Home Справочник Речник на термините
Български (България)English (United Kingdom)

Речник на термините ПДФ Печат Е-мейл



AB - Представка поставена пред имената на електрическите устройства, за да означи че съответната единица е в  C.G.S. електромагнитна система (emu), напр. abampere, abvolt.

Абсолютна температура - Temperature reckoned from the absolute zero (the zero of Kelvin scale corresponds to -273,15 °C).

Абсолютна нула - The theoretical temperature at which molecular motion vanishes and a body would have no heat energy. It is the zero point of the Kelvin and Rankine temperature scales at -273,15 °C (0 K) and -459,67 F (0 °R) respectively.

Ускорение - The time rate of change of velocity. If the initial velocity is Vo and after the time t it is vt, the acceleration is:

a=Vt - Vo

SI unit is m/s² (IP unit is ft/s²).

Гравитационно ускорение - The acceleration of a body freely falling in a vacuum. The standard value adopted is: 9,80665 m/s² (32,174 ft/s²).

Адиабата - Attitude of a body to change its conditions without gain or loss of heat.

Адиабатн процес - A thermodynamic change of state of a system in which there is no transfer of heat or mass across the boundaries of the system. In an adiabatic process, compression always results in a warming, expansion in cooling.

Променлив ток (AC) - Current in wich the charge-flow periodically reverses. Alternating current usually implies a sinusoidal variation of current and voltage.

Ампер - Unit of electric current of SI System. It is the constant current which, if manteined in two straight parallel conductors of infinite length, of negligible circular section, and placed 1 m apart in a vacuum, will produce between these conductors a force equal to 2 x 10-7 newton (0,000 000 2 N) per meter of length.

Единица Атомна маса - The atomic mass unit, equal to 1/12 the mass of the carbon atom of mass 12. 1 AMU = 1,660565 x 10-27 kg.

Ъглово ускорение - The time rate of change of angular velocity. If the initial angular velocity is ωo and the velocity after the time t is ωt, the angular acceleration is:

ωt - ωo


The SI unit is rad/s².

Атом - The smallest particle of an element wich can enter into a chemical combination.

Атомна маса - The mass of an atom related to the atomic mass unit.

Единица Атомна маса (AMU) - A measure of atomic mass, defined as equal to 1/12 the mass of a carbon atom of mass 12.

Закон на Авогадро - Equal volumes of different gases at the same pressure and temperature contain the same number of molecules.

Число на Авогадро - The number of molecules in one mole or gram-molecular mass of a substance (estimated about 6,022045 x 1023 gmol).

Бар - Permitted pressure unit of SI system. 1bar =105 Pa = 0,9869 atm = 750,07 mm Hg.

Бари - Pressure unit of C.G.S. system. 1 barye = 1 dyne/cm².

Теорема на Бернули - A theorem stating that in a flow of incompressible fluid the sum of the static pressure and the dynamic pressure along a streamline is constant if gravity and frictional effect are disregarded. Thus is valid the following relationship:




This theorem can be considered a particular form of the conservation of energy principle.

Константа на Болцман - The ratio of the universal gas constant to Avogadro number, equal to 1,38054 x 10-16 erg/K.

Закон за газовете на Бойл - At a constant temperature the volume of a given quantity of any gas varies inversely as the pressure to wich the gas is subjected. For a perfect gas, changing from pressure p and volume v to pressure p' and volume v' without change of temperature.
pv = p'v'

Закон на Бойл-Мариот - For so-called perfect gases, the product of pressure p an volume V is constant in an isothermal process:

Британска топлинна калория (BTU) - The quantity of heat required to raise 1ºF the temperature of one pound of water at, or near, its point of maximum density (39,1°F). The Btu is equal 1055,06 J or 0,252 kcal.

Bulk modulus - The modulus of volume elasticity:


p2 - p1


where p1, p2 and v1, v2 are the initial and final pressure and volume respectively.
It is the reciprocal of the coefficient of compressibility.

Калория - The quantity of heat necessary to raise 1 °C one gramme of water at 15°C.
It is equal to 4,1868 J, to 0,00163 Wh and to 0,0039683 Btu.

