Job directory.
Part 2

In the process of boiling a liquid, preheated to the boiling point, the energy imparted to it goes

1) to increase average speed molecular motion

2) to increase the average speed of movement of molecules and to overcome the forces of interaction between molecules

3) to overcome the forces of interaction between molecules without increasing the average speed of their movement

4) to increase the average speed of movement of molecules and to increase the forces of interaction between molecules

Solution.

When boiling, the temperature of the liquid does not change, but there is a process of transition to another state of aggregation. The formation of another state of aggregation occurs with overcoming the forces of interaction between molecules. The constancy of temperature also means the constancy of the average velocity of the molecules.

Answer: 3

Source: GIA in Physics. main wave. Option 1313.

An open vessel with water is placed in a laboratory, which maintains a certain temperature and humidity. The rate of evaporation will be equal to the rate of condensation of water in the vessel

1) only if the temperature in the laboratory is more than 25 °C

2) only under the condition that the humidity in the laboratory is 100%

3) only on condition that the temperature in the laboratory is less than 25 ° C, and the air humidity is less than 100%

4) at any temperature and humidity in the laboratory

Solution.

The rate of evaporation will be equal to the rate of condensation of water in the vessel only if the humidity in the laboratory is 100%, regardless of temperature. In this case, dynamic equilibrium will be observed: how many molecules evaporated, the same number condensed.

The correct answer is numbered 2.

Answer: 2

Source: GIA in Physics. main wave. Option 1326.

1) to heat 1 kg of steel by 1 °C, it is necessary to spend 500 J of energy

2) to heat 500 kg of steel by 1 °C, it is necessary to expend 1 J of energy

3) to heat 1 kg of steel by 500 °C, it is necessary to expend 1 J of energy

4) to heat 500 kg of steel by 1 °C, it is necessary to spend 500 J of energy

Solution.

Specific heat capacity characterizes the amount of energy that must be imparted to one kilogram of a substance for the one of which the body consists, in order to heat it by one degree Celsius. Thus, to heat 1 kg of steel by 1 °C, it is necessary to expend 500 J of energy.

The correct answer is numbered 1.

Answer: 1

Source: GIA in Physics. main wave. Far East. Option 1327.

The specific heat capacity of steel is 500 J/kg °C. What does this mean?

1) when 1 kg of steel is cooled by 1 ° C, energy of 500 J is released

2) when 500 kg of steel is cooled by 1 ° C, energy of 1 J is released

3) when cooling 1 kg of steel at 500 ° C, energy of 1 J is released

4) when cooling 500 kg of steel, 500 J of energy is released by 1 ° C

Solution.

Specific heat capacity characterizes the amount of energy that must be imparted to one kilogram of a substance in order to heat it by one degree Celsius. Thus, to heat 1 kg of steel by 1 °C, it is necessary to expend 500 J of energy.

The correct answer is numbered 1.

Answer: 1

Source: GIA in Physics. main wave. Far East. Option 1328.

Regina Magadeeva 09.04.2016 18:54

In the eighth grade textbook, my definition of specific heat capacity looks like this: physical quantity, numerically equal to the amount of heat that must be transferred to a body with a mass of 1 kg in order for its temperature! to change! by 1 degree. The decision states that specific heat needed to heat up by 1 degree.

Study of the rate of cooling of water in a vessel

under various conditions

Executed the command:

Team number:

Yaroslavl, 2013

a brief description of study parameters

Temperature

The concept of body temperature seems at first glance simple and understandable. Everyone knows from everyday experience that there are hot and cold bodies.

Experiments and observations show that when two bodies come into contact, of which we perceive one as hot and the other as cold, changes in the physical parameters of both the first and second bodies occur. “The physical quantity measured by a thermometer and the same for all bodies or parts of the body that are in thermodynamic equilibrium with each other is called temperature.” When the thermometer is brought into contact with the body under study, we see various kinds of changes: a “column” of liquid moves, the volume of gas changes, etc. But soon thermodynamic equilibrium necessarily sets in between the thermometer and the body - a state in which all quantities characterizing these bodies: their masses, volumes, pressures, and so on. From this point on, the thermometer shows not only its own temperature, but also the temperature of the body being studied. AT Everyday life The most common way to measure temperature is with a liquid thermometer. Here, the property of liquids to expand when heated is used to measure temperature. To measure the temperature of a body, a thermometer is brought into contact with it, a heat transfer process is carried out between the body and the thermometer until thermal equilibrium is established. In order for the measurement process not to noticeably change the body temperature, the mass of the thermometer must be significantly less than the mass of the body whose temperature is being measured.

