What does n mean in physics 10. School curriculum: what is n in physics? Derived physical quantities

Studying physics at school lasts several years. At the same time, students are faced with the problem that the same letters represent 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 notation so that identical letters would not appear in the same formula. Most often, students encounter the Latin n. It can be lowercase or uppercase. Therefore, the question logically arises about what n is in physics, that is, in a certain formula encountered by the student.

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 meanings - the power and strength of a normal support reaction. Naturally, these concepts do not overlap, because they are used in different sections of mechanics and are measured in different units. Therefore, you always need to define exactly what n is in physics.

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

The normal ground reaction force is the force exerted on the body by the support or suspension. In addition to the 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 influence is made. The unit of this N is newton (N).

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

Avogadro's constant;

magnification of the optical device;

substance concentration;

Debye number;

total radiation power.

What does the lowercase letter n stand for in physics?

The list of names that may be hidden behind it is quite extensive. The notation n in physics is used for the following 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;

rotation frequency (used to replace the Greek letter "nu", since 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 quantities already indicated? It turns out that it hides the fundamental quantum number (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 with the Latin “en”. It will be discussed below.

What physical quantity can be denoted by n and N?

Its name comes from the Latin word numerus, translated as “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 certain task.

Moreover, “quantity” is one of the few physical quantities that do not have a unit of measurement. It's just a number, without a name. For example, if the problem involves 10 particles, then n will simply be equal to 10. But if it turns out that the lowercase “en” is already taken, then you have to use a capital letter.

Formulas containing capital N

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

In molecular physics there is such a thing as the chemical amount of a substance. Designated Greek letter"nude". To count it, you should divide the number of particles by Avogadro's 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 will need the formula:

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

The letter “en” appears in the formula for the circulation period:

Formulas containing lowercase n

In a school physics course, this letter is most often associated with the refractive index of a substance. 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 a vacuum, v is its speed in a refractive medium.

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

n 21 = v 1: v 2 = 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? A formula will help us with this, which requires knowing the angles of incidence and refraction of the beam, that is, n 21 = sin α: sin γ.

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

Usually the tables give values ​​for absolute refractive indices various substances. Do not forget that this value depends not only on the properties of the medium, but also on the wavelength. Table 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 task

№1. During plowing, the tractor pulls the plow evenly. At the same time, he applies a force of 10 kN. With this movement, it covers 1.2 km within 10 minutes. It is necessary to determine the power it develops.

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

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

Solution. Let's first calculate the work and then the power. Then the first action gives 10,000 × 1,200 = 12,000,000 J. The second action gives 12,000,000: 600 = 20,000 W.

Answer. The tractor power is 20,000 W.

Refractive index problems

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

There is no need to convert data to SI.

When choosing formulas, you need to focus on this one: n = c: v.

Solution. From this formula it is clear that v = c: n. This means that the speed of light in glass is equal to the speed of light in a vacuum divided by the refractive index. That is, it decreases by one and a half times.

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

№3. There are two transparent media available. The speed of light in the first of them is 225,000 km/s, in the second it is 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 non-system units. However, when substituted into formulas, they will be reduced. Therefore, there is no need to convert speeds to m/s.

Selecting the formulas necessary to solve the problem. You will need to use the law of light refraction: n 21 = sin α: sin γ. And also: n = с: v.

Solution. In the first formula, n 21 is the ratio of the two refractive indices of the substances in question, that is, n 2 and n 1. If we write down the second indicated formula for the proposed media, we get the following: n 1 = c: v 1 and n 2 = c: v 2. If we make the ratio of the last two expressions, it turns out that n 21 = 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 refraction angle: sin γ = sin α × (v 2: v 1).

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

Answer. The refraction angle is 26º.

Tasks for the circulation period

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

You only need to convert time to SI units for 1 hour. It will be equal to 3,600 seconds.

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

Solution. From the above 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. An airplane propeller rotates at a frequency of 25 Hz. How long will it take the propeller to make 3,000 revolutions?

All data is given in SI, so there is no need to translate anything.

Required Formula: frequency ν = N: t. From it you only need to derive the 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 gives the number 120. It will be measured in seconds.

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

Let's sum it up

When a student encounters a formula containing n or N in a physics problem, he needs deal with two points. The first is from what branch of physics the equality is given. This may be clear from the title in the textbook, reference book, or the words of the teacher. Then you should decide what is hidden behind the many-sided “en”. Moreover, the name of the units of measurement helps with this, if, of course, its value is given. Another option is also allowed: look carefully at the remaining letters in the formula. Perhaps they will turn out to be familiar and will give a hint on the issue at hand.

Each measurement is a comparison of the measured quantity with another homogeneous quantity, which is considered unitary. Theoretically, the units for all quantities in physics can be chosen to be independent of each other. But this is extremely inconvenient, since for each value one should enter its own standard. In addition, in all physical equations that reflect the relationship between different quantities, numerical coefficients would arise.

