Heat transfer. Types of heat transfer. Thermal conductivity. Internal energy. Work and heat transfer as ways to change the internal energy of a body. Law of conservation of energy in thermal processes
Thermal conductivity is the transfer of delta Q energy from more heated T1 parts of the body to less heated T2.
Law of thermal conductivity: heat delta Q transferred through an element of area delta S during time delta t is proportional to the temperature gradient dT/dx, area delta S and time delta t
Delta Q = -X * (dT/dx) * delta S * delta t
X - thermal conductivity coefficient.
The essence of thermal conductivity
Thermal conduction occurs due to the movement of heat and the interaction of its constituent particles with each other. The process of thermal conduction causes the temperature of the entire body to be the same.
Typically, the energy to be transferred is defined as a heat flux density proportional to the temperature gradient. This proportionality coefficient is called the thermal conductivity coefficient.
Thermal conductivity is the property of bodies to transfer heat, based on the heat exchange that occurs between atoms and molecules of the body.
With thermal conduction, there is no transfer of substance from one end of the body to the other. Liquids have low thermal conductivity, with the exception of mercury and molten metals.
All this is due to the fact that molecules are located far from each other, unlike solids. Gases have even less thermal conductivity because its molecules are at an even greater distance than those of liquids.
Wool, hair, and paper have poor thermal conductivity. This is due to the fact that there is air between the fibers of these substances. The thermal conductivity of different substances is different
Houses are built from bricks and logs because they have poor thermal conductivity and can keep the room cool or warm. Plastic handles are made for frying pans to prevent people from getting burned, because they have poor thermal conductivity.
The essence of convection
Convection is another type of heat transfer in which energy is transferred by the jets of liquids and gases themselves.
Example: in a heated room due to convention warm air goes up and the cold one goes down.
Heat flow Q - the amount of heat W, J passing through time T,S through a given surface in the direction of the normal to it
If the amount of transferred heat W is related to the surface area F and time T, then we obtain the value:
Heat flux density is measured in W/m2
There are two types of convection - natural and forced.
Towards natural convection refers to heating the room, heating the body during the heat (naturally).
Toward forced convection This includes stirring tea with a spoon, using a fan to cool the room (unnaturally)
Convection does not occur if liquids are heated from above (correctly from below), because the heated layers cannot fall below the cold ones because they are heavier.
Heat exchange is a process of change internal energy without doing work on the body or the body itself.
Heat exchange always occurs in a certain direction: from bodies with a higher temperature to bodies with a lower temperature.
When body temperatures equalize, heat exchange stops.
Heat exchange can be carried out in three ways:
- thermal conductivity
- convection
- radiation
Thermal conductivity
Thermal conductivity- the phenomenon of transfer of internal energy from one part of the body to another or from one body to another upon their direct contact.
Metals have the greatest thermal conductivity- they have it hundreds of times more than water. The exceptions are mercury and lead., but here the thermal conductivity is tens of times greater than that of water.
When lowering a metal knitting needle into a glass with hot water very soon the end of the knitting needle also became hot. Consequently, internal energy, like any type of energy, can be transferred from one body to another. Internal energy can be transferred from one part of the body to another. So, for example, if one end of a nail is heated in a flame, then its other end, located in the hand, will gradually heat up and burn the hand.
Heating of a pan on an electric stove occurs through thermal conductivity.
Let's study this phenomenon by performing a series of experiments with solids, liquids and gases.
Let's bring the end of a wooden stick into the fire. It will ignite. The other end of the stick, located outside, will be cold. Means, wood has poor thermal conductivity.
Let's bring the end of a thin glass rod to the flame of the alcohol lamp. After some time it will heat up, but the other end will remain cold. Therefore, and glass has poor thermal conductivity.
If we heat the end of a metal rod in a flame, then very soon the entire rod will become very hot. We will no longer be able to hold it in our hands.
Means, metals conduct heat well, that is, they have high thermal conductivity. Silver and copper have the greatest thermal conductivity.
The thermal conductivity of different substances is different.
Wool, hair, bird feathers, paper, cork and other porous bodies have poor thermal conductivity. This is due to the fact that air is contained between the fibers of these substances. Vacuum (space freed from air) has the lowest thermal conductivity. This is explained by the fact that thermal conductivity is the transfer of energy from one part of the body to another, which occurs during the interaction of molecules or other particles. In a space where there are no particles, thermal conduction cannot occur.
