Notation in physics - units of measurement of physical quantities

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"

What is strength?

If a body accelerates, then something acts on it. How to find this “something”? For example, what kind of forces act on a body near the surface of the earth? This is the force of gravity directed vertically downward, proportional to the mass of the body and for heights much smaller than the radius of the earth ${\large R}$, almost independent of the height; it is equal

${\large F = \dfrac {G \cdot m \cdot M}{R^2} = m \cdot g }$

Where

${\large g = \dfrac {G \cdot M}{R^2} }$

the so-called acceleration of gravity. In the horizontal direction the body will move at a constant speed, but the movement in the vertical direction is according to Newton's second law:

${\large m \cdot g = m \cdot \left ( \dfrac {d^2 \cdot x}{d \cdot t^2} \right ) }$

after contracting ${\large m}$, we find that the acceleration in the direction ${\large x}$ is constant and equal to ${\large g}$. This is the well-known motion of a freely falling body, which is described by the equations

${\large v_x = v_0 + g \cdot t}$

${\large x = x_0 + x_0 \cdot t + \dfrac {1}{2} \cdot g \cdot t^2}$

Table of units of measurement “Periodic phenomena, oscillations and waves”

Some practical issues and inductor designs

In practice, various designs of inductors are used. Depending on the purpose and area of ​​application, the devices can be made in different ways, but the effects that occur in real coils must be taken into account.

Inductor quality factor

A real coil, in addition to inductance, has several other parameters, and one of the most important is the quality factor. This value determines the losses in the coil and depends on:

• ohmic losses in the winding wire (the higher the resistance, the lower the quality factor);
• dielectric losses in wire insulation and winding frame;
• screen losses;
• core losses.

All these quantities determine the loss resistance, and the quality factor is a dimensionless quantity equal to Q=ωL/Rloss, where:

• ω = 2*π*F – circular frequency;
• L – inductance;
• ωL is the reactance of the coil.

Table of units of measurement “Thermal phenomena”

Examples of typical currents

Current values ​​can be read on information plates on electrical receivers or in the manuals for these devices. The table below shows typical values ​​of electric currents for various electrical receivers.

Table of units of measurement "Molecular physics"

What does voltage depend on?

Current unit

The voltage indicator recorded on a section of the electrical circuit depends on a number of factors, for example, on the connected load (resistance). Also influenced by the characteristics of the material from which the conductor element is made, the ambient temperature and the network components themselves.

Josephson effect

This is the name for the phenomenon of superconducting current passing through a layer of thin dielectric material, isolating one superconducting object from another. In the scientific work of the scientist whose name the effect is named, it was suggested that this phenomenon is observed only when using a superthin layer (significantly smaller than the superconducting coherence length). Later experiments demonstrated that it also manifests itself when using much thicker layers.

The use of this phenomenon will allow for high-precision measurements of voltage, as well as magnetic fields. The latter is made possible due to the huge dependence of the electric current, critical for the connection used in the interferometer, on the external magnetic field. When a Josephson junction is maintained at a constant voltage, it can act as a generator of electromagnetic wave radiation. You can also organize an installation with the opposite, absorbing effect. Moreover, both generation and reception are capable of operating in a frequency range inaccessible to other means.

Research is also underway into the effect under consideration and the magnetic field transfer phenomena based on it for the transmission and accumulation of data (quantum computers). The first experimental processor of this type was designed by Japanese engineers. In 2014, employees of the physics department of Moscow State University designed a microcircuit for a computer using the properties of superconductors and this effect.

Table of units of measurement "Electricity and magnetism"

Moment of power

The moment of force is the vector product of the radius vector drawn from the axis of rotation to the point of application of the force and the vector of this force. Those. According to the classical definition, the moment of force is a vector quantity. Within the framework of our problem, this definition can be simplified to the following: the moment of force ${\large \overrightarrow{F}}$ applied to a point with coordinate ${\large x_F}$, relative to the axis located at point ${\large x_0 }$ is a scalar quantity equal to the product of the force modulus ${\large \overrightarrow{F}}$ and the force arm - ${\large \left | x_F — x_0 \right |}$. And the sign of this scalar quantity depends on the direction of the force: if it rotates the object clockwise, then the sign is plus, if counterclockwise, then the sign is minus.

It is important to understand that we can choose the axis arbitrarily - if the body does not rotate, then the sum of the moments of forces about any axis is zero. The second important note is that if a force is applied to a point through which an axis passes, then the moment of this force about this axis is equal to zero (since the arm of the force will be equal to zero).

Let us illustrate the above with an example in Fig. 2. Let us assume that the system shown in Fig. 2 is in equilibrium. Consider the support on which the loads stand. It is acted upon by 3 forces: ${\large \overrightarrow{N_1},\ \overrightarrow{N_2},\ \overrightarrow{N},}$ points of application of these forces A , B and C , respectively. The figure also contains forces ${\large \overrightarrow{N_{1}^{gr}},\ \overrightarrow{N_2^{gr}}}$. These forces are applied to the loads, and according to Newton's 3rd law

${\large \overrightarrow{N_{1}} = — \overrightarrow{N_{1}^{gr}}}$

${\large \overrightarrow{N_{2}} = — \overrightarrow{N_{2}^{gr}}}$

Now consider the condition for the equality of the moments of forces acting on the support relative to the axis passing through point A (and, as we agreed earlier, perpendicular to the plane of the drawing):

${\large N \cdot l_1 — N_2 \cdot \left ( l_1 +l_2 \right ) = 0}$

Please note that the moment of the force ${\large \overrightarrow{N_1}}$ was not included in the equation, since the arm of this force relative to the axis in question is equal to ${\large 0}$. If for some reason we want to choose an axis passing through point C , then the condition for equality of moments of force will look like this:

${\large N_1 \cdot l_1 — N_2 \cdot l_2 = 0}$

It can be shown that, from a mathematical point of view, the last two equations are equivalent.

Table of units of measurement “Optics, electromagnetic radiation”

Voltage determination

When performing electrical installation work, a specialist is faced with different types of voltage. For example, sockets in apartments and private houses are sources of alternating voltage. It can be lowered or raised by a transformer, or rectified by a special device. Friction stress is measured in laboratory conditions using the electrochemical method. The technician needs to know about the features of measuring different types of voltage.

Constant pressure

It can be measured using magnetoelectric devices. Nowadays you can find high-precision instruments equipped with a digital display on sale. The easiest way is to directly connect the device to the area where you want to measure. In this case, the following rules must be observed:

1. The limit value must exceed the intended maximum. In the case when measuring work is carried out without knowledge of this parameter, it is necessary to set a maximum limit and gradually reduce it.
2. Pay attention to the polarity of the connection. Otherwise, the pointer on a dial instrument will tilt in the opposite direction, and on a digital instrument, a negative number will appear on the screen.

Laboratory voltmeter

AC voltage

In this case, different types of measuring instruments are used, with the exception of magnetoelectric ones. They work with such devices only by connecting to the output of a rectifier.

Table of units of measurement "Acoustics"

Table of units of measurement “Atomic and nuclear physics. Radioactivity"

Permissible doses of radiation

• 
  0.57 μSv/hour for five consecutive years

In subsequent years, background radiation should not exceed 0.12 μSv/hour

• the maximum permissible total annual dose received from all technogenic sources is
  1 mSv/year

The value of 1 mSv/year should in total include all episodes of man-made exposure to radiation on humans. This includes all types of medical examinations and procedures, including fluorography, dental x-rays, and so on. This also includes flying on airplanes, going through security at the airport, obtaining radioactive isotopes from food, and so on.