, Grade 10
Subject: "
Temperature and thermal equilibrium
»
thermal phenomena
What types of heat transfer do you know?
Convection;
Thermal conductivity;
Radiation.
What is thermal conductivity?
Answer: heat transfer during the interaction of particles.
Which substances have the highest and lowest thermal conductivity?
Answer: the largest - for metals, the smallest - for gases.
What is the phenomenon of convection?
Answer: heat transfer by liquid or gas flows.
What explains convection?
Answer: the movement of flows of warm gas and liquid is explained by the Archimedean force.
What types of convection do you know?
Answer: natural and forced.
The energy that a body receives or loses during heat transfer is called ...
the amount of warmth.
1. What is the remote heat capacity of a substance?
- a value showing how much heat is required to change the temperature of a substance weighing 1 kg by 1 0C.
2. Different substances have specific heat capacity ...
3. For substances in different states of aggregation (ice, water, steam), specific heat capacity ...
Task. Calculate the amount of heat required to heat a copper part with a mass of 2 kg to change its temperature by 100 0C.
You can download the presentation by clicking on the text Download presentation and installing Microsoft PowerPoint.
Sent by teacher Miroshnichenko.
In the previous paragraph, we found out that when a metal needle was lowered into a glass of hot water, very soon the end of the spoke also became hot. Consequently, internal energy, like any kind of energy, can be transferred from one body to another. Internal energy can also 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, which is in the hand, will gradually heat up and burn the hand.
The phenomenon of transfer of internal energy from one part of the body to another or from one body to another when they are in direct contact is called thermal conductivity.
Let us study this phenomenon by doing a series of experiments with solids, liquids and gases.
Let's put the end of a wooden stick into the fire. It will ignite. The other end of the stick, which is outside, will be cold. So the tree has poor thermal conductivity.
We bring the end of a thin glass rod to the flame of the spirit lamp. After a while, it will heat up, while the other end will remain cold. Consequently, glass also 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 can no longer hold it in our hands.
This means that metals conduct heat well, that is, they have great thermal conductivity. Silver and copper have the highest thermal conductivity.
Consider the transfer of heat from one part of a solid body to another in the following experiment.
We fix one end of a thick copper wire in a tripod. Attach a few carnations to the wire with wax. When the free end of the wire is heated in the flame of an alcohol lamp, the wax will melt. The carnations will start to fall off gradually (Fig. 5). First, those that are closer to the flame will disappear, then all the rest in turn.
Rice. 5. Transfer of heat from one part of a solid body to another
Let's find out how energy is transferred along the wire. The speed of the oscillatory movement of metal particles increases in that part of the wire that is closer to the flame. Since particles constantly interact with each other, the speed of movement of neighboring particles increases. The temperature of the next piece of wire begins to rise, and so on.
It should be remembered that during heat conduction there is no transfer of matter from one end of the body to the other.
Consider now the thermal conductivity of liquids. Take a test tube with water and begin to heat its upper part. The water at the surface will soon boil, and at the bottom of the test tube, during this time, it will only heat up (Fig. 6). This means that liquids have low thermal conductivity, with the exception of mercury and molten metals.
Rice. 6. Thermal conductivity of liquid
This is due to the fact that in liquids the molecules are located at greater distances from each other than in solids.
We investigate the thermal conductivity of gases. We put a dry test tube on a finger and heat it in the flame of an alcohol lamp with the bottom up (Fig. 7). The finger will not feel warm for a long time.
Rice. 7. Gas thermal conductivity
This is due to the fact that the distance between gas molecules is even greater than that of liquids and solids. Therefore, the thermal conductivity of gases is even less.
So, thermal conductivity of different substances is different.
The experience shown in Figure 8 shows that the thermal conductivity of different metals is not the same.
Rice. 8. Thermal conductivity of different metals
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, heat conduction cannot take place.
If there is a need to protect the body from cooling or heating, then substances with low thermal conductivity are used. So, for pots, pans, handles are made of plastic. Houses are built from logs or bricks, which have poor thermal conductivity, which means that they protect the premises from cooling.
