index is somewhat more complicated:

n 21 \u003d v 1: v 2 \u003d n 2: n 1,

where n 1 and n 2 are the absolute refractive indices of the first and second medium, v 1 and v 2 are the speeds of the light wave in these substances.

How to find n in physics? The formula will help us with this, in which we need to know the angles of incidence and refraction of the beam, that is, n 21 \u003d sin α: sin γ.

What is n equal to in physics if it is the index of refraction?

Typically, tables give values for the absolute refractive indices of various substances. Do not forget that this value depends not only on the properties of the medium, but also on the wavelength. Tabular values of the refractive index are given for the optical range.

So, it became clear what n is in physics. To avoid any questions, it is worth considering some examples.

Power Challenge

№1. During plowing, the tractor pulls the plow evenly. In doing so, it applies a force of 10 kN. With this movement for 10 minutes, he overcomes 1.2 km. It is required to determine the power developed by it.

Convert units to SI. You can start with force, 10 N equals 10,000 N. Then the distance: 1.2 × 1000 = 1200 m. The time left is 10 × 60 = 600 s.

Choice of formulas. As mentioned above, N = A: t. But in the task there is no value for work. To calculate it, another formula is useful: A \u003d F × S. The final form of the formula for power looks like this: N \u003d (F × S): t.

Solution. We calculate first the work, and then the power. Then in the first action you get 10,000 × 1,200 = 12,000,000 J. The second action gives 12,000,000: 600 = 20,000 W.

Answer. Tractor power is 20,000 watts.

Tasks for the refractive index

№2. The absolute refractive index of glass is 1.5. The speed of light propagation in glass is less than in vacuum. It is required to determine how many times.

There is no need to convert data to SI.

When choosing formulas, you need to stop at this one: n \u003d c: v.

Solution. It can be seen from this formula that v = c: n. This means that the speed of light in glass is equal to the speed of light in vacuum divided by the refractive index. That is, it is reduced by half.

Answer. The speed of light propagation in glass is 1.5 times less than in vacuum.

№3. There are two transparent media. The speed of light in the first of them is 225,000 km / s, in the second - 25,000 km / s less. A ray of light goes from the first medium to the second. The angle of incidence α is 30º. Calculate the value of the angle of refraction.

Do I need to convert to SI? Speeds are given in off-system units. However, when substituting into formulas, they will be reduced. Therefore, it is not necessary to convert speeds to m/s.

The choice of formulas needed to solve the problem. You will need to use the law of light refraction: n 21 \u003d sin α: sin γ. And also: n = c: v.

Solution. In the first formula, n 21 is the ratio of the two refractive indices of the substances under consideration, that is, n 2 and n 1. If we write down the second indicated formula for the proposed environments, then we get the following: n 1 = c: v 1 and n 2 = c: v 2. If you make the ratio of the last two expressions, it turns out that n 21 \u003d v 1: v 2. Substituting it into the formula for the law of refraction, we can derive the following expression for the sine of the angle of refraction: sin γ \u003d sin α × (v 2: v 1).

We substitute the values of the indicated velocities and the sine of 30º (equal to 0.5) into the formula, it turns out that the sine of the angle of refraction is 0.44. According to the Bradis table, it turns out that the angle γ is 26º.

Answer. The value of the angle of refraction is 26º.

Tasks for the period of circulation

№4. The blades of a windmill rotate with a period of 5 seconds. Calculate the number of revolutions of these blades in 1 hour.

To convert to SI units, only the time is 1 hour. It will be equal to 3600 seconds.

Selection of formulas. The period of rotation and the number of revolutions are related by the formula T \u003d t: N.

Solution. From this formula, the number of revolutions is determined by the ratio of time to period. Thus, N = 3600: 5 = 720.

Answer. The number of revolutions of the mill blades is 720.

№5. The aircraft propeller rotates at a frequency of 25 Hz. How long does it take the screw to complete 3,000 revolutions?