The main feature of the currently used systems of units is that there are certain relationships between units of different quantities. These relationships are established by the physical laws (definitions) that relate the measured quantities to each other. Thus, the unit of speed is chosen in such a way that it is expressed in terms of units of distance and time. When selecting speed units, the speed definition is used. The unit of force, for example, is established using Newton's second law.

When constructing a specific system of units, several physical quantities are selected, the units of which are set independently of each other. Units of such quantities are called basic. The units of other quantities are expressed in terms of the basic ones, they are called derivatives.

Table of units of measurement "Space and time"

Table of units of measurement "Mechanics"

Table of units of measurement "Periodic phenomena, oscillations and waves"

Units table " Thermal phenomena"

Units table " Molecular physics"

Units table " Electricity and magnetism"

Units table " Optics, electromagnetic radiation"

Table of units of measurement "Acoustics"

Units table " Atomic and nuclear physics. Radioactivity"

Physics notation with multiple letters

The differential is indicated by a small letter

Before the name of the quantity, for example .

Special symbols

Diacritics

Diacritics are added to the symbol of a physical quantity to indicate certain differences. Below, diacritics have been added to the letter x as an example.

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STATE SECURITY SYSTEM
UNITS OF MEASUREMENT

UNITS OF PHYSICAL QUANTITIES

GOST 8.417-81

(ST SEV 1052-78)

USSR STATE COMMITTEE ON STANDARDS

Moscow

STATE STANDARD OF THE USSR UNION

from 01/01/1982

This standard establishes units of physical quantities (hereinafter referred to as units) used in the USSR, their names, designations and rules for the use of these units. The standard does not apply to units used in scientific research and in the publication of their results, if they do not consider and use the results measurements of specific physical quantities, as well as units of quantities assessed on conventional scales*. * Conventional scales mean, for example, the Rockwell and Vickers hardness scales and the photosensitivity of photographic materials. The standard complies with ST SEV 1052-78 in terms of general provisions, units of the International System, units not included in SI, rules for the formation of decimal multiples and submultiples, as well as their names and designations, rules for writing unit designations, rules for the formation of coherent derived SI units (see reference appendix 4).

1. GENERAL PROVISIONS

2. UNITS OF THE INTERNATIONAL SYSTEM

Table 1

table 2

Table 3

Examples of derived SI units, the names of which are formed from the names of basic and additional units

Table 4

Derived SI units with special names

Table 5

Examples of derived SI units, the names of which are formed using the special names given in table. 4

3. UNITS NOT INCLUDED IN SI

Table 6

Non-system units allowed for use along with SI units

Table 7

Units temporarily approved for use

4. RULES FOR THE FORMATION OF DECIMAL MULTIPLES AND MULTIPLE UNITS, AS WELL AS THEIR NAMES AND DESIGNATIONS

Table 8

Factors and prefixes for the formation of decimal multiples and submultiples and their names

5. RULES FOR WRITING UNIT DESIGNATIONS

APPLICATION 1

Mandatory

RULES FOR FORMATION OF COHERENT DERIVATIVE SI UNITS

v = s/t,

Where v- speed; s- length of the traveled path; t- time of movement of the point. Substitution instead s And t their SI units gives

[v] = [s]/[t] = 1 m/s.

Therefore, the SI unit of speed is meter per second. It is equal to the speed of a rectilinearly and uniformly moving point, at which this point moves a distance of 1 m in a time of 1 s. If the communication equation contains a numerical coefficient different from 1, then to form a coherent derivative of an SI unit, values ​​with values ​​in SI units are substituted into the right-hand side, giving, after multiplication by the coefficient, a total numerical value equal to the number 1. Example. If the equation is used to form a unit of energy

Where E- kinetic energy; m is the mass of the material point; v is the speed of motion of a point, then the coherent SI unit of energy is formed, for example, as follows:

Therefore, the SI unit of energy is the joule (equal to the newton meter). In the examples given, it is equal to the kinetic energy of a body weighing 2 kg moving at a speed of 1 m / s, or a body weighing 1 kg moving at a speed

APPLICATION 2

Information

Correlation of some non-systemic units with SI units

APPLICATION 3

Information

Table 1

table 2

APPLICATION 4

Information

INFORMATION DATA ABOUT COMPLIANCE WITH GOST 8.417-81 ST SEV 1052-78

Cheat sheet with formulas in physics for the Unified State Exam

and more (may be needed for grades 7, 8, 9, 10 and 11).

First, a picture that can be printed in a compact form.

