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The times when the current was detected with the help of personal sensations of scientists who passed it through themselves are long gone. Now, special devices called ammeters are used for this.
An ammeter is a device used to measure current. What is meant by current?
Let's turn to Figure 21, b. It highlights the cross section of the conductor through which charged particles pass in the presence of an electric current in the conductor. In a metallic conductor, these particles are free electrons. In the course of their movement along the conductor, the electrons carry some charge. The more electrons and the faster they move, the more charge they will transfer in the same time.
The current strength is a physical quantity that shows how much charge passes through the cross section of the conductor in 1 s.
Let, for example, for a time t = 2 s, current carriers transfer a charge q = 4 C through the cross section of the conductor. The charge carried by them in 1 s will be 2 times less. Dividing 4 C by 2 s, we get 2 C/s. This is the power of the current. It is denoted by the letter I:
I - current strength.
So, to find the current strength I, it is necessary to divide the electric charge q, which passed through the cross section of the conductor in time t, by this time:
The unit of current strength is called the ampere (A) in honor of the French scientist A. M. Ampère (1775-1836). The definition of this unit is based on the magnetic effect of the current, and we will not dwell on it. If the strength of the current I is known, then you can find the charge q passing through the cross section of the conductor in time t. To do this, you need to multiply the current by the time:
The resulting expression allows you to determine the unit of electric charge - the pendant (C):
1 Cl \u003d 1 A 1 s \u003d 1 A s.
1 C is the charge that passes in 1 s through the cross section of the conductor at a current of 1 A.
In addition to the ampere, other (multiple and submultiple) units of current strength are often used in practice, for example, milliampere (mA) and microampere (μA):
1 mA = 0.001 A, 1 µA = 0.000001 A.
As already mentioned, the current strength is measured using ammeters (as well as milli- and microammeters). The demonstration galvanometer mentioned above is a conventional microammeter.
There are different designs of ammeters. An ammeter intended for demonstration experiments at school is shown in Figure 28. The same figure shows its symbol (a circle with the Latin letter "A" inside). 
When included in the circuit, the ammeter, like any other measuring device, should not have a noticeable effect on the measured value. Therefore, the ammeter is designed so that when it is turned on, the current strength in the circuit almost does not change.
Depending on the purpose in technology, ammeters with different scale divisions are used. On the scale of the ammeter, you can see what the highest current strength it is designed for. It is impossible to include it in a circuit with a higher current strength, as the device may deteriorate.
To turn on the ammeter in the circuit, it is opened and the free ends of the wires are connected to the terminals (clamps) of the device. In this case, the following rules must be observed:
1) the ammeter is connected in series with the circuit element in which the current is measured;
2) the ammeter terminal with the "+" sign should be connected to the wire that comes from the positive pole of the current source, and the terminal with the "-" sign - with the wire that comes from the negative pole of the current source.
When an ammeter is connected to the circuit, it does not matter on which side (left or right) of the element under study it is connected. This can be verified by experience (Fig. 29). As you can see, when measuring the strength of the current passing through the lamp, both ammeters (both the one on the left and the one on the right) show the same value. 
1. What is the current strength? What letter is it? 2. What is the formula for the current strength? 3. What is the unit of current called? How is it designated? 4. What is the name of the device for measuring current strength? How is it indicated on the diagrams? 5. What rules should be followed when connecting an ammeter to a circuit? 6. What is the formula for the electric charge passing through the cross section of the conductor, if the strength of the current and the time of its passage are known?
phscs.ru
Basic physical quantities, their letter designations in physics.
It's no secret that there are special designations for quantities in any science. Letter designations in physics prove that this science is no exception in terms of identifying quantities using special symbols. 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 the first scientific revolution took place, it was thanks to it 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 of the general laws of 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, amount of heat, 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 designation is in the form of the letter v, for mechanical work - A, and for energy - E. Electric charge is usually denoted by the letter q, and magnetic flux is F.
