Presentation: Temperature and thermal equilibrium - knowledge hypermarket. Presentation: Temperature and thermal equilibrium - Hypermarket Knowledge What bodies have the greatest and smallest thermal conductivity

Inner energy, like any, other type of energy, can be transmitted from one body to another. We already considered one of the examples of such a transfer. - Transmission of energy from hot water to a cold spoon. This type of heat transfer is called: thermal conductivity.

Thermal conductivity can be observed in the following experience. One end of thick copper wire is fixed in a tripod, and several carnations are attached to the wire (Fig. 183). For heating the free end of the wire in the flame alcohol wax Melts, and carnations gradually disappear from the wire. First, those who are located closer to the flame will be disappeared, then in turns all the rest.

How is the transmission of energy on the wire?

First, the hot flame causes an increase in the oscillatory movement of metal particles at one end of the wire and its temperature rises. Then this increase in motion is transmitted 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 rate of oscillation of the following particles is increasing, etc. It is very important to note that during thermal conductivity itself does not move from one end of the body to another.

Various substances have different thermal conductivity. This can be seen on the experience in which the energy is transmitted by rods from different metals (Fig. 184). And from life experience, we know that some substances have greater thermal conductivity than others.Iron nail, for example, can not be heated for a long time, holding in your hand, and you can keep the burning match until the flame is touched by hands.

Metals, especially silver and copper, have greater thermal conductivity.

In liquids, with the exception of molten metals, such as mercury, thermal conductivity is small. Gas thermal conductivity even less. After all molecules are far from each other. And the transfer of movement from one molecule to another is difficult.

Wool, fluff, fur and other porous bodies between their fibers contain air and therefore possess bad thermal conductivity. That's why wool, fur, fluff protect animals from cooling. Protects animals from cooling and a fat layer, which is available from waterfowl, whale, walruses, seals.

The smallest thermal conductivity has a vacuum - strongly hot gas. It is explained by the fact that thermal conductivity, i.e., energy transfer from one part of the body to another Molecules or other particles are carried out - consequently, where there are no particles, the thermal conductivity cannot be carried out.

Substances with low thermal conductivity are used where it is necessary to maintain energy. For example, brick walls help keep indoor indoor energy. Can protect body, and from heating, such as ice in the cellar maintain,landing the pellar of straw, sawdust and earth, which have bad thermal conductivity.

Questions. one.What experience can you observe the transmission of internal energy with a solid body? 2. How is the transmission of energy for metal wire? 3. What substances have the greatest and smallest thermal conductivity? Where are they used?

Exercises. one.Why deep loose snow protects winter bread from freezing? 2. Explain why straw, hay, dry leaves have bad thermal conductivity. 3. It is estimated that the thermal conductivity of pine boards is 3.7 times larger than pine sawdust, the thermal conductivity of ice is 21.6 times more than freshly-haired snow (the snow consists of small ice crystals). How to explain this difference? 4. Why the expression "fur coat" is incorrect? 5. Scissors and pencils lying on the table have the same temperature. Why does the scissors seem colder? 6. Explain how fur, fluff, feathers on the body of animals, as well as human clothes protect from cold.

In the previous paragraph, we found out that when the metal knitting needles is lowered into a glass with hot water, the end of the knitting needles became also hot. Consequently, internal energy, like any kind of energy, can be transferred from some bodies to others. Internal energy can be transmitted from one part of the body to another. For example, if one end of the nail is heated in the flame, then its other end, which is in hand, gradually heats up and burn hand.

The phenomenon of transmission of internal energy from one part of the body to another or from one body to another with direct contact is called thermal conductivity.

We study this phenomenon, having done a number of experiments with solid bodies, liquid and gas.

We will enter the end of a wooden stick. He will ignite. The other end of the stick, which is outside, will be cold. So the tree has bad thermal conductivity.