If there is a need to protect the body from cooling or heating, then substances with low thermal conductivity are used. So, for pots and pans, handles are made of plastic. Houses are built from logs or bricks, which have poor thermal conductivity, which means they are protected from cooling.
Convection
Convection is a heat transfer process carried out by transferring energy by flows of liquid or gas.
Example of convection phenomenon: a small paper pinwheel placed over a candle flame or light bulb begins to rotate under the influence of rising heated air. This phenomenon can be explained this way. Air coming into contact with a warm lamp heats up, expands and becomes less dense than the cold air surrounding it. The Archimedes force, which acts on warm air from the side of cold air from bottom to top, is greater than the force of gravity, which acts on warm air. As a result, the heated air “floats”, rises, and cold air takes its place.
During convection, energy is transferred by the jets of gas or liquid themselves.
There are two types of convection:
- natural (or free)
- forced
In order for convection to occur in liquids and gases, they must be heated from below.
Convection cannot occur in solids.
Radiation
Radiation - electromagnetic radiation, emitted due to internal energy by a substance at a certain temperature.
The power of thermal radiation from an object that meets the blackbody criteria is described by Stefan-Boltzmann law.
The relationship between the emissive and absorptive abilities of bodies is described Kirchhoff's radiation law.
Energy transfer by radiation differs from other types of heat transfer: it can be carried out in complete vacuum.
All bodies emit energy: both highly heated and weakly heated ones, for example the human body, a stove, an electric light bulb, etc. But the higher the temperature of a body, the more energy it transmits by radiation. In this case, the energy is partially absorbed by these bodies and partially reflected.
When energy is absorbed, bodies heat up differently, depending on the state of the surface. Bodies with a dark surface absorb and emit energy better than bodies with a light surface. At the same time, bodies with a dark surface cool faster by radiation than bodies with a light surface. For example, in a light teapot hot water lasts longer high temperature
than in the dark. Heat transfer is a way of changing the internal energy of a body when transferring energy from one part of the body to another or from one body to another without doing work. There are the following types of heat transfer
Thermal conductivity
Thermal conductivity: thermal conductivity, convection and radiation. is the process of transferring energy from one body to another or from one part of the body to another thanks to thermal movement
particles. It is important that during thermal conduction there is no movement of matter; energy is transferred from one body to another or from one part of the body to another.
Different substances have different thermal conductivities. If you put a piece of ice at the bottom of a test tube filled with water and place its upper end over the flame of an alcohol lamp, then after a while the water in the upper part of the test tube will boil, but the ice will not melt. Consequently, water, like all liquids, has poor thermal conductivity.
Gases have even poorer thermal conductivity. Let's take a test tube containing nothing but air, and place it over the flame of an alcohol lamp. A finger placed in a test tube will not feel any heat. Consequently, air and other gases have poor thermal conductivity. Metals are good conductors of heat, while highly rarefied gases are the worst. This is explained by the peculiarities of their structure. Molecules of gases are located at distances from each other that are greater than molecules of solids, and collide much less frequently. Therefore, the transfer of energy from one molecules to others in gases does not occur as intensely as in solids
Convection
As is known, gases and liquids conduct heat poorly. At the same time, the air is heated from steam heating batteries. This occurs due to a type of thermal conductivity called convection.
If a pinwheel made of paper is placed over a heat source, the pinwheel will begin to rotate. This happens because the heated, less dense layers of air rise upward under the action of the buoyant force, and the colder ones move down and take their place, which leads to the rotation of the turntable.
Convection- a type of heat transfer in which energy is transferred through layers of liquid or gas. Convection is associated with the transfer of matter, so it can only occur in liquids and gases; Convection does not occur in solids.
Radiation
The third type of heat transfer is radiation. If you bring your hand to the spiral of an electric stove plugged into the network, to a burning light bulb, to a heated iron, to a radiator, etc., then you can clearly feel the heat.
Experiments also show that black bodies absorb and emit energy well, while white or shiny bodies emit and absorb it poorly. They reflect energy well. Therefore, it is understandable why people wear light-colored clothes in the summer, and why they prefer to paint houses in the south white.