SI: general information
The International System of Units (SI) is a system of physical units based on the International System of Units, including the names and designations of physical units. 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 designations for various 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.
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| Area (Latin area), vector potential, work (German Arbeit), amplitude (Latin amplitudo), degeneracy parameter, work function (German Austrittsarbeit), Einstein coefficient for spontaneous emission, mass number | |
| Acceleration (lat. acceleratio), amplitude (lat. amplitudo), activity (lat. activitas), thermal diffusivity, rotational ability, Bohr radius | |
| Magnetic induction vector, baryon number, specific gas constant, virial coefficient, Brillion function, interference fringe width (German Breite), brightness, Kerr constant, Einstein coefficient for stimulated emission, coefficient Einstein for absorption, the rotational constant of the molecule | |
| Magnetic induction vector, beauty/bottom quark, Veena constant, width (German Breite) | |
| capacitance, heatcapacity, constant of integration (lat. constans), charm (eng. charm), Clebsch-Gordan coefficients, Cotton-Mouton constant (eng. Cotton-Mouton constant), curvature (Latin curvatura) | |
| Speed of light (lat. celeritas), speed of sound (lat. celeritas), heat capacity (English heat capacity), magic quark (English charm quark), concentration (English concentration), first radiative constant, Second radiative constant | |
| electric displacement field, diffusion coefficient, dioptric power, transmission coefficient, quadrupole electric moment tensor, angular dispersion of a spectral device, linear dispersion of a spectral device, transparency coefficient of a potential barrier, de-plus meson (English Dmeson), de-zero meson (English Dmeson), diameter (Latin diametros, other Greek διάμετρος) | |
| Distance (lat. distantia), diameter (lat. diametros, other Greek διάμετρος), differential (lat. differentia), down quark, dipole moment, grating period, thickness (German Dicke) | |
| Energy (lat. energīa), electric field strength (eng. electric field), electromotive force (eng. electromotive force), magnetomotive force, illumination (fr. éclairement lumineux), emissivity of the body, Young's modulus | |
| 2.71828…, electron, elementary electric charge, electromagnetic interaction constant | |
| Force (Latin fortis), Faraday constant, Helmholtz free energy (German freie Energie), atomic scattering factor, electromagnetic field strength tensor, magnetomotive force, shear modulus | |
| Frequency (Latin frequentia), function (Latin functia), volatility (German Flüchtigkeit), force (Latin fortis), focal length (English focal length), oscillator strength, coefficient of friction | |
| Gravitational constant, Einstein tensor, Gibbs free energy, space-time metric, virial, partial molar value, adsorbate surface activity, shear modulus, total field momentum, gluon ), Fermi constant, conduction quantum, electrical conductivity, weight (German Gewichtskraft) | |
| Gravitational acceleration, gluon, Lande factor, degeneracy factor, weight concentration, graviton, constant Gauge interactions | |
| Magnetic field strength, equivalent dose, enthalpy ), Higgs boson, exposition, Hermite polynomials | |
| Height (German Höhe), Planck's constant (German Hilfsgröße), helicity (English helicity) | |
| current strength (fr. intensité de courant), sound intensity (lat. intēnsiō), light intensity (lat. intēnsiō), radiation strength, light intensity, moment of inertia, magnetization vector | |
| Imaginary unit (lat. imaginarius), unit vector | |
| Current density, angular momentum, Bessel function, moment of inertia, polar moment of inertia of the section, internal quantum number, rotational quantum number, luminous intensity, J/ψ-meson | |
| Imaginary unit, current density, unit vector, internal quantum number, 4-vector of current density | |
| Kaon (English kaons), thermodynamic equilibrium constant, coefficient of electronic thermal conductivity of metals, bulk modulus, mechanical momentum, Josephson constant | |
| Coefficient (German: Koeffizient), Boltzmann constant, thermal conductivity, wave number, unit vector | |
| Angular momentum, inductance, Lagrangian function, classical Langevin function, Lorenz number, sound pressure level, Laguerre polynomials, orbital quantum number, energy brightness, brightness (English luminance) | |