Кандела - SI unit of luminous intensity. It is equal to the luminous intensity, in the direction of the normal, of a black body surface 1/600 000 m² in area, at the temperature of solidification of platinum (2045 K) under a pressure of 102325 N/m².

Капацитет - It is measured by the charge which must be communicated to a conductor to raise its potential one unit. SI capacitance unit is farad (F), which is the capacitance of a conductor which requires one coulomb of electricity (C) to raise its potential one volt (V).

Цикъл на Карно - An idealized reversible thermodynamic cycle. It consist of four stages:

(a) an isothermal expansion of the gas at temperature T1;
(b) an adiabatic expansion to temperature T2;
(c) an isothermal compression at temperature T2;
(d) an adiabatic compression to the original state of the gas.

In a Carnot cycle, the net work done is the difference between the heat input Q1, at the higher temperature T1, and the heat extracted Q2, at the lower temperature T2.

Целзий - Alternative appellation of centigrade temperature scale.

Centigrade temperature scale (abbr.°C) - A temperature scale with the ice point at 0° and the boiling point of water at 100°.
Conversion to the Fahrenheit temperature scale is according the following formula:

F=(9/5°C) + 32

Centipoise (cP) - The submultiple of C.G.S. dynamic viscosity unit, the poise, equal to 1/100 of such unit. Water at 20°C has a viscosity of 1,002 cP or 0,01002 P.

Charles-Gay Lussac law - In a gaseous system of perfect gases at constant pressure, the temperature increase and the relative volume increase stand in approximately the same proportion.
If V1 and V2 are the volumes of the same mass of gas at absolute temperature T1 and T2 respectively, the following relationship is valid:

V1/V2 = T1/T2

The following formula is also valid:

Vt=Vo×(1 + αt)


Vt is the volume at temperature t
Vo is the volume at 0°C
α is the thermal expansion coefficient

For perfect gases α = 1/273.

Coefficient of compressibility - The relative decrease of the volume of a gaseous system with increasing pressure in a isothermal process. It is the reciprocal of the Bulk modulus.

Conductance - The reciprocal of resistance. The SI unit is the Siemens (S), which corresponds to the conductance of a body through which a current of 1 ampere flows when the potential difference is 1 volt (1 S = 1 A/V)

Conductivity, thermal - It is the attitude of a substance to transfer heat by conduction. The SI unit is W/mK and is equal to the quantity of heat transferred by conduction through unit thickness, across unit area for unit difference of temperature.

Conservation of energy, law of - Energy can neither be created nor destroyed and therefore the total amount of energy in the universe remain constant.

Coulomb (C) - SI unit of quantity of electricity, equal to the quantity of electricity transported in one second by a current of one ampere.

Critical temperature - 1) temperature above wich a substance cannot exist in liquid state, regardless of the pressure. - 2) As applied to materials, the temperature at wich a change in phase takes place causing an appreciable change in the properties of the material.

Current (electric) - The rate of transfer of electricity. SI unit of current is the ampere (A) equal to a transfer of one coulomb per second.

Dalton's law of partial pressures - The pressure exerted by a mixture of gases is equal to the sum of the separate pressures wich each gas would exert if it alone occupaied the whole volume.

Decibel - A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P1/P2.
In acustics, the decibel is equal to 20 times the commom logarithm of the sound pressure ratio with the reference pressure as 0,0002 dyn/cm².

Degree - Angle subtended at the center by a circular arc wich is 1/360 of the circumference.

Density - Concentration of matter, measured by the mass per unit volume. SI unit is the kg/m³.

Dewpoint - The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur. Any further cooling usually results in the formation of dew or frost.

Dynamic pressure - The pressure of a fluid resulting from its motion, equal to one-half the fluid density (p)times the fluid velocity squared (q = 1/2ρv²).
In incompressible flow, dinamic pressure is the difference between total pressure and static pressure.

