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, convection and radiation.
Thermal conductivity
Thermal conductivity is the process of transferring energy from one body to another or from one part of a body to another due to the thermal movement of 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. The thermal conductivity of a liquid is intermediate between the thermal conductivity of gases and 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 coil of an electric stove plugged in, to a burning light bulb, to a heated iron, to a radiator, etc., 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.
THERMAL CONDUCTIVITY Hot water was poured into aluminum and glass pans of equal capacity. Which pan will heat up faster to the temperature of the water poured into it? Aluminum conducts heat faster than glass, so an aluminum pan will heat up faster to the temperature of the water poured into it.
CONVECTION In industrial refrigerators, air is cooled using pipes through which cooled liquid flows. Where should these pipes be located: at the top or bottom of the room? To cool the room, the pipes through which the cooled liquid flows must be located at the top. Hot air, in contact with cold pipes, will cool and fall down under the influence of the Archimedes force.
Type of heat transfer Features of heat transfer Figure Thermal conductivity Requires a certain time Substance does not move Atomic-molecular energy transfer Convection Substance is transferred by jets Observed in liquid and gas Natural, forced Warm up, cold down Radiation Radiated by all heated bodies Carried out in a complete vacuum Emitted, reflected, absorbed
Heat transfer is a spontaneous irreversible process of energy transfer from more heated bodies or parts of the body to less heated ones. Heat transfer is a way of changing the internal energy of a body or system of bodies. Heat transfer determines and accompanies processes in nature, technology and everyday life. There are three types of heat transfer: conduction, convection and radiation.
Theory: Thermal conductivity is 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.
The closer the molecules are located to each other, the better the thermal conductivity of the body. (Thermal conductivity depends on the specific heat capacity of the body)
Consider an experiment in which nails are attached to a metal rod using wax. At one end, an alcohol lamp was brought to the rod, the heat spreads over time along the rod, the wax melts and the carnations fall off. This is due to the fact that molecules begin to move faster when heated. The flame of the alcohol lamp heats one end of the rod, the molecules from this end begin to vibrate faster, collide with neighboring molecules, and transfer part of their energy to them, so internal energy is transferred from one part to another.
Convection is the transfer of internal energy with layers of liquid or gas. Convection in solids is impossible.
Radiation is the transfer of internal energy by rays (electromagnetic radiation).
Exercise:
Solution:
Answer: 2.
1) A tourist lit a fire at a rest stop in calm weather. Being at some distance from the fire, the tourist feels the warmth. What is the main way to transfer heat from a fire to a tourist?
1) by thermal conduction
2) by convection
3) by radiation
4) by thermal conduction and convection
Solution (thanks to Alena): by radiation. Since energy in this case was not transferred by thermal conductivity, because between the person and the fire there was air - a poor conductor of heat. Convection here also cannot be observed, since the fire was next to the person, and not under him; therefore, in this case, the transfer of energy occurs by radiation.
Answer: 3
Exercise: Which substance has the best thermal conductivity under normal conditions?
1) water 2) steel 3) wood 4) air
Solution: Air has poor thermal conductivity because the distance between molecules is large. Steel has the lowest heat capacity.
Answer: 2.
OGE assignment in physics (fipi): 1) The teacher conducted the following experiment. Two rods of the same size (the copper one is located on the left, and the steel one on the right) with nails attached to them using paraffin were heated from the end using an alcohol lamp (see figure). When heated, the paraffin melts and the carnations fall off. 
Select two statements from the proposed list that correspond to the results of the experimental observations. Indicate their numbers.
1) Heating of metal rods occurs mainly by radiation.
2) Heating of metal rods occurs mainly by convection.
3) Heating of metal rods occurs mainly by thermal conductivity.
4) The density of copper is less than the density of steel.
5) The thermal conductivity of copper is greater than the thermal conductivity of steel
Solution: Heating of metal rods occurs mainly by thermal conduction; internal energy is transferred from one part of the rod to another. The thermal conductivity of copper is greater than the thermal conductivity of steel, since copper heats up faster.