| Length (eng. length), mean free path (eng. length), orbital quantum number, radiative length | |
| Moment of force, magnetization vector, torque, Mach number, mutual inductance, magnetic quantum number, molar mass | |
| Mass (Latin massa), magnetic quantum number, magnetic moment, effective mass, mass defect, Planck mass | |
| Quantity (lat. numerus), Avogadro's constant, Debye number, total radiation power, magnification of an optical instrument, concentration, power | |
| Refractive index, amount of matter, normal vector, unit vector, neutron (English neutron), quantity (English number), basic quantum number, rotation frequency, concentration, polytropic index, Loschmidt constant | |
| Origin (lat. origo) | |
| Power (lat. potestas), pressure (lat. pressūra), Legendre polynomials, weight (fr. poids), gravity, probability (lat. probabilitas), polarizability, transition probability, 4-momentum | |
| Momentum (Latin petere), proton (English proton), dipole moment, wave parameter | |
| Electric charge (English quantity of electricity), quantity of heat (English quantity of heat), generalized force, radiation energy, light energy, quality factor (English quality factor), zero Abbe invariant, quadrupole electric moment (English quadrupole moment) , nuclear reaction energy | |
| Electric charge, generalized coordinate, quantity of heat, effective charge, quality factor | |
| Electrical resistance, gas constant, Rydberg constant, von Klitzing constant, reflectance, radiation resistance, resolution, luminosity, particle range, distance | |
| Radius (lat. radius), radius vector, radial polar coordinate, specific heat of phase transition, specific heat of fusion, specific refraction (lat. rēfractiō), distance | |
| Surface area, entropy, action, spin, spin quantum number, strangeness, Hamilton principal function, scattering matrix , evolution operator, Poynting vector | |
| Movement (ital. b s "postamento), strange quark (eng. strange quark), path, space-time interval (eng. spacetime interval), optical path length | |
| Temperature (lat. temperātūra), period (lat. tempus), kinetic energy, critical temperature, term, half-life, critical energy, isospin | |
| Time (lat. tempus), true quark (eng. true quark), truthfulness (eng. truth), Planck time | |
| Internal energy, potential energy, Umov vector, Lennard-Jones potential, Morse potential, 4-speed, electric voltage | |
| Up quark, velocity, mobility, specific internal energy, group velocity | |
| Volume (fr. volume), voltage (eng. voltage), potential energy, visibility of the interference fringe, constant Verdet (eng. Verdet constant) | |
| Velocity (lat. vēlōcitās), phase velocity, specific volume | |
| Mechanical work (English work), work function, W boson, energy, binding energy of the atomic nucleus, power | |
| Velocity, Energy Density, Internal Conversion Rate, Acceleration | |
| Reactance, longitudinal magnification | |
| Variable, displacement, Cartesian coordinate, molar concentration, anharmonicity constant, distance | |
| Hypercharge, force function, linear increase, spherical functions | |
| Cartesian coordinate | |
| Impedance, Z boson, atomic number or nuclear charge number (German Ordnungszahl), partition function (German Zustandssumme), Hertzian vector, valence, electrical impedance, angular magnification, vacuum impedance | |
| Cartesian coordinate | |
| Thermal expansion coefficient, alpha particles, angle, fine structure constant, angular acceleration, Dirac matrices, expansion coefficient, polarization, heat transfer coefficient, dissociation coefficient, specific thermoelectromotive force, Mach angle, absorption coefficient, natural light absorption coefficient, body emissivity, damping constant | |
| Angle, beta particles, particle velocity divided by the speed of light, quasi-elastic force coefficient, Dirac matrices, isothermal compressibility, adiabatic compressibility, damping factor, angular interference fringe width, angular acceleration | |
| Gamma function, Christophel symbols, phase space, adsorption value, circulation rate, energy level width | |
| Angle, Lorentz factor, photon, gamma rays, specific gravity, Pauli matrices, gyromagnetic ratio, thermodynamic pressure coefficient, surface ionization coefficient, Dirac matrices, adiabatic exponent | |
| Change in magnitude (e.g.), Laplace operator, dispersion, fluctuation, degree of linear polarization, quantum defect | |