By radiation, energy is transferred from the Sun to the Earth. Since the space between the Sun and the Earth is a vacuum (the height of the Earth’s atmosphere is much less than the distance from it to the Sun), energy cannot be transferred either by convection or by thermal conduction. Thus, the transfer of energy by radiation does not require the presence of any medium; this heat transfer can also be carried out in a vacuum.
Goals:
- repeat methods of changing internal energy;
- to actualize students’ personal meaning to study the topic;
- promote the development of the ability to compare facts;
- create conditions for increasing interest in the material being studied;
- develop research and creative skills;
- help students understand the practical significance and usefulness of the acquired knowledge and skills;
- create conditions for the development of communication skills and joint activities.
Demos:
- No. 1. Heat transfer through a metal rod
- No. 2. Heat transfer through rods of different metals
- No. 3. The movement of the paper of the electric plume over the switched on tile
- No. 4. Heating manganese in a flask with water
- No. 5. Natural and forced convection
- No. 6. Interaction of the radiation source with the heat sink.
During the classes
1. Organizational moment.(matched up, hello, sit down)
On the table: diary, notebook, textbook, pen, pencil, ruler. Sit down correctly and listen carefully.
The purpose of the lesson- repeat your homework, get acquainted with the types of heat transfer and explain them based on molecular kinetic theory (knowledge about internal structure substances) and apply the acquired knowledge in practice.
2. Checking homework(frontal survey).
1. Verbally checked No. 921 L.
2. Experimental task No. 1, p. 10.
3. Ways to change the internal energy of the body using examples.
Teacher: We opened our notebooks, wrote down the date and topic of the lesson “Types of heat transfer: thermal conductivity, convection, radiation.”
3. Studying new material.
So, today in the lesson we will get acquainted with three types of heat transfer.
Plan for studying the type of heat transfer.
1. Definition;
2. Features;
3. Where and how you can observe;
4. Use in practice, accounting.
(At the stage of updating knowledge, a problematic situation is posed.)
Teacher:(gives several students a feel for scissors and a pencil).
They have the same temperature because... have been in class for a long time.
Why do scissors feel cooler to the touch than a pencil?
Student:(many versions are put forward, but most often they are incorrect).
Teacher: Why are beautifully designed heating radiators not placed in the room near the ceiling?
Student:(1-2 students give the correct answer to this question).
Teacher: Why on a hot sunny summer day do we wear light and light clothes, cover our heads with a light hat, Panama hat, etc.?
Student:(there are also many versions, but rarely the correct one).
Teacher: To answer these and others correctly interesting questions Let's turn to experiments. In your notebook, write down the first type of heat transfer. Pay attention to the plan for studying types of heat transfer, which is on the screen.
Thermal conductivity.
Experience Demonstration #1: We heat a steel rod with matches on plasticine at one end.
Teacher: What will happen? How is heat transferred? Does the shape of the rod change?
There is a heated discussion of these issues and as a result, students themselves define thermal conductivity and write it down in their notebooks.
Thermal conduction is a type of heat transfer in which energy is transferred from particles with higher energy to particles with lower energy (from a heated part of the body to a cold one).
Teacher: Next, let's find out how it happens? (The teacher involves students in clarifying this issue from the point of view of the internal structure of bodies. Discussion result: particles transfer energy as a result of thermal motion and particle interaction (written by students in a notebook).
Experience Demonstration #2: We heat 2 rods: steel and copper at the same time.
Teacher: The substances are different. Do they transfer heat equally? During the experiment, they see that copper heats up faster than iron. The result of these two experiments allows us to formulate together the features of this type of heat transfer, recording it in a notebook.
Peculiarities:
1) the substance itself is not tolerated;
2) different substances have different thermal conductivities
(for metals - good; for liquids - low; for gases - almost none)
Teacher: Let's answer the question asked at the beginning of the lesson. Why do scissors feel cooler to the touch than a pencil?
Student: The thermal conductivity of metal is greater, it absorbs heat from the hand faster, so we feel cool.
Teacher: Where is the knowledge we have acquired used in practice? Find the answer on page 13.
Teacher: We write down the second type of heat transfer.
2. Convection.
Experience Demonstration #3: a switched-on electric stove, on top of which an electric plume is brought.
Teacher: Why does the paper move? As a result of the discussion, the conclusion is that the heated air rises up (floats up according to Archimedes’ law) and moves the paper.
Experience Demonstration #4: a flask with water and a grain of manganese is heated from below.