We bring to the flame alcohol end thin glass sticks. After some time, he warms up, the other end will remain cold. Consequently, the glass has bad thermal conductivity.

If we heat the end of the metal rod in the flame, then very soon the whole rod is very hot. Hold it in your hands we can no longer.

It means that the metals are well carried out heat, i.e. have great thermal conductivity. Silver and copper have the greatest thermal conductivity.

Consider the transfer of heat from one part of the solid body to the other at the next experience.

Fight one end of thick copper wire in a tripod. To the wire we will attach a few carnations wax. When heating the free end of the wire in the flame of the alcohol wax will melt. Carnations will begin to fall off gradually (Fig. 5). First, those who are located closer to the flame will be disappeared, then in turns all the rest.

Fig. 5. Heat transmission from one part of the solid body to another

Find out how the power is transmitted on the wire. The speed of the oscillatory motion of the metal particles increases in the part of the wire, which is closer to the flame. Since the particles constantly interact with each other, the speed of movement of neighboring particles increases. Begins to increase the temperature of the next part of the wire, etc.

It should be remembered that with thermal conductivity, there is no transfer of the substance from one end of the body to another.

Consider now the thermal conductivity of liquids. Take a test tube with water and get it over its upper part. The surface of the surface will soon boil, and at the bottom of the test tube during this time it is just heated (Fig. 6). It means that liquids thermal conductivity is small, with the exception of mercury and molten metals.

Fig. 6. Thermal conductivity of fluid

This is explained by the fact that in liquids of the molecule is located at large distances from each other than in solid bodies.

We explore the thermal conductivity of gases. Dry tube put on a finger and heated in a flame of an alcohol with a snug up (Fig. 7). Finger at the same time will not feel warmth.

Fig. 7. Gas thermal conductivity

This is due to the fact that the distance between gas molecules is even greater than that of liquids and solid bodies. Consequently, the thermal conductivity of gases is even less.

So, thermal conductivity in various substances is different.

The experience depicted in Figure 8 shows that the thermal conductivity of various metals is not the same.

Fig. 8. Thermal conductivity of different metals

Wool, hair, bird feathers, paper, plug and other porous bodies have bad thermal conductivity. This is due to the fact that there are air between the fibers of these substances. The lowest thermal conductivity has a vacuum (space liberated from air). It is explained by the fact that thermal conductivity is the transfer of energy from one part of the body to another, which occurs when the interaction of molecules or other particles. In space, where there are no particles, the thermal conductivity cannot be carried out.

If there is a need to protect the body from cooling or heating, then apply substances with low thermal conductivity. So, for the saucepan, pens are made from plastic. At home are built from briced or bricks with poor thermal conductivity, and therefore protecting places from cooling.

Questions

How is the transmission of energy on metal wire?
Explain the experience (see Fig. 8), showing that the thermal conductivity of copper is greater than the thermal conductivity of steel.
What substances have the greatest and smallest thermal conductivity? Where are they used?
Why the fur, fluff, feathers on the body of animals and birds, as well as human clothes protect from the cold?

Exercise 3.

Why deep loose snow protects winter bread from freezing?
It is estimated that the thermal conductivity of pine boards is 3.7 times more than pine sawdust. How to explain this difference?
Why water does not freeze under a thick layer of ice?
Why is the expression "fur coat warming" incorrectly?

The task

Take a cup with hot water and at the same time lower the metal and wooden spoons into the water. Which of the spoons will warm up faster? What is the heat exchange between water and spoons? How does the internal energy of water and spoons change?

, Grade 10
Subject: " Temperature and thermal equilibrium »

Heat phenomena

What types of heat transfer do you know?

Convection;

Thermal conductivity;

Radiation.

What is thermal conductivity?

Answer: Heat transfer in the interaction of particles.

What substances have the greatest and smallest thermal conductivity?

Answer: The greatest - from metals, the smallest - in gases.

What is the phenomenon of convection?

Answer: Heat transfer by fluid or gas flows.