| Small displacement, Dirac delta function, Kronecker delta | |
| Electric constant, angular acceleration, unit antisymmetric tensor, energy | |
| Riemann zeta function | |
| Efficiency, dynamic coefficient of viscosity, metric Minkowski tensor, coefficient of internal friction, viscosity, scattering phase, eta meson | |
| Statistical temperature, Curie point, thermodynamic temperature, moment of inertia, Heaviside function | |
| Angle to the X axis in the XY plane in spherical and cylindrical coordinate systems, potential temperature, Debye temperature, nutation angle, normal coordinate, measure of wetting, Cabbibo angle, Weinberg angle | |
| Extinction coefficient, adiabatic index, magnetic susceptibility of the medium, paramagnetic susceptibility | |
| Cosmological constant, Baryon, Legendre operator, lambda-hyperon, lambda-plus-hyperon | |
| Wavelength, specific heat of fusion, linear density, mean free path, Compton wavelength, operator eigenvalue, Gell-Man matrices | |
| Coefficient of friction, dynamic viscosity, magnetic permeability, magnetic constant, chemical potential, Bohr magneton, muon, erected mass, molar mass, Poisson's ratio, nuclear magneton | |
| Frequency, neutrino, kinematic viscosity coefficient, stoichiometric coefficient, amount of matter, Larmor frequency, vibrational quantum number | |
| Grand canonical ensemble, xy-null-hyperon, xi-minus-hyperon | |
| Coherence length, Darcy coefficient | |
| Product, Peltier coefficient, Poynting vector | |
| 3.14159…, pi bond, pi plus meson, pi zero meson | |
| Resistivity, Density, Charge Density, Radius in Polar Coordinates, Spherical and Cylindrical Coordinates, Density Matrix, Probability Density | |
| Summation operator, sigma-plus-hyperon, sigma-zero-hyperon, sigma-minus-hyperon | |
| Electrical conductivity, mechanical stress (measured in Pa), Stefan-Boltzmann constant, surface density, reaction cross section, sigma bond, sector velocity, surface tension coefficient, photoconductivity, differential scattering cross section, shielding constant, thickness | |
| Lifetime, tau-lepton, time interval, lifetime, period, linear charge density, Thomson coefficient, coherence time, Pauli matrix, tangential vector | |
| Y-boson | |
| Magnetic flux, electrical displacement flux, work function, ide, Rayleigh dissipative function, Gibbs free energy, wave energy flux, lens optical power, radiation flux, luminous flux, magnetic flux quantum | |
| Angle, electrostatic potential, phase, wave function, angle, gravitational potential, function, golden ratio, body force field potential | |
| X-boson | |
| Rabi frequency, thermal diffusivity, dielectric susceptibility, spin wave function | |
| Wave function, interference aperture | |
| Wave function, function, current function | |
| Ohm, solid angle, number of possible states of a statistical system, omega-minus-hyperon, angular velocity of precession, molecular refraction, cyclic frequency | |
| Angular frequency, meson, state probability, precession Larmor frequency, Bohr frequency, solid angle, flow velocity |
dik.academic.ru
| Value | Designation | SI unit | |
| Current strength | I | ampere | BUT |
| current density | j | ampere per square meter | A/m2 |
| Electric charge | Q, q | pendant | Cl |
| Electric dipole moment | p | coulomb meter | C ∙ m |
| Polarization | P | pendant per square meter | C/m2 |
| Voltage, potential, emf | U, φ, ε | volt | AT |
| Electric field strength | E | volt per meter | V/m |
| Electrical capacitance | C | farad | F |
| Electrical resistance | R, r | ohm | Ohm |
| Specific electrical resistance | ρ | ohm meter | Ohm ∙ m |
| electrical conductivity | G | Siemens | Cm |
| Magnetic induction | B | tesla | Tl |
| magnetic flux | F | weber | wb |
| Magnetic field strength | H | ampere per meter | A/m |
| Magnetic moment | pm | ampere square meter | A ∙ m2 |
| Magnetization | J | ampere per meter | A/m |
| Inductance | L | Henry | gn |
| electromagnetic energy | N | joule | J |
| Bulk energy density | w | joule per cubic meter | J/m3 |
| Active power | P | watt | Tue |
| Reactive power | Q | var | var |
| Full power | S | watt-ampere | W ∙ A |
tutata.ru
Physical quantities of electric current
Hello, dear readers of our site! We continue the series of articles on beginner electricians. Today we will briefly consider the physical quantities of electric current, types of connections and Ohm's law.