Answer: 35
OGE assignment in physics (fipi): Two identical blocks of ice were brought from the cold into a warm room. The first block was wrapped in a woolen scarf, and the second was left open. Which bar will heat up faster? Explain your answer.
Solution: The second block will heat up faster; the woolen scarf will prevent the transfer of internal energy from the room to the block. Wool is a poor conductor of heat and has poor thermal conductivity, which means the ice block will heat up more slowly.
OGE assignment in physics (fipi): A hot kettle of which color - black or white - will, all other things being equal, cool down faster and why?
1) white, since it absorbs thermal radiation more intensely
2) white, since the thermal radiation from it is more intense
3) black, since it absorbs thermal radiation more intensely
4) black, since the thermal radiation from it is more intense
Solution: Black bodies absorb thermal radiation better; for example, in the sun, water in a black tank will heat up faster than in a white one. The reverse process is also true; black bodies cool faster.
Answer: 4
OGE assignment in physics (fipi): In solids, heat transfer can be carried out by
1) thermal conductivity
2) convection
3) convection and thermal conductivity
4) radiation and convection
Solution: In solids, heat transfer can only be accomplished by thermal conductivity. In a solid body, molecules are near the equilibrium position and can only oscillate around it, so convection is impossible.
Answer: 1
OGE assignment in physics (fipi): Which mug - metal or ceramic - is easier to drink hot tea from without burning your lips? Explain why.
Solution: The thermal conductivity of a metal mug is higher, and the heat from hot tea will be transferred to the lips faster and burn more strongly.
Back forward
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Lesson objectives:
- Introduce students to the types of heat transfer.
- To develop the ability to explain the thermal conductivity of bodies from the point of view of the structure of matter; be able to analyze video information; explain observed phenomena.
Lesson type: combined lesson.
Demos:
1. Heat transfer along a metal rod.
2. Video demonstration of an experiment comparing the thermal conductivity of silver, copper and iron.
3. Rotate a paper pinwheel over a switched-on lamp or tile.
4. Video demonstration of the occurrence of convection currents when heating water with potassium permanganate.
5. Video demonstration of radiation from bodies with dark and light surfaces.
DURING THE CLASSES
I. Organizational moment
II. Communicating the topic and objectives of the lesson
In the previous lesson, you learned that internal energy can be changed by doing work or by heat transfer. Today in the lesson we will look at how internal energy changes through heat transfer.
Try to explain the meaning of the word “heat transfer” (the word “heat transfer” implies the transfer of thermal energy). There are three ways to transfer heat, but I will not name them; you will name them yourself when you solve the puzzles.
Answers: thermal conductivity, convection, radiation.
Let's get acquainted with each type of heat transfer separately, and let the motto of our lesson be the words of M. Faraday: “Observe, study, work.”
III. Learning new material
1. Thermal conductivity
Answer the questions:(slide 3)
1. What happens if we put a cold spoon into hot tea? (It will warm up after a while.)
2. Why did the cold spoon get hot? (The tea gave part of its heat to the spoon, and part to the surrounding air).
Conclusion: From the example it is clear that heat can be transferred from a body that is more heated to a body that is less heated (from hot water to a cold spoon). But energy was transferred along the spoon itself - from its heated end to the cold one.
3. What causes heat transfer from the heated end of the spoon to the cold one? (As a result of the movement and interaction of particles)
Heating a spoon in hot tea is an example of conduction.
Thermal conductivity– transfer of energy from more heated parts of the body to less heated ones, as a result of thermal movement and interaction of particles.
Let's conduct an experiment:
Secure the end of the copper wire to the tripod leg. Studs are attached to the wire with wax. We will heat the free end of the wire with candles or on the flame of an alcohol lamp.
Questions:(slide 4)
1. What are we seeing? (The carnations begin to gradually fall off one by one, first those closest to the flame).
2. How does heat transfer occur? (From the hot end of the wire to the cold end).
3. How long will it take for heat to transfer through the wire? (Until the entire wire heats up, that is, until the temperature throughout the entire wire is equalized)
4. What can be said about the speed of movement of molecules in the area located closer to the flame? (The speed of movement of molecules increases)
5. Why does the next section of wire heat up? (As a result of the interaction of molecules, the speed of movement of molecules in the next section also increases and the temperature of this part increases)
6. Does the distance between molecules affect the rate of heat transfer? (The smaller the distance between the molecules, the faster the heat transfer occurs)
7. Remember the arrangement of molecules in solids, liquids and gases. In which bodies will the process of energy transfer occur faster? (Faster in metals, then in liquids and gases).