Dyne - C.G.S. force unit equal the force necessary to give acceleration of one centimetre per second per second to one gram of mass (1 dyne = 10-5 N)

Electric power - For alternating current the pover in W is stated by the following relation:

P=E·I cosφ

where E and I are the real values of the tension and current respectively and φ is the phase angle between the tension and the current.
The ratio P/EI = cosφ is called "power factor".
For a direct current of I ampere, flowing in a conductor having a resistance of R ohm, when a potential of E volt is applied to its terminals the power in watt is:

P=E·I = R·I²

Elements - Substances which cannot be decomposed by the ordinary types of chemical changes, or made by chemical union.

Energy - The capability of doing work. Potential energy is energy due to position of one body with respect to another or to the relative parts of the same body. Kinetic energy is energy due to motion.
SI unit of energy, the joule (J), is the energy expended when a force of one newton (N) acts through a distance of one meter.
The potential energy of a mass M (kg), raised through a distance h (m) is:

E = M g h

where g is the acceleration due to the gravity.
The kinetic energy of a mass M (kg), moving with a velocity v (m/s) is:

E=1/2 M v²

Engler - Empirical method of viscosity measurement for oils. Conventionally the viscosity of an oil expressed in degree engler is the ratio between the time of flux through a restriction of a given volume of oil to the time of flux of the same volume of water at 20°C temperature.

Enthalpy, or heat content - It is a thermodynamic quantity equal to the sum of the internal energy of a system plus the product od the pressure-volume work done on the system. Thus:
H = E + pv

H =enthalpy or heat content
E = internal energy of the system
p = pressure
v = volume

Entropy - A measure of the extent to which the energy of a system is unavailable. A mathematically defined thermodynamic function of state, the increase in wich gives a measure of the energy of a system wich has ceased to be available for work during a certain process. In an adiabatic process, the entropy increases if the process is irreversible and remain unchanged if the process is reversible. Thus, since all natural processes are irreversible, it is said that in an isolated system the entropy is always increasing as the system tends toward equilibrium.

Erg - The unit of energy or work in the centimetre-gram-second system (C.G.S.); it is the work performed by a force of 1 dyne actig through a distance of 1 centimetre.

Expansion of gases - See Charles-Gay Lussac's law. Also the general law for gases is valid:

pt vt = po Vo(1 + t/273)

where po, vo, pt, vt represent the pressure and volume at 0° and t°C respectively. The same law can be expressed by the formula:

p1 v1

=p2 v2


where p1, V1, T1 and p2, v1, T2 represent pressure, volume and absolute temperature of the same quantity of gas in two different conditions or:

pv= R m T

where m is the mass of gas at absolute temperature T and R the gas constant.

Fahrenheit temperature scale (F) - A temperature scale with the ice point at 32° and the boiling point of water at 212°.
The conversion to centigrade degree (Celsius) is by formula:

°C = 5/9(F-32)

Farad - SI unit of electric capacitance. It is the capacitance of a capacitor between the plates of wich a difference of potential of 1 V appears when it is charged by a quantity of electricity equal to 1 coulomb.

Force - That wich change the state of rest or motion of a body. SI unit is the newton (N) equal to the force wich will produce an acceleration of 1 m/s² in a body having a mass of 1 kg.

Frequency - Rate of oscillation. Unit is the hertz = 1 cycle/s.

Friction, coefficient of - The coefficient of friction between two surfaces is the ratio of the force required to move one over the other to the total force pressing the two together. If F is the force required to move one surface over anothero-ne and W the force pressing the surfaces together, the coefficient of friction is:

k = F / W

Flow coefficient Cv - The flowrate in gpm of water at 15°C (59 F) passing through a valve or other device, with a pressure drop of one psi.

Flow coefficient Kv - The flowrate in m³/h of water at 15°C passing through a valve or other device, with a pressure drop of one bar.

Gas - A state of matter in wich the molecules are pratically unrestricted by cohesive forces. A gas has neither definite shape nor volume.

Gas constant (R) - The constant factor in the equation of state for perfect gases. Its value is:

R = 8,3143 J/ mole K

The gas constant for a particular gas, specific gas constant, is:

r= R / m

Where m is the molecular mass of the gas.

Gases - Kinetic theory of - Gases are considered to be made up to minute, perfectly elastic particles wich are ceaselessly moving about with high velocity, colliding with each other and with the walls of the containing vessel. The pressure exerted by a gas is due to the combined effect of the impacts of the moving molecules upon the walls of the containing vessel, the magnitude of the pressure being dependent upon the kinetic energy of the molecules an their number.

