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Nauki ścisłe priorytetem społeczeostwa opartego na wiedzy
Artykuły na platformę CMS
Author: Ryszard Świda
Introduction to thermodynamics. Temperature and heat.
All objects that surround us and can be seen or detected by the available instruments, are
called substances. Substances are divided, given the mechanical features, into the solids and liquids.
However, taking into account the thermodynamic features they can be divided into solids, liquids
and volatiles body. Hence the name of the aggregation state, or the state of the phase. By studying
the chemical properties, we can see that the substances with the same characteristics are present as
solids or liquids, or as volatile body. Substances occurring in one state may move their focus to
another state, and a change of state is also called a phase transition. While observing the phase
states, we note that under certain conditions difference between the liquid and gaseous state
disappears - this state is called a critical state, and it is defined by: critical temperature (Tk), critical
pressure (pk) and critical density (ρk). Here is an example of critical values of selected substances.
Substancja
woda
tlen
azot
dwutlenek węgla
Tk(K)
647,15
154,35
128,05
304,35
pk(MPa)
22,06
5,036
3,394
7,387
ρk(kg/m3)
400
430
311
460
Table 1 *
This volatilized gases and vapors - the gas is part of the volatile body heated to a temperature
above the critical temperature.
In order to explain the phenomena in nature we have to accept certain assumptions of the
materials’ construction. Kinetic-molecular theory is based on two fundamental assumptions:
-substances are made up of microscopic particles known as molecules,
-molecules are in a continuous and random motion.
Projekt współfinansowany przez Unię Europejską
w ramach Europejskiego Funduszu Społecznego
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Fig.1
The substance in gaseous occupies the entire volume of the vessel, the particles are moving at
high speed from collision to collision (also with the walls of the vessel).
Fig.2
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The liquid fills the part of the vessel, from the bottom to a certain height, the particles move at
lower speeds than in the gaseous state. In this state intermolecular forces play a greater role.
Figure 3
In the solid-state molecules are in a high density, and their movement is limited to the
oscillating motion (when the molecules are made up of many atoms, it must also be taken into
account the rotational movement). The substance in the solid state usually has a described shape.
The main difference in the construction is the distance between the molecules, which means the
density of molecules. The bodies of volatile concentration is lowest (distances are the biggest) fig.1, in the liquid bigger – fig. 2, and in solids the biggest– fig. 3. Density determines the
restrictions in the molecules movement, and the interaction between them. With the increasing
density decreases so called average free path, which is the distance between collisions of molecules.
Depending on the concentration, the rates at which the particles move (in the drawings is the length
of the arrow) are different. In the volatile bodies the largest and in the solid bodies the smallest
Temperature.
In everyday language, we use the terms: cold, warm, lukewarm, hot. Using them express a
comparison of the perceived characteristics of the body by the appropriate human sense.
With two bodies A and B, and touching them, we can say that:
- either the body A is warmer than body B,
or body A is colder than body B,
or bodies A and B are equally warm.
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For comparison of this quality in physics it is introduced quantity called temperature, which is
indicative of the energy contained in the body.
To determine the temperature we use thermometers which use changes in physical features which
occur in bodies due to temperature changes such as:
a) the deformation of the bimetal (permanently connected to the two metal strips with different
features)
b) producing a potential difference at the terminals of different metals and at different temperatures,
c) a change in resistance,
d) changes in the volume of liquid,
e) the color change of heat radiation
f) and others.
Different scales are used to record the temperature.
The most commonly used are:
- Kelvin scale (used in the study, the unit is 1K)
- Celsius scale (used colloquially, the unit is 1 ° C)
- Fahrenheit scale (used today for example in the USA,
the unit is 1 º F) ..
Kelvin scale, also called absolute temperature scale is based on the ideal expanding gas. We
assume that the lowest possible temperature is 0 ° C and 1 ° C is equal to 1 ° C.
Celsius scale is based on the two temperatures, a melting point of ice to 0 ° C and the boiling
point of water under a pressure of 1013 hPa equal of 100 ° C.