First, let's remember what types of current exist:
Alternating current (letter designation AC) - is produced due to the magnetic effect. This is the same current that we have in our homes. It does not have any poles because it changes them many times per second. This phenomenon (reversal of polarity) is called frequency and is expressed in hertz (Hz). At the moment, our network uses an alternating current of 50 Hz (that is, a direction change occurs 50 times per second). The two wires that enter the dwelling are called phase and zero, since there are no poles here.
Direct current (letter designation DC) is the current that is obtained by a chemical method (for example, batteries, accumulators). It is polarized and flows in a certain direction.
Basic physical quantities:
- Potential difference (designation U). Since generators act on electrons like a water pump, there is a difference in its terminals, which is called the potential difference. It is expressed in volts (designation B). If you and I measure the potential difference at the input and output connections of an electrical appliance with a voltmeter, we will see readings of 230-240 V on it. Usually this value is called voltage.
- Current strength (designation I). For example, when a lamp is connected to a generator, an electrical circuit is created that passes through the lamp. A stream of electrons flows through the wires and through the lamp. The strength of this current is expressed in amperes (designation A).
- Resistance (designation R). Resistance is usually understood as a material that allows electrical energy to be converted into heat. Resistance is expressed in ohms (notation Ohm). Here we can add the following: if the resistance increases, then the current decreases, since the voltage remains constant, and vice versa, if the resistance decreases, then the current increases.
- Power (designation P). Expressed in watts (notation W) - it determines the amount of energy consumed by the device that is currently connected to your outlet.
Types of consumer connections
Conductors, when included in a circuit, can be connected to each other in various ways:
- Consistently.
- Parallel.
- mixed way
A connection is called serial, in which the end of the previous conductor is connected to the beginning of the next.
A connection is called parallel, in which all the beginnings of the conductors are connected at one point, and the ends at another.
A mixed conductor connection is a combination of series and parallel connections. Everything we have told in this article is based on the basic law of electrical engineering - Ohm's law, which states that the current strength in a conductor is directly proportional to the applied voltage at its ends and inversely proportional to the resistance of the conductor.
In the form of a formula, this law is expressed as follows:
fazaa.ru
It's no secret that there are special designations for quantities in any science. Letter designations in physics prove that this science is no exception in terms of identifying quantities using special symbols. 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 the first scientific revolution took place, it was thanks to it 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 of the general laws of 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, amount of heat, 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 designation is in the form of the letter v, for mechanical work - A, and for energy - E. Electric charge is usually denoted by the letter q, and magnetic flux is F.
SI: general information
The International System of Units (SI) is a system of physical units based on the International System of Units, including the names and designations of physical units. 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 designations for various 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.
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 the 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. This 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 it hides the basic quantum number (quantum physics), concentration and the 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 the 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 amount of a substance. Denoted by the Greek letter "nu". To calculate it, 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 electric charge, the formula is required:
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?
Tables usually give values for absolute refractive index 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.
Decision. 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.
Decision. 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.
Decision. 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. blades 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.
Decision. 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.
Decision. 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.