All data is given with SI, so nothing needs to be translated.

Required Formula: frequency ν = N: t. From it it is only necessary to derive a formula for the unknown time. It is a divisor, so it is supposed to be found by dividing N by ν.

Solution. Dividing 3,000 by 25 results in the number 120. It will be measured in seconds.

Answer. An airplane propeller makes 3000 revolutions in 120 s.

Summing up

When a student encounters a formula containing n or N in a physics problem, he needs to deal with two things. The first is from which section of physics the equality is given. This may be clear from the heading in a textbook, reference book, or the teacher's words. Then you should decide what is hidden behind the many-sided "en". Moreover, the name of the units of measurement helps in this, if, of course, its value is given. Another option is also allowed: carefully look at the rest of the letters in the formula. Perhaps they will be familiar and will give a hint in the issue being resolved.

Building drawings is not an easy task, but without it in modern world no way. After all, in order to make even the most ordinary object (a tiny bolt or nut, a book shelf, the design of a new dress, and the like), you first need to make the appropriate calculations and draw a drawing of the future product. However, it is often made by one person, and another is engaged in the manufacture of something according to this scheme.

In order to avoid confusion in understanding the depicted object and its parameters, it is accepted all over the world conventions length, width, height and other quantities used in the design. What are they? Let's find out.

Quantities

Area, height and other designations of a similar nature are not only physical, but also mathematical quantities.

Their single letter designation (used by all countries) was established in the middle of the twentieth century by the International System of Units (SI) and is used to this day. It is for this reason that all such parameters are indicated in Latin, and not in Cyrillic letters or Arabic script. In order not to create separate difficulties, when developing standards for design documentation in most modern countries, it was decided to use almost the same symbols that are used in physics or geometry.

Any school graduate remembers that depending on whether a two-dimensional or three-dimensional figure (product) is shown in the drawing, it has a set of basic parameters. If there are two dimensions - this is the width and length, if there are three - the height is also added.

So, for starters, let's find out how to correctly indicate the length, width, height in the drawings.

Width

As mentioned above, in mathematics, the quantity under consideration is one of the three spatial dimensions of any object, provided that its measurements are made in the transverse direction. So what is the famous width? It is designated with the letter "B". This is known all over the world. Moreover, according to GOST, the use of both capital and lowercase Latin letters is permissible. The question often arises as to why such a letter was chosen. After all, usually the reduction is made according to the first Greek or English name quantities. In this case, the width in English will look like "width".

Probably, the point here is that this parameter was originally most widely used in geometry. In this science, describing figures, often the length, width, height are denoted by the letters "a", "b", "c". According to this tradition, when choosing, the letter "B" (or "b") was borrowed by the SI system (although non-geometric symbols began to be used for the other two dimensions).

Most believe that this was done in order not to confuse the width (designated by the letter "B" / "b") with the weight. The fact is that the latter is sometimes referred to as "W" (short for the English name weight), although the use of other letters ("G" and "P") is also acceptable. According to the international standards of the SI system, the width is measured in meters or multiples (longitudinal) of their units. It is worth noting that in geometry it is sometimes also acceptable to use "w" to denote width, but in physics and other exact sciences this notation is generally not used.

Length

As already mentioned, in mathematics, length, height, width are three spatial dimensions. Moreover, if the width is a linear dimension in the transverse direction, then the length is in the longitudinal direction. Considering it as a quantity of physics, one can understand that this word means a numerical characteristic of the length of lines.

IN English language this term is called length. It is because of this that this value is indicated by the capital or lowercase initial letter of this word - “L”. Like width, length is measured in meters or their multiples (longitudinal) units.

Height

The presence of this value indicates that one has to deal with a more complex - three-dimensional space. Unlike length and width, height quantifies the size of an object in the vertical direction.