Heat exchange

Almost all phenomena outside world and various changes in the human body are accompanied by a change in temperature. The phenomena of heat transfer accompany all our daily life.

At the end of the 17th century, the famous English physicist Isaac Newton hypothesized: “The rate of heat transfer between two bodies is the greater, the more their temperatures differ (by the rate of heat transfer we mean the change in temperature per unit time). Heat transfer always occurs in a certain direction: from bodies with more high temperature to bodies with a lower one. We are convinced of this by numerous observations, even at the household level (a spoon in a glass of tea heats up, and the tea cools down). When the temperature of the bodies equalizes, the heat transfer process stops, i.e., thermal equilibrium sets in.

A simple and understandable statement that heat independently transfers only from bodies with a higher temperature to bodies with a lower temperature, and not vice versa, is one of the fundamental laws in physics, and is called the II law of thermodynamics, this law was formulated in the 18th century by the German scientist Rudolf Clausius.

Studycooling rate of water in a vessel under various conditions

Hypothesis: We assume that the rate of cooling of water in a vessel depends on the layer of liquid (oil, milk) poured onto the surface of the water.

Target: Determine whether the surface layer of oil affects and surface layer milk on the rate of cooling of water.

Tasks:
1. Study the phenomenon of water cooling.

2. Determine the dependence of the cooling temperature of water with the surface layer of oil on time, write the results in a table.

3. Determine the dependence of the cooling temperature of water with a surface layer of milk on time, write the results in a table.

4. Build dependency graphs, analyze the results.

5. Make a conclusion about which surface layer on the water has a greater influence on the rate of cooling of water.

Equipment: laboratory glass, stopwatch, thermometer.

Experiment plan:
1. Determination of the division value of the thermometer scale.

2. Measure the water temperature during cooling every 2 minutes.

3. Measure the temperature when the water with the surface layer of oil cools every 2 minutes.

4. Measure the temperature when the water with the surface layer of milk cools down every 2 minutes.

5. Record the measurement results in a table.

6. According to the table, draw graphs of the dependences of the water temperature on time.

8. Analyze the results and give their rationale.

9. Make a conclusion.

Completing of the work

First, we heated water in 3 glasses to a temperature of 71.5⁰C. Then we poured vegetable oil into one of the glasses and milk into the other. The oil spread over the surface of the water, forming an even layer. Vegetable oil is a product extracted from vegetable raw materials and consisting of fatty acids and related substances. Milk mixed with water (forming an emulsion), this indicated that the milk was either diluted with water and did not correspond to the fat content stated on the package, or it was made from a dry product, and in both cases the physical properties of the milk change. Natural milk undiluted with water in water is collected in a clot and does not dissolve for some time. To determine the cooling time of liquids, we fixed the cooling temperature every 2 minutes.

Table. Study of the cooling time of liquids.

According to the table, we see that the initial conditions in all experiments were the same, but after 20 minutes of the experiment, the liquids have different temperatures, which means they have different cooling rates of the liquid.

This is shown more clearly in the graph.

In the coordinate plane with the axes temperature and time marked points displaying the relationship between these quantities. Averaging the values, draw a line. The chart turned out linear dependence cooling temperature of water from cooling time under various conditions.

Calculate the rate of cooling of water:

a) for water

0-10 min (ºС/min)

10-20 min (ºС/min)
b) for water with a surface layer of oil

0-10 min (ºС/min)

10-20 min (ºС/min)
b) for water with milk

0-10 min (ºС/min)

10-20 min (ºС/min)

As can be seen from the calculations, water with oil cooled the slowest. This is due to the fact that the oil layer does not allow water to intensively exchange heat with air. This means that the heat exchange of water with air slows down, the rate of cooling of water decreases, and the water remains hotter longer. This can be used when cooking, for example, when cooking pasta, after boiling water, add oil, the pasta will cook faster and will not stick together.

Water without any additives has the highest cooling rate, which means it will cool faster.

Conclusion: thus, we have experimentally verified that the surface layer of oil has a greater effect on the rate of cooling of water, the rate of cooling decreases and the water cools more slowly.

The same substance in the real world, depending on the surrounding conditions, can be in different states. For example, water can be in the form of a liquid, in the idea of ​​a solid body - ice, in the form of a gas - water vapor.

• These states are called aggregate states of matter.

Molecules of a substance in different states of aggregation do not differ from each other. A specific state of aggregation is determined by the arrangement of molecules, as well as the nature of their movement and interaction with each other.

Gas - the distance between molecules is significantly more sizes the molecules themselves. Molecules in a liquid and in a solid are quite close to each other. AT solids even closer.