Watch a demonstration of the experiment and prepare to answer my questions.
Questions:(slide 5)
1. Along which plate does heat spread faster, and along which slower?
2. Draw a conclusion about the thermal conductivity of these metals. (The best thermal conductivity is for silver and copper, somewhat worse for iron)
Please note that when heat is transferred in this case, there is no body transfer.
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.
Let's write down the main thermal conductivity features:(slide 7)
- in solids, liquids and gases;
- the substance itself is not tolerated;
- leads to equalization of body temperature;
- different bodies - different thermal conductivity
Examples of thermal conductivity: (slide 8)
1. Snow is a porous, loose substance; it contains air. Therefore, snow has poor thermal conductivity and protects the soil, winter crops, and fruit trees well from freezing.
2. Kitchen oven mitts are made of material that has poor thermal conductivity. Handles of teapots and pots are made from materials with poor thermal conductivity. All this protects your hands from burns when touching hot objects.
3. Substances with good thermal conductivity (metals) are used to quickly heat bodies or parts.
2. Convection
Guess the riddles:
1) Look under the window -
There is an accordion stretched out there,
But he doesn’t play the harmonica -
Warms our apartment... (battery)2) Our fat Fedora
will not be full soon.
But when I’m full,
From Fedora - warmth... (stove)
Batteries, stoves, and heating radiators are used by humans to heat living spaces, or rather, to heat the air in them. This happens thanks to convection, the next type of heat transfer.
Convection- This is the transfer of energy by jets of liquid or gas. (Slide 9)
Let's try to explain how convection occurs in residential premises.
The air, in contact with the battery, is heated by it, while it expands, its density becomes less than the density of cold air. Warm air, being lighter, rises upward under the influence of Archimedes' force, and heavy cold air sinks down.
Then again: colder air reaches the battery, heats up, expands, becomes lighter and rises upward under the influence of Archimedean force, etc.
Thanks to this movement, the air in the room warms up.
A paper pinwheel placed over the switched-on lamp begins to rotate. (Slide 10)
Try to explain how this happens? (Cold air, when heated by the lamp, becomes warm and rises, while the turntable rotates).
The liquid is heated in the same way. Watch an experiment on observing convection currents when heating water (using potassium permanganate). (Slide 11)
Please note that, unlike thermal conduction, convection involves the transfer of matter and convection does not occur in solids.
There are two types of convection: natural And forced.
Heating a liquid in a pan or the air in a room are examples of natural convection. For it to occur, substances must be heated from below or cooled from above. Why is this so? If we heat from above, then where will the heated layers of water move, and where will the cold ones? (Answer: nowhere, since the heated layers are already on top, and the cold layers will remain below)
Forced convection occurs when a liquid is stirred with a spoon, pump, or fan.
Features of convection:(slide 12)
- occurs in liquids and gases, is impossible in solids and vacuum;
- the substance itself is transferred;
- Substances need to be heated from below.
Examples of convection:(slide 13)
1) cold and warm sea and ocean currents,
2) in the atmosphere, vertical air movements lead to the formation of clouds;
3) cooling or heating of liquids and gases in various technical devices, for example in refrigerators, etc., water cooling of engines is provided
internal combustion.
3. Radiation
(Slide 14)
Everyone knows that The sun is the main source of heat on Earth. The earth is located at a distance of 150 million km from it. How is heat transferred from the Sun to Earth?
Between the Earth and the Sun outside our atmosphere, all space is a vacuum. And we know that thermal conductivity and convection cannot occur in a vacuum.
How does heat transfer occur? Another type of heat transfer occurs here – radiation.
Radiation - This is heat exchange in which energy is transferred by electromagnetic rays.