Gay-Lussac's law - See Charles' law.

Gram mole - Mass in gram numerically equal to the molecular weight.

Gravity - Force imparted by the earth to a mass wich is at rest relative to the earth. Since the earth is rotating, the gravity is the resultant of the force of gravitation and the centrifugal force araising from this rotation. The magnitude of the force of gravity at sea level decreases from the poles, where the centrifugal force is zero, to the equator, where the centrifugal force is maximum.
This difference is accentuated by the shape of the earth, wich is nearly that of a spheroid slightly depressed at the poles. Also, because of the asimmetric distribution of the mass of the earth, the force of gravity is not directed precisely toward the earth's centre.

Heat - Energy transferred by a thermal process.
The SI unit is the joule (J), that of ST (technical) is the kilo-calorie (Cal) and that of IP system the British thermal unit (Btu).

Heat of combustion - It is the amount of heat evolved by the combustion of 1 g mol mass of a substance.

Hertz - The measure of frequency, defined as equal to 1 cycle/s.

Horsepower (BHP) - A IP unit of power equal to 550 ft lbi / s and to 745,7 W.

Horsepower (CV) - A metric unit of power equal to 75,0 kg m/s and 735,5 W.

Humidity, absolute - Mass of water vapour present in unit mass of dry air, usually measured as gram per kilogram of dry air or gram per cubic metre of dry air.

Hydrostatic pressure - The pressure at a depth h from the surface of a liquid of a density ρ

P = ρ g h

The value of P is in pascal (Pa) if h is in m, ρ in kg/m³ and g is the acceleration due to gravity in m/s².

Inductance - The change in magnetic field, due to the variation of a current in a conducting circuit, wich causes an induced counter electomotive force in the circuit itself. Unit of inductance is the henry (H) wich is the inductance in wich an induced electromotive force of one volt is produced when the inducing current is changed at the rate of one ampere per second.

International System of units (SI) - The metric system of unit based on the metre, kilogram, second, ampere, Kelvin degree, mole and candela.

Isobaric - Applied to a thermodynamic process at constant pressure.

Isoentalpic - Applied to a thermodynamic process at constant entalpy.

Isoentropic - Applied to a thermodynamic process at constant entropy.

Isothermal - Applied to thermodynamic process at constant temperature.

Joule - The unit of energy, work, entalpy or heat of SI system. It is the work done when the point of application of one newton force is displaced a distance of one metre in the direction of the force.

1 joule = 107 erg = 1 W s

Kelvin - It is the unit of the thermodynamic temperature of SI system, equal the fraction 1/273,16 of the thermodynamic temperature of the triple point of water. The absolute temperature scale is based on this unit. See also absolute zero and Kelvin temperature scale.

Kelvin - International temperature scale - The scale is defined by a set of interpolation equations based on the following reference temperatures:

Oxygen, liquid-gas equilibrium90,18-182,97
Water, solid-liquid equilibrium273,150,00
Water, solid-liquid-gas equilibrium273,16 0,01
Water, liquid-gas equilibrium373,15100,00
Zinc, solid-liquid equilibrium692,655 419,505
Sulphur, liquid-gas equilibrium717,75444,60
Silver, solid-liquid equilibrium1233,95960,80
Gold, solid-liquid equilibrium1336,151063,00


Kinematic viscosity - A coefficient defined as the ratio of the dynamic viscosity of a fluid to its density . The SI unit is the m²/s but still used is the stoke (1 St = 10-4 m²/s) or the submultiple centistoke (1 cSt= 10-6 m²/s).

Latent heat of vaporization - The quantity of thermal energy (heat) necessary to change one unit mass of liquid to vapour without change of temperature. The SI unit is the J/kg.

Liquid - A state of matter in wich the molecules are relative free to change their position with respect to each other, but restricted by cohesive forces so as to mantain a relatively fixed volume without a proper shape.

Lumen - The lumen is the unit of luminous flux. It is equal to the luminous flux from a uniform point souce of one candle through a unit-solid angle (steradian).