Mechanics

1. Pressure P=F/S
2. Density ρ=m/V
3. Pressure at liquid depth P=ρ∙g∙h
4. Gravity Ft=mg
5. 5. Archimedean force Fa=ρ f ∙g∙Vt
6. Equation of motion for uniformly accelerated motion

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

1. Velocity equation for uniformly accelerated motion υ =υ 0 +a∙t
2. Acceleration a=( υ -υ 0)/t
3. Circular speed υ =2πR/T
4. Centripetal acceleration a= υ 2/R
5. Relationship between period and frequency ν=1/T=ω/2π
6. Newton's II law F=ma
7. Hooke's law Fy=-kx
8. Law Universal gravity F=G∙M∙m/R 2
9. Weight of a body moving with acceleration a P=m(g+a)
10. Weight of a body moving with acceleration а↓ Р=m(g-a)
11. Friction force Ftr=µN
12. Body momentum p=m υ
13. Force impulse Ft=∆p
14. Moment of force M=F∙ℓ
15. Potential energy of a body raised above the ground Ep=mgh
16. Potential energy of an elastically deformed body Ep=kx 2 /2
17. Kinetic energy of the body Ek=m υ 2 /2
18. Work A=F∙S∙cosα
19. Power N=A/t=F∙ υ
20. Efficiency η=Ap/Az
21. Oscillation period of a mathematical pendulum T=2π√ℓ/g
22. Oscillation period of a spring pendulum T=2 π √m/k
23. Equation of harmonic vibrations Х=Хmax∙cos ωt
24. Relationship between wavelength, its speed and period λ= υ T

Molecular physics and thermodynamics

1. Amount of substance ν=N/Na
2. Molar mass M=m/ν
3. Wed. kin. energy of monatomic gas molecules Ek=3/2∙kT
4. Basic MKT equation P=nkT=1/3nm 0 υ 2
5. Gay-Lussac's law (isobaric process) V/T =const
6. Charles's law (isochoric process) P/T =const
7. Relative humidity φ=P/P 0 ∙100%
8. Int. energy ideal. monatomic gas U=3/2∙M/µ∙RT
9. Gas work A=P∙ΔV
10. Boyle–Mariotte law (isothermal process) PV=const
11. Amount of heat during heating Q=Cm(T 2 -T 1)
12. Amount of heat during melting Q=λm
13. Amount of heat during vaporization Q=Lm
14. Amount of heat during fuel combustion Q=qm
15. Equation of state of an ideal gas PV=m/M∙RT
16. First law of thermodynamics ΔU=A+Q
17. Efficiency of heat engines η= (Q 1 - Q 2)/ Q 1
18. Efficiency is ideal. engines (Carnot cycle) η= (T 1 - T 2)/ T 1

Electrostatics and electrodynamics - formulas in physics

1. Coulomb's law F=k∙q 1 ∙q 2 /R 2
2. Electric field strength E=F/q
3. Electrical tension point charge field E=k∙q/R 2
4. Surface charge density σ = q/S
5. Electrical tension fields of an infinite plane E=2πkσ
6. Dielectric constant ε=E 0 /E
7. Potential energy of interaction. charges W= k∙q 1 q 2 /R
8. Potential φ=W/q
9. Point charge potential φ=k∙q/R
10. Voltage U=A/q
11. For a uniform electric field U=E∙d
12. Electric capacity C=q/U
13. Electric capacity of a flat capacitor C=S∙ ε ∙ε 0 /d
14. Energy of a charged capacitor W=qU/2=q²/2С=CU²/2
15. Current strength I=q/t
16. Conductor resistance R=ρ∙ℓ/S
17. Ohm's law for the circuit section I=U/R
18. Laws of the last. connections I 1 =I 2 =I, U 1 +U 2 =U, R 1 +R 2 =R
19. Laws parallel. conn. U 1 =U 2 =U, I 1 +I 2 =I, 1/R 1 +1/R 2 =1/R
20. Electric current power P=I∙U
21. Joule-Lenz law Q=I 2 Rt
22. Ohm's law for a complete circuit I=ε/(R+r)
23. Short circuit current (R=0) I=ε/r
24. Magnetic induction vector B=Fmax/ℓ∙I
25. Ampere power Fa=IBℓsin α
26. Lorentz force Fl=Bqυsin α
27. Magnetic flux Ф=BSсos α Ф=LI
28. Law electromagnetic induction Ei=ΔФ/Δt
29. Induction emf in a moving conductor Ei=Вℓ υ sinα
30. Self-induction EMF Esi=-L∙ΔI/Δt
31. Coil magnetic field energy Wm=LI 2 /2
32. Oscillation period no. circuit T=2π ∙√LC
33. Inductive reactance X L =ωL=2πLν
34. Capacitance Xc=1/ωC
35. Effective current value Id=Imax/√2,
36. Effective voltage value Uд=Umax/√2
37. Impedance Z=√(Xc-X L) 2 +R 2

Optics

1. Law of light refraction n 21 =n 2 /n 1 = υ 1 / υ 2
2. Refractive index n 21 =sin α/sin γ
3. Thin lens formula 1/F=1/d + 1/f
4. Lens optical power D=1/F
5. max interference: Δd=kλ,
6. min interference: Δd=(2k+1)λ/2
7. Differential grid d∙sin φ=k λ

The quantum physics

1. Einstein's formula for the photoelectric effect hν=Aout+Ek, Ek=U z e
2. Red border of the photoelectric effect ν k = Aout/h
3. Photon momentum P=mc=h/ λ=E/s

Physics of the atomic nucleus