Fahrenheit scale is based on the three points, 0 ° F to freezing mixture of water, sal ammoniac and
ice in proportions of 1:1:1, 32 ° F is the temperature of the mixture of ice and water in proportions
of 1: 1 and 212 ° F is the temperature of boiling water.
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The relationship between the temperature values in these scales give the following formulas:
9
5
t F=
t+ 32 F
t= (t F− 32 )C
5
9
T = (t + 273,15)K,
,
.
(
)
Where T - temperature in Kelvin scale, T - temperature in Celsius scale and tF – Fahrenheit .
Here are a few examples of the temperature values expressed in different scales:
Table 2
Temperatura
-zera bezwzględnego
-zera Fahrenheita
-zera Celsjusza
-ciała człowieka
-wrzenia wody
-efektywna powierzchni S łońca
Kelvina (K)
0
255,37
273,15
309,8
373,15
5800
Celsjusza(˚C)
-273,15
-17,78
0
36,6
100
5526
Fahrenheita(˚F)
-459,07
0
32
98,2
212
9980
Sometimes we use the following terms:
- temperature in normal conditions (a temperature equals to 0 ° C),
- room temperature (i.e. 25 ° C previously 20 ° C).
The temperature is related to the notion of comfort or range of temperatures for well-being of
dressed people. In winter this is a range between 20 ° C and 23 ° C, in summer between 24 ° C and
28 ° C.
At the same time when cooling the rooms (including a car), we should make sure that the
temperature difference between the room and the outside should not be more than 5 ° C to 7 ° C.
Higher difference risks of colds.
If the two bodies will be closed with each other for too long, their temperatures equalize, then we
say that they are in thermal equilibrium.
Before confronting two bodies with each, we cannot tell whether they are at thermal equilibrium or
not? It turns out that to fix this we will need the third body, usually it is the thermometer. Before we
proceed to measure we will learn about a new law called the zero law of thermodynamics.
If the body A is in thermal equilibrium with the body C and the body B is in thermal equilibrium
with the body C, the bodies A and B are also in thermal equilibrium.
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A
równowaga
termiczna
C
Jeżeli
A
to
B
równowaga
termiczna
C
równowaga termiczna
B
Tasks.
1) Have the actual air temperature at different scales temperature.
2) Find formulas determining the temperature dependence of the Kelvin scale from the temperature
in Fahrenheit, and the temperature dependence of Fahrenheit from the temperature in Kelvin.
3) Bartek and Maciek are eight years old, Jack and Maciek are also eight years old. Does it mean
that Bartek and Jacek are of the same age?
Heat.
When we considered of energy in the mechanics, we have found that the change of energy in a
mechanical, electrical, magnetic or another way is connected with the work. In mechanics, there is
conservation of energy, which can be expressed by the formula ΔE = W, where ΔE is the sum of
increases in energy, and the amount of work done by the outside force. Doing the work on the
system caused a change in speed, the height, deformation or other visible effects. But in some cases,
the result of executing the work is invisible, eg moving the body to the level track against the force
of friction. Could the work ‘disappear’? Based on many observations of life we can answer that the
energy received from work turned into heat Q. This inexact, but an illustrative way of speaking is
adopted for historical reasons (caloric theory), we often say: the heat is taken or given by the
system.
Heat Q is an important physical quantity used to describe the phenomena associated with the
change in temperature. It is a form of transferring internal energy Ew ( Ew - the sum of all forms of
energy which are particles of bodies, atoms, and the components of atoms) of the body or system of
bodies at a temperature above the body temperature or the bodies set of the lower temperature. We
do not include the internal energy of the mechanical energy of the body as a whole.
The change in internal energy of the body we conclude on the basis of changes in the parameters
defining the thermodynamic state, ie.: volume V, pressure p and temperature T.
Reflections on the work and the heat led to the formulation of the so-called the first law of
thermodynamics, which the following sentence expresses the meaning:
The change in internal energy of the body or system of bodies is equal to the algebraic sum of
the work done by external forces and replaced heat of the environment.
Δ Ew = W + Q
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Task.
1.Analise different cases and laws of thermodynamics:
a) where W = 0
b) when Q = 0
c) where Δ Ew = 0
* Data based on: Mieczysław Jeżewski and Józef Kalisz Boards of physical quantities.

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