Teacher: What are we seeing? Students clearly see that the heated, tinted layers of liquid rise up, and cold ones take their place. So what is convection?
Student: Convection is a type of heat exchange in which heat is transferred by the jets of gas or liquid themselves.
Teacher: write it down in your notebook.
Experience Demonstration #5: one flask with water and a grain of manganese heats up on its own, while the other flask is heated and gradually mixed.
Teacher: What is the difference? Convection occurs in both. So what? As a result of the discussion, a conclusion is drawn and written down in a notebook.
Student: There are 2 types of them: natural and forced.
Teacher: What features did you see?
Student:
1) the substance itself is transferred;
2) exists only in liquids and gases, it does not exist in solids,
3) for it to happen, it needs to be heated from below.
Teacher: Write down the features in your notebook. We have come to the answer to the second question: “Why are beautifully designed heating radiators not placed in the room near the ceiling?”
Student: Heating of the air in the room occurs as a result of convection, and for this to occur, it must be heated from below, which means that the heating radiators must be below, under the window, i.e. in the coldest part of the room.
Experience Demonstration #6: a switched-on electric stove, to which a heat receiver connected to a liquid pressure gauge is brought to the side.
Teacher: What are we seeing? Why did the water level in the pressure gauge change?
Student: The air in the heat sink heated up, expanded, and in this elbow of the pressure gauge the liquid dropped and in the other it rose.
Teacher: How was the air heated in the heat sink? Is there thermal conductivity here? Convection?
Student: There is no thermal conductivity, because There is air between it and the tile, and it has very little thermal conductivity. There is no convection either, because... the heat sink is not above the tile, but next to it.
Teacher: This is truly a new type of heat exchange - radiation (radiant heat exchange).
An example is the sun's rays and heat rays emitted by heated bodies. We wrote down the third type of heat transfer in our notebook.
Radiation - This is heat exchange in which energy is transferred by electromagnetic rays.
Peculiarities:
1) radiate from all heated bodies (solid, liquid, gaseous),
2) occurs in a vacuum,
3) depends on the color of the surfaces (a dark surface emits and absorbs heat better, a light surface does the opposite).
Now we can answer the question posed at the beginning of the lesson:
“Why on a hot sunny summer day do we wear light and light clothes, cover our heads with a light hat, Panama hat, etc.?”
The issue is discussed and a conclusion is drawn.
Student: Cloth light color It heats up less on a hot sunny summer day and we don't feel as hot.
Teacher: Where is the knowledge we have acquired used in practice? Find the answer on page 17, last paragraph and beyond.
It turns out practical use substances with different thermal conductivities.
Students comprehend the practical significance and usefulness of the acquired knowledge.
3. Homework.
Students who wish can prepare for the next lesson short reports on the use of heat transfer in nature and technology (“Types of heat transfer in everyday life, in aviation, in agriculture”, etc.); No. 979 from the collection of problems by V.I. Lukashika, E.V. Ivanova.
4. Consolidation of the studied material.
Teacher: Consolidation of the studied material (exercise 1(1), 2(1), 3(1) according to the textbook by A.V. Peryshkin).
Summing up the work in the lesson:
- Was everything clear in the lesson?
- Was the lesson interesting?
- Is the topic of the lesson understood?
Teacher: If there are no questions, then we solve the test. (Option 3 - “3”)
Any material body has such a characteristic as heat, which can increase and decrease. Heat is not a material substance: as part of the internal energy of a substance, it arises as a result of the movement and interaction of molecules. Since the heat of different substances may differ, a process of heat transfer occurs from a hotter substance to a substance with less heat. This process is called heat transfer. We will consider the main mechanisms of their action in this article.
Definition of Heat Transfer
Heat exchange, or the process of temperature transfer, can occur both within matter and from one substance to another. In this case, the intensity of heat transfer largely depends on physical properties matter, temperature of substances (if several substances are involved in heat exchange) and laws of physics. Heat transfer is a process that always occurs one-way. Main principle heat transfer is that the hottest body always gives off heat to an object with a lower temperature. For example, when ironing clothes, a hot iron transfers heat to pants, and not vice versa. Heat transfer is a time-dependent phenomenon that characterizes the irreversible spread of heat in space.
Heat transfer mechanisms
Mechanisms of thermal interaction of substances can acquire different shapes. Three types of heat exchange are known in nature:
- Thermal conductivity is a mechanism for intermolecular heat transfer from one part of the body to another or to another object. The property is based on the temperature inhomogeneity in the substances in question.