It differs from conduction and convection in that heat in this case can be transferred through a vacuum.
Watch the video about radiation (slide 15).
All bodies emit energy: the human body, a stove, an electric lamp.
The higher the body temperature, the stronger its thermal radiation.
Bodies not only emit energy, but also absorb it.
(slide 16) Moreover, dark surfaces absorb and emit energy better than bodies with a light surface.
Features of radiation(slide 17):
- occurs in any substance;
- the higher the body temperature, the more intense the radiation;
- occurs in a vacuum;
- dark bodies absorb radiation better than light bodies and emit radiation better.
Examples of using body radiation(slide 18):
The surfaces of rockets, airships, balloons, satellites, and airplanes are painted with silver paint so that they are not heated by the Sun. If, on the contrary, it is necessary to use solar energy, then parts of the devices are painted dark.
People wear dark clothes (black, blue, cinnamon) in winter, which are warmer, and light clothes (beige, white) in summer. Dirty snow melts faster in sunny weather than clean snow, because bodies with a dark surface absorb solar radiation better and heat up faster.
IV. Consolidating the acquired knowledge using examples of problems
Game "Try, Explain", (slides 19-25).
In front of you is a playing field with six tasks, you can choose any one. After completing all the tasks, a wise saying and the one who very often pronounces it from TV screens will be revealed to you.
1. Which house is warmer in winter if the wall thickness is the same? It is warmer in a wooden house, since wood contains 70% air, and brick 20%. Air is a poor conductor of heat. Recently, “porous” bricks have been used in construction to reduce thermal conductivity.
2. How is energy transferred from the heat source to the boy? To a boy sitting by the stove, energy is mainly transferred by thermal conductivity.
3. How is energy transferred from the heat source to the boy?
To a boy lying on the sand, energy is transferred from the sun by radiation, and from the sand by thermal conductivity.
4. In which of these cars are perishable products transported? Why? Perishable products are transported in wagons painted white, since such a wagon is less heated by the sun's rays.
5. Why don't waterfowl and other animals freeze in winter?
Fur, wool, and down have poor thermal conductivity (the presence of air between the fibers), which allows the animal’s body to retain the energy generated by the body and protect itself from cooling.
6. Why are window frames made double?
Between the frames there is air, which has poor thermal conductivity and protects against heat loss.
“The world is more interesting than we think”, Alexander Pushnoy, Galileo program.
V. Lesson summary
– What types of heat transfer did we get acquainted with?
– Determine which type of heat transfer plays a major role in the following situations:
a) heating water in a kettle (convection);
b) a person warms himself by the fire (radiation);
c) heating of the table surface from the switched on table lamp (radiation);
d) heating a metal cylinder immersed in boiling water (thermal conductivity).
Solve the crossword puzzle(slide 26):
1. The value on which the radiation intensity depends.
2. A type of heat transfer that can be carried out in a vacuum.
3. The process of changing internal energy without doing work on the body or the body itself.
4. The main source of energy on Earth.
5. Mixture of gases. Has poor thermal conductivity.
6. The process of converting one type of energy into another.
7. Metal with the best thermal conductivity.
8. Rarefied gas.
9. A quantity that has the property of conservation.
10. Type of heat transfer, which is accompanied by the transfer of matter.
Having solved the crossword puzzle, you got another word that is synonymous with the word “heat transfer” - this word... (“heat exchange”). “Heat transfer” and “heat exchange” are the same words. Use them by replacing one with the other.
VI. Homework
§ 4, 5, 6, Ex. 1 (3), Ex. 2(1), Ex. 3(1) – in writing.
VII. Reflection
At the end of the lesson, we invite students to discuss the lesson: what they liked, what they would like to change, and evaluate their participation in the lesson.
The bell is now ringing,
The lesson has come to an end.
Goodbye friends,
It's time to rest.
Subject: Physics and Astronomy
Class: 8 rus
Subject: Thermal conduction, convection, radiation.
Lesson type: Combined
Purpose of the lesson:
Educational: introduce the concept of heat transfer, types of heat transfer, explain that heat transfer with any type of heat transfer always goes in one direction; that depending on the internal structure, the thermal conductivity of various substances (solid, liquid and gaseous) is different, that a black surface is the best emitter and the best absorber of energy.
