1 Heating of conductors and devices in the event of a short circuit
Short mode
short circuit (short circuit) in the circuit for the most part
is an emergency, and it is usually
eliminated in a short period of time
- seconds and fractions of a second. During
this period of time allocation
heat is so great that the temperature
conductors and apparatus goes beyond
limits set for normal
mode.
Even short-term
temperature rise of conductors and
devices during a short circuit can lead to
softening and melting of metal,
burning insulation, destruction of contacts
and other damage. For reliable
operation of the electrical system is necessary
avoid damage such as
achieved by choosing the appropriate
dimensions of current-carrying parts and settings
relay protection.
Ability
apparatus and conductor resist
short-term thermal effect
short-circuit current without damage, preventing
further work is called thermal
tenacity. Thermal
resistance is the final temperature,
which is limited to mechanical
metal strength, deformation
parts of devices, as well as heat resistance
isolation. Permissible end temperatures
for conductors in case of short circuit are given in
table 2.1.
Specific heat
specific heat capacity, specific heat capacity class 8Specific heat - the ratio of heat capacity to mass, the heat capacity of a unit mass of a substance (different for different substances); a physical quantity numerically equal to the amount of heat that must be transferred to a unit mass of a given substance in order for its temperature to change by one.
In the International System of Units (SI), specific heat is measured in joules per kilogram per kelvin, J / (kg K). Sometimes non-systemic units are also used: calorie / (kg K), etc.
Specific heat capacity is usually denoted by the letters c or C, often with subscripts.
The value of specific heat is affected by the temperature of the substance and other thermodynamic parameters. For example, measuring the specific heat capacity of water will give different results at 20°C and 60°C.
In addition, the specific heat capacity depends on how the thermodynamic parameters of the substance (pressure, volume, etc.) are allowed to change.
); for example, the specific heat at constant pressure (CP) and at constant volume (CV) are generally different.
The formula for calculating the specific heat capacity: where c is the specific heat capacity, Q is the amount of heat received by the substance during heating (or released during cooling), m is the mass of the heated (cooling) substance, ΔT is the difference between the final and initial temperatures of the substance. Specific heat can depend (and in principle, strictly speaking, always - more or less strongly - depends) on temperature, so the following formula with small (formally infinitesimal) and is more correct:
- 1 Specific heat capacity values for some substances
- 2 See also
- 3 notes
- 4 Literature
- 5 Links
The values of the specific heat capacity of some substances
| air (dry) | gas | 1,005 |
| air (100% humidity) | gas | 1,0301 |
| aluminum | solid | 0,903 |
| beryllium | solid | 1,8245 |
| brass | solid | 0,377 |
| tin | solid | 0,218 |
| copper | solid | 0,385 |
| molybdenum | solid | 0,250 |
| steel | solid | 0,462 |
| diamond | solid | 0,502 |
| ethanol | liquid | 2,460 |
| gold | solid | 0,129 |
| graphite | solid | 0,720 |
| helium | gas | 5,190 |
| hydrogen | gas | 14,300 |
| iron | solid | 0,444 |
| lead | solid | 0,130 |
| cast iron | solid | 0,540 |
| tungsten | solid | 0,134 |
| lithium | solid | 3,582 |
| Mercury | liquid | 0,139 |
| nitrogen | gas | 1,042 |
| petroleum oils | liquid | 1,67 — 2,01 |
| oxygen | gas | 0,920 |
| quartz glass | solid | 0,703 |
| water 373 K (100 °C) | gas | 2,020 |
| water | liquid | 4,187 |
| ice | solid | 2,060 |
| beer wort | liquid | 3,927 |
| asphalt | 0,92 |
| solid brick | 0,84 |
| silicate brick | 1,00 |
| concrete | 0,88 |
| kronglas (glass) | 0,67 |
| flint (glass) | 0,503 |
| window glass | 0,84 |
| granite | 0,790 |
| soapstone | 0,98 |
| gypsum | 1,09 |
| marble, mica | 0,880 |
| sand | 0,835 |
| steel | 0,47 |
| the soil | 0,80 |
| wood | 1,7 |
see also
- Heat capacity
- Volumetric heat capacity
- Molar heat capacity
- Latent heat
- Heat capacity of an ideal gas
- Specific heat of vaporization and condensation
- Specific heat of fusion
Notes
-
↑ For an inhomogeneous (in terms of chemical composition) sample, specific heat is a differential characteristic that varies from point to point.