Lux - A photometric unit of illumination equal to 1 lumen/m².

Mass - Quantity of matter. The SI basic unit is the kg, equal to the mass of the platinum-iridium prototype kept at BIPM.

Metre - The SI basic unit for length. The metre is the length of 1 650 763,73 wavelength in a vacuum of the radiation corresponding to the transition between levels 2P10 and 5d5 of the atom of krypton 86.

Minute of arc - 1/60 of a degree.

Mole - Unit for quantity of elementary entities (atoms, molecules, electrons). It is the amount of substance wich contain as many elementary entities as there are atoms in 12 gram of carbon 12, thus equal to Avogadro's number (6,022045 x 1O23).

Molecular mass - The sum of the atomic masses of all the atoms in a molecule.

Molecule - The smallest unit quantity of matter wich can exist by itself and retain all the proprties of the original substance.

Newton - The SI unit for force. It is the force wich gives to a mass of one kg an acceleration of one m/s².

Normal atmosphere (atm) - This is defined the pressure exerted by a vertical column of 760 mm of mercury of density 13 595,1 kg/m³ at a place where the gravitational acceleration is 9,80665 m/s².
1 atm =1,01325 x 105 Pa
1 atm = 14,696 psi

Ohm (Ω) - Unit for electric resistance. The electric resistance between two points of a conductor when a constant difference of potential of one volt, applied between these two points, produces in this conductor a current of one ampere.

Partial pressure - The pressure exerted by a designated component or components of a gaseous mixture.

Pascal (Pa) - The unit of pressure in the SI system. One pascal equals one newton per square metre.

Pascal's law - "Pressure exerted at any point upon a confined liquid is transmitted undiminished in all directions".

Perfect gas - A gas having the following characteristics:

a) it obeys the Boyle-Mariotte law and the Charles- Gay Lussac law;
b) its internal energy is a function of temperature only;
c) its thermal capacity is indipendent from temperature.

Poise (P) - The unit of dynamic viscosity in the C.G.S. system. It is defined as the tangential force per unit area (dyn/cm²) required to mantain unit difference of velocity (1 cm/s) between two parallel planes 1 cm apart each other.

Poiseuille's law - It states that, for a steady laminar flow of a fluid through a horizontal capillary tube, the relation among the viscosity μ, the volume of liquid V, passing in the time t through the capillary of radius r and length l with a pressure drop Δp, is controlled by the following formula:

μ = π Δp r4 t / (8 V l)

Pound - A unit of mass and force still used in the U.S. and Anglo-Saxon countries.
1 pound = 0,45359237 kg.

Poundal - Force necessary to give 1 pound mass acceleration of 1 ft/s².

Power - The time rate at wich work is done. If the work L is done in the time t, the power required is:

P = L / t.

The unit of SI system is the watt (W) equal to J/s.

Pressure - Force applied to, or distributed over a surface, measured as force per unit area.
The pressure due to force F distributed over area A is

p = F / A.

SI unit, the pascal (Pa), is 1 newton per square metre.
ST unit is kgf/cm² (at); IP unit is lbt/in² (psi)
1 Pa= 1,0197 x 105 at = 0,145 x 10-3 psi.

Radian - The angle substended at the center of a circle by an arc equal to the radius of the circle. It is equal to 360°/2π or approximately 57 degree 17 minutes 44,8 seconds. The radian is the SI unit for plane angle.

Radiation - The emission and propagation of energy through space or a material medium in the form of waves.

Radiation laws - The four physical laws wich, together, fundamentally describe the behavior of a black-body radiation:

a) the Kirchoff law is essentially a thermodynamic relationship between emission and adsorption of any given wavelength at a given temperature;
b) the Plank law describes the variation of intensity of black-body radiation at a given temperature, as a function of wavelength;
c) the Stefan-Boltzman law relates the time rate of radiant energy emission from a black-body to its absolute temperature;
d) the Wien law relates the wavelength of maximum intensity emitted by a black-body to its absolute temperature.

Rankine scale of temperature - The Rankine scale, with its size of degree equal to that of Fahrenheit scale, also has the zero at absolute zero of temperature. Thus.T°R = t F + 459,67. Therefore 0°R = -459,67 F and the water boiling temperature is 671,67 °R.