Developmental: develop cognitive interest in the subject.
Educational: to develop a sense of responsibility, the ability to competently and clearly express one’s thoughts, be able to behave and work in a team
Intersubject communication: chemistry, mathematics
Visual aids: 21-30 drawings, thermal conductivity table
Technical training aids: ___________________________________________________
_______________________________________________________________________
Lesson structure
1. ABOUTlesson organization(2 minutes.)
Greeting students
Checking student attendance and class readiness for class.
2. Homework survey (15 min) Topic: Internal energy. Ways to change internal energy.
3. Explanation of new material. (15 minutes)
A method of changing internal energy in which particles of a more heated body, having greater kinetic energy, upon contact with a less heated body, transfer energy directly to the particles of a less heated body is calledheat transfer There are three methods of heat transfer: thermal conductivity, convection and radiation.
These types of heat transfer have their own characteristics, however, heat transfer with each of them always goes in the same direction: from a more heated body to a less heated one . In this case, the internal energy of a hotter body decreases, and that of a colder body increases.
The phenomenon of energy transfer from a more heated part of the body to a less heated one or from a more heated body to a less heated one through direct contact or intermediate bodies is calledthermal conductivity.
In a solid body, particles are constantly in oscillatory motion, but do not change their equilibrium state. As the temperature of a body increases when it is heated, the molecules begin to vibrate more intensely, as their kinetic energy increases. Part of this increased energy is gradually transferred from one particle to another, i.e. from one part of the body to neighboring parts of the body, etc. But not all solids transfer energy equally. Among them there are so-called insulators, in which the mechanism of thermal conduction occurs quite slowly. These include asbestos, cardboard, paper, felt, granite, wood, glass and a number of other solids. Medb and silver have greater thermal conductivity. They are good heat conductors.
Liquids have low thermal conductivity. When a liquid is heated, internal energy is transferred from a more heated region to a less heated one during collisions of molecules and partly due to diffusion: faster molecules penetrate into a less heated region.
In gases, especially rarefied ones, the molecules are located at fairly large distances from each other, so their thermal conductivity is even less than that of liquids.
The perfect insulator is vacuum , because it lacks particles to transfer internal energy.
Depending on the internal state, the thermal conductivity of different substances (solid, liquid and gaseous) is different.
Thermal conductivity depends on the nature of energy transfer in a substance and is not related to the movement of the substance itself in the body.
It is known that the thermal conductivity of water is low, and when the upper layer of water is heated, the lower layer remains cold. Air is an even worse conductor of heat than water.
Convection - is a heat transfer process in which energy is transferred by jets of liquid or gas. Convection in Latin means"mixing". Convection does not exist in solids and does not occur in a vacuum.
Widely used in everyday life and technology, covection is natural or free .
When liquids or gases are mixed with a pump or stirrer to uniformly mix them, convection is called forced.
A heat sink is a device that is a flat cylindrical container made of metal, one side of which is black and the other shiny. There is air inside it, which, when heated, can expand and escape out through the hole.
In the case when heat is transferred from a heated body to a heat sink using heat rays invisible to the eye, the type of heat transfer is calledradiation or radiant heat transfer
Absorption called the process of converting radiation energy into internal energy of the body
Radiation (or radiant heat transfer) is the process of transferring energy from one body to another using electromagnetic waves.
The higher the body temperature, the higher the radiation intensity. The transfer of energy by radiation does not require a medium: heat rays can also propagate through a vacuum.
Black surface-best emitter and best absorber, followed by rough, white and polished surfaces.
Good energy absorbers are good energy emitters, and bad energy absorbers are bad energy emitters.
4. Consolidation:(10 min) Self-test questions, assignments and exercises
specific tasks: 1) Comparison of the thermal conductivity of metal and glass, water and air, 2) Observation of convection in a living room.
6. Assessment of student knowledge. (1 min)
Basic literature: Physics and astronomy grade 8
Additional reading: N. D. Bytko “Physics” parts 1 and 2