In principle, it also depends on temperature (although in many cases it changes quite weakly with sufficiently large changes in temperature), while strictly speaking it is determined - following the heat capacity - as a differential quantity and along the temperature axis, i.e.
Strictly speaking, one should consider the change in temperature in the definition of specific heat not by one degree (especially not by some larger unit of temperature), but by a small one with the corresponding amount of heat transferred. (See main text below).
- ↑ Kelvins (K) here can be replaced by degrees Celsius (°C), since these temperature scales (absolute and Celsius scale) differ from each other only in the starting point, but not in the value of the unit of measurement.
Links
- Tables of physical quantities. Handbook, ed. I. K. Kikoina, M., 1976.
- Sivukhin DV General course of physics. - T. II. Thermodynamics and molecular physics.
- E. M. Lifshits Heat capacity // under. ed. AM Prokhorova Physical Encyclopedia. - M .: "Soviet Encyclopedia", 1998. - T. 2.
Heat capacity of materials table
In construction, a very important characteristic is the heat capacity of building materials. The thermal insulation characteristics of the walls of the building depend on it, and, accordingly, the possibility of a comfortable stay inside the building
The thermal insulation characteristics of the walls of the building depend on it, and, accordingly, the possibility of a comfortable stay inside the building.
Before proceeding to get acquainted with the thermal insulation characteristics of individual building materials, it is necessary to understand what the heat capacity is and how it is determined.
Specific heat capacity of materials
Heat capacity is a physical quantity that describes the ability of a material to accumulate temperature from a heated environment.
Quantitatively, the specific heat is equal to the amount of energy, measured in J, required to heat a body of mass 1 kg by 1 degree.
Below is a table of the specific heat capacity of the most common building materials.
In order to calculate the heat capacity of a material, it is necessary to have such data as:
- type and volume of heated material (V);
- an indicator of the specific heat capacity of this material (Court);
- specific gravity (msp);
- initial and final temperatures of the material.
Heat capacity of building materials
The heat capacity of materials, the table of which is given above, depends on the density and thermal conductivity of the material.
And the coefficient of thermal conductivity, in turn, depends on the size and closure of the pores. A finely porous material with a closed system of pores has greater thermal insulation and, accordingly, lower thermal conductivity than a coarsely porous one.
This is very easy to follow on the example of the most common materials in construction. The figure below shows how the coefficient of thermal conductivity and the thickness of the material affect the heat-shielding qualities of external fences.
The figure shows that building materials with a lower density have a lower thermal conductivity.
However, this is not always the case. For example, there are fibrous types of thermal insulation for which the opposite pattern applies: the lower the density of the material, the higher the thermal conductivity.
Therefore, one cannot rely solely on the indicator of the relative density of the material, but it is worth considering its other characteristics.
Comparative characteristics of the heat capacity of the main building materials
In order to compare the heat capacity of the most popular building materials, such as wood, brick and concrete, it is necessary to calculate the heat capacity for each of them.
First of all, you need to determine the specific gravity of wood, brick and concrete. It is known that 1 m3 of wood weighs 500 kg, brick - 1700 kg, and concrete - 2300 kg. If we take a wall whose thickness is 35 cm, then by simple calculations we get that the specific gravity of 1 sq.
m of wood will be 175 kg, brick - 595 kg, and concrete - 805 kg. Next, we select the temperature value at which the accumulation of thermal energy in the walls will occur. For example, this will happen on a hot summer day with an air temperature of 270C.