- Convection is heat exchange between fluids (liquid, air).
- Radiation exposure is the transfer of heat from bodies (sources) heated and heated due to their energy in the form electromagnetic waves with a constant spectrum.
Let us consider the listed types of heat transfer in more detail.
Thermal conductivity
Most often, thermal conductivity is observed in solids. If, under the influence of any factors, areas with different temperatures, then thermal energy from the hotter area will move to the colder one. In some cases, a similar phenomenon can be observed even visually. For example, if you take a metal rod, say, a needle, and heat it over a fire, then after some time we will see how thermal energy is transferred along the needle, forming certain area glow. Moreover, in a place where the temperature is higher, the glow is brighter and, conversely, where the temperature is lower, it is darker. Thermal conduction can also be observed between two bodies (a mug of hot tea and a hand)
The intensity of heat flow transfer depends on many factors, the relationship of which was identified by the French mathematician Fourier. These factors include, first of all, the temperature gradient (the ratio of the temperature difference at the ends of the rod to the distance from one end to the other), the cross-sectional area of the body, as well as the coefficient of thermal conductivity (it is different for all substances, but the highest is observed in metals). The most significant thermal conductivity coefficient is observed in copper and aluminum. It is not surprising that these two metals are most often used in the manufacture of electrical wires. Following Fourier's law, the amount of heat flow can be increased or decreased by changing one of these parameters.
Convection types of heat transfer
Convection, characteristic mainly of gases and liquids, has two components: intermolecular thermal conductivity and movement (propagation) of the medium. The mechanism of action of convection occurs as follows: when the temperature of a fluid substance increases, its molecules begin to become more active movement and in the absence of spatial restrictions, the volume of matter increases. Consequence this process there will be a decrease in the density of the substance and its upward movement. A striking example Convection is the movement of air heated by the radiator from the battery to the ceiling.
There are free and forced convective types of heat transfer. Heat transfer and mass movement in the free type occurs due to the heterogeneity of the substance, that is, hot liquid rises above cold liquid naturally without the influence of external forces (for example, heating a room by central heating). With forced convection, mass movement occurs under the influence of external forces, for example, stirring tea with a spoon.
Radiant heat transfer
Radiant or radiation heat transfer can occur without contact with another object or substance, therefore it is possible even in Radiation heat transfer is inherent in all bodies to a greater or lesser extent and manifests itself in the form of electromagnetic waves with a continuous spectrum. Bright to that An example is the sun's rays. The mechanism of action is as follows: the body continuously radiates a certain amount of heat into the space surrounding it. When this energy hits another object or substance, part of it is absorbed, the second part passes through, and the third is reflected in environment. Any object can both emit and absorb heat, with dark substances being able to absorb more heat than light ones.
Combined heat transfer mechanisms
In nature, types of heat transfer processes rarely occur separately. Much more often they can be observed together. In thermodynamics, these combinations even have names, say, thermal conductivity + convection - this is convective heat transfer, and thermal conductivity + thermal radiation is called radiation-conductive heat transfer. In addition, there are such combined types heat transfer as:
- Heat transfer is the movement of thermal energy between a gas or liquid and a solid.
- Heat transfer is the transfer of t from one matter to another through a mechanical obstacle.
- Convective-radiative heat exchange is formed by combining convection and thermal radiation.
Types of heat transfer in nature (examples)
Heat transfer in nature plays a huge role and is not limited to heating globe sun rays. Extensive convection currents, such as the movement of air masses, largely determine the weather throughout our planet.
Thermal conductivity of the Earth's core leads to the appearance of geysers and the eruption of volcanic rocks. This is just a small part of on a global scale. Together, they form the types of convective heat transfer and radiation-conduction types of heat transfer necessary to support life on our planet.
The use of heat transfer in anthropological activities
Heat is an important component of almost all manufacturing processes. It is difficult to say which type of heat exchange by humans is used most in national economy. Probably all three at the same time. Thanks to heat transfer processes, metals are smelted and a huge number of goods are produced, starting with objects everyday use and ending with space ships.
Of utmost importance for civilization are thermal units, capable of transforming thermal energy into useful force. Among them are gasoline, diesel, compressor, and turbine units. For their work they use different kinds heat exchange.