Relative humidity - The ratio of the quantity of waer vapour present in the air to that wich would saturate it at the same temperature. It is also the ratio of the pressure of the water vapours present in the air to the pressure of water vapour at saturation condition at the same temperature.

Resistance - The property of conductors depending on their dimensions, material and temperature wich determine the current produced by a given differential of potential. The unit of resistance, the ohm (Ω), is that resistance through wich a difference of potential of one volt will produce a current of one ampere.
The international ohm is the resistance offerd to a constant current by a column of mercury of constant cross-sectional area, 106,30 cm in length, 14,4521 kg in mass at 0°C.

Resistivity - In electricity, a characteristic proportionality factor equal to the resistance of a conductor of a subtance having unit length and section to the passage of an electric current perpendicular to two parallel faces. Also called specific resistance.
If R is the resistance of a conductor, I its length, A its cross-sectional area and ρ its resistivity:

R = ρ (l / A)

Reynolds number - Dimensionless number dynamically considered as the ratio between the dynamic forces and the viscous forces of a aflow of fluid in a pipe. If ω is the average velocity, ρ the mass per unit volume and μ the dynamic (absolute) viscosity of fluid flowing in a pipe of diameter d:



Kinematically can be defined as the ratio between the mass velocity among the layers of fluid. In this case:




where ν is the kinematic viscosity of fluid.
When Re < 2000 the flow is laminar while if Re > 4000 the flow is turbulent.

Second - Unit of time in the SI system.
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two levels of the ground state of the atom of cesium 133.
It correspond exactly to the "ephemeris second" based on the earth's motion equal to 1/31 556 923,9747 of tropical year or to 1/86 400 of average solar day.

Sideral month - The average period of revolution of the moon with respect to the stars, a period of 27 days 7 hours 43 minutes 11,5 seconds, or approximately 271/3 days.

Sideral time - Time based upon the rotation of the earth relative to the vernal equinox.

Sideral year - The period of one apparent revolution of the earth around the sun, with respect to the stars, averaging 365 days 6 hours 9 minutes 9,55 seconds in 1955, and increasing at the rate of 0,000095 seconds annually.

Specific gravity - The ratio of the mass of a body to the mass of an equal volume of water at 4°C or other specified temperature.

Specific heat - The ratio of thermal capacity of a substance to that of water at 15°C.

Spontaneous-ignition temperature - The lowest temperature of a plate or other solid surface adequate to cause ignition in air of a fuel upon the surface.

Static pressure - The pressure with respect to a stationary surface tangent to the mass-flow velocity vector.

Stefan Boltzmann constant (σ) - A universal constant of proportionality between the radiant emittance of a black-body and the fourth power of the body's absolute temperature.

Stefan Boltzmann law - The total radiation E from a black-body is given by:

E = σ T4

where T is the absolute temperature of the black-body and σ = 5,6697 x 10-5 erg/cm²sK4 is the Stefan Boltzmann constant.

Steradian - The SI unit of the solid angle. It is the solid angle wich, having its vertex in the centre of a sphere, cuts off an area on the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere. There are 4 π steradian in a sphere.

Stoke - The unit of kinematic viscosity of C.G.S. system. It is the ratio between the dynamic viscosity and the density of the fluid (see viscosity).

Stokes' law - Gives the rate of fall of a small sphere in a viscous fluid. When a small sphere falls under the action of gravity through a viscous medium it ultimately acquires a constant velocity given by the following formula:

2gr² (d1 - d2)

where: V = rate of falling of the sphere (cm/s); g = acceleration due to gravity (cm/s²); r is the radius of the sphere (cm); d1 and d2 the densities of the sphere and the medium respectively (g/cm³); η = the absolute viscosity of the medium (poise).

Temperature - May be defined as the condition of a body wich determines the transfer of heat to or from other bodies. Particularly it is a manifestation of the average translational kinetic energy of the molecules of a substance due to the heat agitation.

Thermal capacity - It is the quantity of heat necessary to produce unit change of temperature in unit mass of a substance. Numerically equivalent to specific heat. The unit of SI system is the kJ/kg.