For the selected conditions, we calculate the heat capacity of the selected materials:
- Wood wall: C=SudhmudhΔT; Cder \u003d 2.3x175x27 \u003d 10867.5 (kJ);
- Concrete wall: C=SudhmudhΔT; Cbet \u003d 0.84x805x27 \u003d 18257.4 (kJ);
- Brick wall: C=SudhmudhΔT; Skirp \u003d 0.88x595x27 \u003d 14137.2 (kJ).
From the calculations made, it can be seen that with the same wall thickness, concrete has the highest heat capacity, and wood has the lowest. What does it say? This suggests that on a hot summer day, the maximum amount of heat will accumulate in a house made of concrete, and the least - from wood.
This explains the fact that in a wooden house it is cool in hot weather and warm in cold weather. Brick and concrete easily accumulate a sufficiently large amount of heat from the environment, but just as easily part with it.
Heat capacity and thermal conductivity of materials
Thermal conductivity is a physical quantity of materials that describes the ability of temperature to penetrate from one wall surface to another.
To create comfortable conditions in the room, it is necessary that the walls have a high heat capacity and a low thermal conductivity. In this case, the walls of the house will be able to accumulate the thermal energy of the environment, but at the same time prevent the penetration of thermal radiation into the room.
Heat capacity for various processes and states of matter
The concept of heat capacity is defined both for substances in various states of aggregation (solids, liquids, gases) and for ensembles of particles and quasiparticles (in metal physics, for example, one speaks of the heat capacity of an electron gas).
Heat capacity of an ideal gas
Main article: Heat capacity of an ideal gas
The heat capacity of a system of non-interacting particles (for example, an ideal gas) is determined by the number of degrees of freedom of the particles.
Molar heat capacity at constant volume:
- CV=dUdT=i2R,{\displaystyle C_{V}={dU \over dT}={\frac {i}{2}}R,}
where R{\displaystyle R} ≈ 8.31 J/(mol K) is the universal gas constant, i{\displaystyle i} is a number.
The molar heat capacity at constant pressure is related to the CV{\displaystyle C_{V}} Mayer relation:
- CP=CV+R=i+22R.{\displaystyle C_{P}=C_{V}+R={{i+2} \over 2}R.}
Heat capacity of crystals
Comparison of the Debye and Einstein models for the heat capacity of a solid
There are several theories of the heat capacity of a solid:
- The Dulong-Petit law and the Joule-Kopp law. Both laws are derived from classical concepts and are valid with a certain accuracy only for normal temperatures (approximately from 15 °C to 100 °C).
- Einstein's quantum theory of heat capacities. The first application of quantum laws to the description of heat capacity.
- Quantum theory of Debye's heat capacities. Contains the most complete description and agrees well with experiment.
Specific, molar and volumetric heat capacities
Main articles: Specific heat, Molar heat capacity and Volumetric heat capacity
Obviously, the greater the mass of the body, the more heat is required to heat it, and the heat capacity of the body is proportional to the amount of substance contained in it. The amount of a substance can be characterized by mass or the number of moles. Therefore, it is convenient to use the concepts of specific heat capacity (heat capacity per unit mass of a body):
- c=Cm{\displaystyle c={C \over m}}
and molar heat capacity (heat capacity of one mole of a substance):
- Cμ=Cν,{\displaystyle C_{\mu }={C \over \nu },}
where ν=mμ{\displaystyle \nu ={m \over \mu }} is the amount of substance in the body; m{\displaystyle m} is body weight; μ{\displaystyle \mu } is the molar mass. Molar and specific heat capacities are related by Cμ=cμ{\displaystyle C_{\mu }=c\mu }.
Volumetric heat capacity (heat capacity per unit volume of a body):
- C'=CV.{\displaystyle C'={C \over V}.}
Thermal conductivity of non-ferrous metals, heat capacity and density of alloys
The table shows the values of thermal conductivity of metals (non-ferrous), as well as the chemical composition of metals and technical alloys in the temperature range from 0 to 600°C.