In English, it is written as "height". Therefore, according to international standards, it is designated by the Latin letter "H" / "h". In addition to the height, in the drawings, sometimes this letter also acts as a depth designation. Height, width and length - all of these parameters are measured in meters and their multiples and submultiples (kilometers, centimeters, millimeters, etc.).

Radius and Diameter

In addition to the parameters considered, when drawing up drawings, one has to deal with others.

For example, when working with circles, it becomes necessary to determine their radius. This is the name of a segment that connects two points. The first one is the center. The second is located directly on the circle itself. In Latin, this word looks like "radius". Hence the lowercase or capital "R"/"r".

When drawing circles, in addition to the radius, one often has to deal with a phenomenon close to it - the diameter. It is also a line segment connecting two points on a circle. However, it must pass through the center.

Numerically, the diameter is equal to two radii. In English, this word is written like this: "diameter". Hence the abbreviation - a large or small Latin letter "D" / "d". Often the diameter in the drawings is indicated with a crossed out circle - “Ø”.

Although this is a common abbreviation, it should be borne in mind that GOST provides for the use of only the Latin "D" / "d".

Thickness

Most of us remember school lessons mathematics. Even then, teachers said that it was customary to designate such a quantity as area with the Latin letter “s”. However, according to generally accepted standards, a completely different parameter is recorded in the drawings in this way - thickness.

Why is that? It is known that in the case of height, width, length, the designation with letters could be explained by their spelling or tradition. That's just the thickness in English looks like "thickness", and in the Latin version - "crassities". It is also not clear why, unlike other quantities, the thickness can be denoted only by a lowercase letter. The "s" designation is also used to describe the thickness of pages, walls, ribs, and so on.

Perimeter and area

Unlike all the quantities listed above, the word "perimeter" did not come from Latin or English, but from Greek. It is derived from "περιμετρέο" ("to measure the circumference"). And today this term has retained its meaning (the total length of the borders of the figure). Subsequently, the word got into the English language ("perimeter") and was fixed in the SI system in the form of an abbreviation with the letter "P".

Area is a quantity showing a quantitative characteristic geometric figure, which has two dimensions (length and width). Unlike everything listed above, it is measured in square meters(as well as in submultiples and multiples of their units). As for the letter designation of the area, it differs in different areas. For example, in mathematics, this is the Latin letter “S”, familiar to everyone since childhood. Why so - there is no information.

Some people unknowingly think that this is due to English spelling the words "square". However, in it the mathematical area is "area", and "square" is the area in the architectural sense. By the way, it is worth remembering that "square" is the name of the geometric figure "square". So you should be careful when studying drawings in English. Due to the translation of "area" in some disciplines, the letter "A" is used as a designation. In rare cases, "F" is also used, but in physics given letter means a quantity called "strength" ("fortis").

Other common abbreviations

The designations of height, width, length, thickness, radius, diameter are the most used in drawing up drawings. However, there are other quantities that are also often present in them. For example, lowercase "t". In physics, this means "temperature", however, according to the GOST of the Unified System for Design Documentation, this letter is a pitch (of helical springs, and the like). However, it is not used when it comes to gears and threads.

The capital and lowercase letter "A" / "a" (according to all the same standards) in the drawings is used to indicate not the area, but the center-to-center and center-to-center distance. In addition to various values, in the drawings it is often necessary to designate angles of different sizes. To do this, it is customary to use lowercase letters Greek alphabet. The most used are "α", "β", "γ" and "δ". However, others can be used as well.

What standard defines the letter designation of length, width, height, area and other quantities?

As mentioned above, so that there is no misunderstanding when reading the drawing, representatives different peoples accepted common standards letter designation. In other words, if you are in doubt about the interpretation of a particular abbreviation, look at GOSTs. Thus, you will learn how to correctly indicate the height, width, length, diameter, radius, and so on.