Thermal expansion, coefficient of - The coefficient of linear expansion is the ratio of the change in length per degree C to the length at 0°C. If Lo is the length at 0°C, α the coefficient of thermal expansion, the length at t°C is:

lt = lo (1 + αt)

The coefficient of volume expansion for solids is approximately three times the linear coefficient. The coefficient of volume expansion for liquids is the ratio of the change in volume per degree to the volume at 0°C. It varies with temperature. If V represents volume and β the coefficient of volume expasion:

Vt =Vo (1 + βt)

The coefficient of volume expansion for a gas under constant pressure is nearly the same for all gases and is equal to 0,00367 per degree C.

Thermodynamics, laws of -

  1. "When mechanical work is transformed into heat or heat into work, the amount of work is always equivalent to the quantity of heat" or " the thermal energy absorbed by a system serves either to raise the internal energy of the system or to do work on the environment".
  2. "It is impossible to transfer heat from a colder to a warmer system without the occurrance of other simultaneous changes in the two systems or in the environment". Consequently, during an adiabatic process, entropy cannot decrease. For reversible adiabatic processes entropy remain constant, and for irreversible adiabatic processes it increases.
  3. "Every substance has a finite positive entropy and the entropy of a crystalline substance is zero at the temperature of absolute zero".

Thermodynamic temperature scale - See "Kelvin".

Time, unit of - See "second".

Torr - International standard term to replace the term "millimetre of mercury" (mm of Hg). The torr is defined as 1/760 of a standard atmosphere equivalent to 1,33322 millibar.

Triple point - The thermodynamic state at wich three phases of a substance exist in equilibrium. The triple point of water occurs at a saturation vapour pressure of 6,11 millibar and at a temperature of 273,16 K.

Vapour - It is the gaseous phase of a substance wich generally exists as a liquid or solid at room temperature.

Vapour pressure - The pressure exerted when a solid or liquid is in equilibrium with its own vapour. The vapour pressure is a function of the substance and of the temperature.

Velocity - Time rate of motion in a fixed direction. If s is the space covered in time t, the velocity is: v = s/t. Velocity unit of SI system is the m/s.

Viscosity - Viscosity is the resistance to the mass-flow of a fluid, that is a sort of internal friction. It is measurable as a function of the force necessary to mantain the difference of velocity between two parallel layers.
If dr is the distance between the layers of same area A having a difference of velocity of dv, the friction force F is proportional to the product of A by the velocity gradient dv/dr and the proportionality coefficient μ is called "dynamic viscosity coefficient". The tangential force τ per unit area is:




The unit of dynamic viscosity of SI system is the Pas equal to 1N·s/m² or 1kg/(m·s). Still used is the poise (1P = 0,1 Pa·s) or its submultiple centipoise (1 cP= 10-3Pa·s).
Kinematic viscosity is the ratio between the dynamic viscosity η and the density ρ of the fluid. The unit of kinematic viscosity of SI system is the m²/s but frequently used is the stoke (1St = 10-4 m²/s) or its submultiple centistoke (1cSt = 10-6 m²/s)
The most common methods of measurement of viscosity for liquids are based on the measurement of time of efflux through a capillary tube of a certain quantity of liquid (see "Engler").

Watt - Unit of power of SI system equal to the power required to rise the production of energy at a rate of one joule per second. 1W= 0,8605 kcal/h = 1,341 x 10-3 BHP = 1,360 x 10-3 CV.

Weight - The force with which a body is attracted toward the earth. The weight P of a body of mass m is given by:

P = m·g

where g is the acceleration due to gravity. The weight unit of SI system is the newton (N). 1N = 0,102 kgf (kg force).

Work - When a force acts against a resistance to produce motion of a body, the force is said to do work. The work is measured by the product of the acting force by the distance moved against the resistance. The unit of work of SI system is the joule (J) equal to the work done by a force of one newton acting through a distance of one meter. The unit of C.G.S. system is the erg (1dyn-cm) equal to 10-7 J. In electricity the joule is the work expended per second by a current of one ampere flowing through a resistance of one ohm.