Questions
- How is energy transferred through a metal wire?
- Explain the experience (see Fig. 8) showing that the thermal conductivity of copper is greater than the thermal conductivity of steel.
- Which substances have the highest and lowest thermal conductivity? Where are they used?
- Why do fur, down, feathers on the body of animals and birds, as well as human clothing protect from the cold?
Exercise 3
- Why does deep loose snow protect winter crops from freezing?
- It is estimated that the thermal conductivity of pine boards is 3.7 times greater than that of pine sawdust. How to explain such a difference?
- Why doesn't water freeze under a thick layer of ice?
- Why is the expression "fur coat warm" incorrect?
Exercise
Take a cup of hot water and dip a metal and wooden spoon into the water at the same time. Which spoon will heat up faster? How is heat exchanged between water and spoons? How does the internal energy of water and spoons change?
Internal energy, like any other type of energy, can be transferred from one body to another. We already considered one of the examples of such a transfer- transfer of energy from hot water to a cold spoon. This type of heat transfer is called conduction.
Thermal conductivity can be observed in the following experiment. They fix one end of a thick copper wire in a tripod, and several carnations are attached to the wire with wax (Fig. 183). At heating the free end of the wire in the flame of an alcohol lamp wax melts, and the studs gradually fall off the wire. First, those that are closer to the flame will disappear, then all the rest in turn.
How does energy transfer through a wire?
First, the hot flame causes an increase in the oscillatory motion of the metal particles at one end of the wire and its temperature rises. Then this increase in motion is transferred to neighboring particles, and the speed of their oscillations also increases, i.e. the temperature of the next part of the wire rises. Then the speed of oscillation of the next particles increases, and so on. It is very important to note that during heat conduction, the substance itself does not move from one end of the body to the other.
Different substances have different thermal conductivity. This can be seen in an experiment in which energy is transferred through rods of different metals (Fig. 184). And from life experience we know that some substances have a greater thermal conductivity than others. An iron nail, for example, cannot be heated for a long time while holding it in the hand, but a burning match can be held until the flame touches the hand.
Metals have a high thermal conductivity, especially silver and copper.
In liquids, with the exception of molten metals, such as mercury, thermal conductivity is low. Gases have a lower thermal conductivity. After all their molecules are far apart and the transfer of motion from one molecule to another is difficult.
Wool, down, fur and other porous bodies contain air between their fibers and therefore have poor thermal conductivity. That's why wool fur, fluff protect animals from cooling. Protects animals from cooling and the fat layer, which is available in waterfowl, whales, walruses, seals.
Vacuum, a highly rarefied gas, has the lowest thermal conductivity. This is explained by the fact that thermal conductivity, i.e., the transfer of energy from one part of the body to another carry out molecules or other particles - therefore, where there are no particles, heat conduction cannot occur.
Substances with low thermal conductivity are used where it is necessary to conserve energy. For example, brick walls help to keep the internal energy in the room. Can to protect the body, and from heating, for example, ice in the cellar is preserved, lining the cellar with straw, sawdust and earth, which have poor thermal conductivity.
Questions. one. What experiment can be used to observe the transfer of internal energy by a solid body? 2. How is the transfer of energy through a metal wire? 3. What substances have the highest and lowest thermal conductivity? Where are they used?
Exercises. one. Why does deep loose snow protect winter crops from freezing? 2. Explain why straw, hay, dry leaves have poor thermal conductivity. 3. It is calculated that the thermal conductivity of pine boards is 3.7 times greater than that of pine sawdust, the thermal conductivity of ice is 21.6 times greater than that of freshly fallen snow (snow consists of small ice crystals). How to explain such a difference? 4. Why is the expression "fur coat warm" incorrect? 5. Scissors and pencils lying on the table have the same temperature. Why do scissors feel colder to the touch? 6. Explain how fur, down, feathers on the body of animals, as well as human clothing protect from the cold.