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It's no secret that there are special designations for quantities in any science. Letter designations in physics prove that given science is no exception in terms of identifying quantities using special characters. There are a lot of basic quantities, as well as their derivatives, each of which has its own symbol. So, letter designations in physics are discussed in detail in this article.

Physics and basic physical quantities

Thanks to Aristotle, the word physics began to be used, since it was he who first used this term, which at that time was considered synonymous with the term philosophy. This is due to the generality of the object of study - the laws of the Universe, more specifically, how it functions. As you know, in the XVI-XVII centuries there was the first scientific revolution, it was thanks to her that physics was singled out as an independent science.

Mikhail Vasilyevich Lomonosov introduced the word physics into the Russian language through the publication of a textbook translated from German - the first textbook on physics in Russia.

So, physics is a branch of natural science devoted to the study general laws nature, as well as matter, its movement and structure. There are not so many basic physical quantities as it might seem at first glance - there are only 7 of them:

length,
weight,
time,
current,
temperature,
amount of substance
the power of light.

Of course, they have their own letter designations in physics. For example, the symbol m is chosen for mass, and T for temperature. Also, all quantities have their own unit of measurement: the intensity of light is candela (cd), and the unit of measurement for the amount of substance is the mole.

Derived physical quantities

There are much more derivative physical quantities than the main ones. There are 26 of them, and often some of them are attributed to the main ones.

So, area is a derivative of length, volume is also a derivative of length, speed is a derivative of time, length, and acceleration, in turn, characterizes the rate of change in speed. Impulse is expressed in terms of mass and velocity, force is the product of mass and acceleration, mechanical work depends on force and length, and energy is proportional to mass. Power, pressure, density, surface density, linear density, heat quantity, voltage, electrical resistance, magnetic flux, moment of inertia, moment of momentum, moment of force - they all depend on mass. Frequency, angular velocity, angular acceleration are inversely proportional to time, and electric charge is directly dependent on time. Angle and solid angle are derived quantities from length.

What is the symbol for stress in physics? Voltage, which is a scalar quantity, is denoted by the letter U. For speed, the symbol is v, for mechanical work- A, and for energy - E. Electric charge It is customary to denote the letter q, and the magnetic flux - Ф.

SI: general information

International system units (SI) is a system physical units, which is based on the International System of Quantities, including the names and designations of physical quantities. It was adopted by the General Conference on Weights and Measures. It is this system that regulates the letter designations in physics, as well as their dimension and units of measurement. For designation, letters of the Latin alphabet are used, in some cases - Greek. It is also possible to use special characters as a designation.

Conclusion

So, in any scientific discipline There are special notations for different kinds of quantities. Naturally, physics is no exception. There are a lot of letter designations: force, area, mass, acceleration, voltage, etc. They have their own designations. There is a special system called the International System of Units. It is believed that the basic units cannot be mathematically derived from others. Derived quantities are obtained by multiplying and dividing from the basic ones.

Cheat sheet with formulas in physics for the exam

and not only (may need 7, 8, 9, 10 and 11 classes).

For starters, a picture that can be printed in a compact form.

Mechanics

Pressure P=F/S
Density ρ=m/V
Pressure at the depth of the liquid P=ρ∙g∙h
Gravity Ft=mg
5. Archimedean force Fa=ρ w ∙g∙Vt
The equation of motion for uniformly accelerated motion