Non-ferrous metals and alloys: nickel Ni, monel, nichrome; nickel alloys (according to GOST 492-58): cupronickel NM81, NM70, constantan NMMts 58.5-1.54, kopel NM 56.5, monel NMZhMts and K-monel, alumel, chromel, manganin NMMts 85-12, invar; magnesium alloys (according to GOST 2856-68), electron, platinum-rhodium; soft solders (according to GOST 1499-70): pure tin, lead, POS-90, POS-40, POS-30, Rose alloy, Wood alloy.
According to the table, it can be seen that magnesium alloys and nickel have high thermal conductivity (at room temperature). Low thermal conductivity is characteristic of nichrome, invar and Wood's alloy.
Thermal conductivity coefficients of aluminum, copper and nickel alloys
The thermal conductivity of metals, aluminum, copper and nickel alloys in the table is given in the temperature range from 0 to 600 ° C in the units of W / (m deg). Metals and alloys: aluminum, aluminum alloys, duralumin, brass, copper, monel, nickel silver, nichrome, ferruginous nichrome, soft steel. Aluminum alloys have greater thermal conductivity than brass and nickel alloys.
Thermal conductivity coefficients of alloys
The table shows the thermal conductivity values of alloys in the temperature range from 20 to 200ºС. Alloys: aluminum bronze, bronze, phosphor bronze, invar, constantan, manganin, magnesium alloys, copper alloys, Rose alloy, Wood's alloy, nickel alloys, nickel silver, platinum-iridium, alloy electron, platinum-rhodium.
The table shows the values of electrical resistivity and CTE of a metal wire made of various metals and alloys.
Wire material: aluminum, tungsten, iron, gold, brass, manganin, copper, nickel, constantan, nichrome, tin, platinum, lead, silver, zinc.
As can be seen from the table, nichrome wire has a high electrical resistivity and is successfully used as incandescent spirals of heating elements in many household and industrial devices.
Specific heat capacity of non-ferrous alloys
The table shows the values of the specific (mass) heat capacity of two-component and multi-component non-ferrous alloys that do not contain iron at temperatures from 123 to 1000K. The heat capacity is indicated in units of kJ/(kg deg).
The heat capacity of the following alloys is given: alloys containing aluminum, copper, magnesium, vanadium, zinc, bismuth, gold, lead, tin, cadmium, nickel, iridium, platinum, potassium, sodium, manganese, titanium, bismuth-lead-tin alloy, alloy bismuth-lead, bismuth-lead-cadmium, alumel, linden alloy, nichrome, rose alloy.
There is also a separate table that shows the specific heat capacity of metals at various temperatures.
Specific heat capacity of multicomponent special alloys
The specific (mass) heat capacity of multicomponent special alloys is given in the table at temperatures from 0 to 1300ºС. The unit of heat capacity is cal/(g deg). Heat capacity of special alloys: alumel, bell-metal, Wood's alloy, invar, linden alloy, manganin, monel, Rose alloy, phosphor bronze, chromel, Na-K alloy, Pb-Bi alloy, Pb - Bi - Sn, Zn - Sn - Ni - Fe - Mn.
Density of alloys
A table of alloy density values at room temperature is presented. The following alloys are given: bronze, tin, phosphorous, duralumin, invar, constantan, brass, magnalium, manganin, monel - metal, platinum - iridium alloy, Wood's alloy, rolled steel, cast.
NOTE: Be careful! The density of the alloys in the table is indicated in the power of 10-3. Don't forget to multiply by 1000! For example, the density of rolled steel varies from 7850 to 8000 kg/m3.
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- Tables of physical quantities. Directory. Ed. acad. I.K. Kikoin. M.: Atomizdat, 1976. - 1008 p.
- Sheludyak Yu.E., Kashporov L.Ya. and other Thermophysical properties of components of combustible systems. M. 1992. - 184 p.
- Industrial ovens. Reference guide for calculations and design. 2nd edition, supplemented and revised, Kazantsev E.I. M.: "Metallurgy", 1975.- 368 p.