X=X0 + υ 0∙t+(a∙t 2)/2 S=( υ 2 -υ 0 2) /2а S=( υ +υ 0) ∙t /2

Velocity equation for uniformly accelerated motion υ =υ 0 +a∙t
Acceleration a=( υ -υ 0)/t
Circular speed υ =2πR/T
Centripetal acceleration a= υ 2/R
Relationship between period and frequency ν=1/T=ω/2π
Newton's II law F=ma
Hooke's law Fy=-kx
Law gravity F=G∙M∙m/R 2
The weight of a body moving with acceleration a P \u003d m (g + a)
The weight of a body moving with acceleration a ↓ P \u003d m (g-a)
Friction force Ffr=µN
Body momentum p=m υ
Force impulse Ft=∆p
Moment M=F∙ℓ
Potential energy of a body raised above the ground Ep=mgh
Potential energy of elastically deformed body Ep=kx 2 /2
Kinetic energy of the body Ek=m υ 2 /2
Work A=F∙S∙cosα
Power N=A/t=F∙ υ
Efficiency η=Ap/Az
Oscillation period of the mathematical pendulum T=2π√ℓ/g
Oscillation period spring pendulum T=2π √m/k
The equation of harmonic oscillations Х=Хmax∙cos ωt
Relationship of the wavelength, its speed and period λ= υ T

Molecular physics and thermodynamics

Amount of substance ν=N/ Na
Molar mass M=m/ν
Wed. kin. energy of monatomic gas molecules Ek=3/2∙kT
Basic equation of MKT P=nkT=1/3nm 0 υ 2
Gay-Lussac law (isobaric process) V/T =const
Charles' law (isochoric process) P/T =const
Relative humidity φ=P/P 0 ∙100%
Int. ideal energy. monatomic gas U=3/2∙M/µ∙RT
Gas work A=P∙ΔV
Boyle's law - Mariotte (isothermal process) PV=const
The amount of heat during heating Q \u003d Cm (T 2 -T 1)
The amount of heat during melting Q=λm
The amount of heat during vaporization Q=Lm
The amount of heat during fuel combustion Q=qm
The equation of state for an ideal gas is PV=m/M∙RT
First law of thermodynamics ΔU=A+Q
Efficiency of heat engines η= (Q 1 - Q 2) / Q 1
Ideal efficiency. engines (Carnot cycle) η \u003d (T 1 - T 2) / T 1

Electrostatics and electrodynamics - formulas in physics

Coulomb's law F=k∙q 1 ∙q 2 /R 2
tension electric field E=F/q
Email tension. field of a point charge E=k∙q/R 2
Surface charge density σ = q/S
Email tension. fields of the infinite plane E=2πkσ
Dielectric constant ε=E 0 /E
Potential energy of interaction. charges W= k∙q 1 q 2 /R
Potential φ=W/q
Point charge potential φ=k∙q/R
Voltage U=A/q
For a uniform electric field U=E∙d
Electric capacity C=q/U
Capacitance of a flat capacitor C=S∙ ε ∙ε 0/d
Energy of a charged capacitor W=qU/2=q²/2С=CU²/2
Current I=q/t
Conductor resistance R=ρ∙ℓ/S
Ohm's law for the circuit section I=U/R
The laws of the last compounds I 1 \u003d I 2 \u003d I, U 1 + U 2 \u003d U, R 1 + R 2 \u003d R
Parallel laws. conn. U 1 \u003d U 2 \u003d U, I 1 + I 2 \u003d I, 1 / R 1 + 1 / R 2 \u003d 1 / R
Power electric current P=I∙U
Joule-Lenz law Q=I 2 Rt
Ohm's law for a complete chain I=ε/(R+r)
Short circuit current (R=0) I=ε/r
Magnetic induction vector B=Fmax/ℓ∙I
Ampere Force Fa=IBℓsin α
Lorentz force Fл=Bqυsin α
Magnetic flux Ф=BSсos α Ф=LI
Law of electromagnetic induction Ei=ΔФ/Δt
EMF of induction in moving conductor Ei=Вℓ υ sinα
EMF of self-induction Esi=-L∙ΔI/Δt
Energy magnetic field coils Wm=LI 2 /2
Oscillation period count. contour T=2π ∙√LC
Inductive reactance X L =ωL=2πLν
Capacitance Xc=1/ωC
The current value of the current Id \u003d Imax / √2,
RMS voltage Ud=Umax/√2
Impedance Z=√(Xc-X L) 2 +R 2