To change the aggregate body condition, he needs to give some energy. For example, in order to convert water into steam, it must be heated. In order for steam to become water again, it must give up energy.

The transition from solid to liquid

The transition of a substance from a solid to a liquid state is called melting. In order for the body to begin to melt, it must be heated to a certain temperature. The temperature at which a substance melts is called the melting point of the substance.

Each substance has its own melting point. For some bodies it is very low, for example, for ice. And some bodies have a very high melting point, for example, iron. In general, melting crystalline body it is a complicated process.

ice melt chart

The figure below shows a graph of the melting of a crystalline body, in this case ice.

• The graph shows the dependence of the temperature of the ice on the time that it is heated. Temperature is plotted on the vertical axis, time is plotted on the horizontal axis.

From the graph, the initial temperature of the ice was -20 degrees. Then they started to heat it up. The temperature started to rise. Section AB is the section of ice heating. Over time, the temperature increased to 0 degrees. This temperature is considered the melting point of ice. At this temperature, the ice began to melt, but at the same time its temperature ceased to increase, although the ice also continued to heat up. The melting area corresponds to the BC section on the graph.

Then, when all the ice melted and turned into a liquid, the temperature of the water began to increase again. This is shown on the graph by ray C. That is, we conclude that during melting, the body temperature does not change, All incoming energy is used for heating.

1. Plot temperature (t i) (for example t 2) versus heating time (t, min). Verify that steady state is reached.

3. Calculate the values ​​of and lnA only for the stationary mode, enter the results of the calculations in the table.

4. Construct a graph of the dependence on x i, taking the position of the first thermocouple x 1 = 0 as the origin (the coordinates of the thermocouples are indicated on the installation). Draw a straight line through the given points.

5. Determine the average tangent of the slope or

6. Using formula (10), taking into account (11), calculate the thermal conductivity of the metal and determine the measurement error.

7. Using a reference book, determine the metal from which the rod is made.

test questions

1. What phenomenon is called thermal conductivity? Write down his equation. What characterizes the temperature gradient?

2. What is the carrier of thermal energy in metals?

3. What mode is called stationary? Get equation (5) describing this mode.

4. Derive formula (10) for the thermal conductivity coefficient.

5. What is a thermocouple? How can it be used to measure the temperature at a certain point on the rod?

6. What is the method for measuring thermal conductivity in this work?

Laboratory work № 11

Fabrication and calibration of a temperature sensor based on a thermocouple

Objective: familiarization with the method of manufacturing a thermocouple; manufacturing and calibration of a temperature sensor based on a thermocouple; using a temperature probe to determine the melting point of Wood's alloy.

Introduction

Temperature is a physical quantity that characterizes the state of thermodynamic equilibrium of a macroscopic system. In equilibrium conditions, the temperature is proportional to the average kinetic energy thermal motion of body particles. The temperature range at which physical, chemical and other processes take place is exceptionally wide: from absolute zero to 10 11 K and above.

Temperature cannot be measured directly; its value is determined by the temperature change, any convenient for measurements physical property substances. Such thermometric properties can be: gas pressure, electrical resistance, thermal expansion of liquid, speed of sound propagation.

When constructing a temperature scale, the temperature value t 1 and t 2 is assigned to two fixed temperature points (the value of the measured physical parameter) x \u003d x 1 and x \u003d x 2, for example, the melting point of ice and the boiling point of water. The temperature difference t 2 - t 1 is called the main temperature interval of the scale. The temperature scale is a specific functional numerical relationship of temperature with the values ​​of the measured thermometric property. An unlimited number of temperature scales is possible, differing in thermometric property, accepted dependence t(x) and temperatures of fixed points. For example, there are scales of Celsius, Réaumur, Fahrenheit, and others. The fundamental disadvantage of empirical temperature scales is their dependence on the thermometric substance. This shortcoming is absent in the thermodynamic temperature scale based on the second law of thermodynamics. For equilibrium processes, the equality is true:

where: Q 1 - the amount of heat received by the system from the heater at temperature T 1; and Q 2 - the amount of heat given to the refrigerator at a temperature of T 2. The ratios do not depend on the properties of the working fluid and make it possible to determine the thermodynamic temperature from the values ​​Q 1 and Q 2 available for measurements. It is customary to consider T 1 \u003d 0 K - at absolute zero temperatures and T 2 \u003d 273.16 K in triple point water. The temperature on the thermodynamic scale is expressed in degrees Kelvin (0 K). The introduction of T 1 = 0 is an extrapolation and does not require the implementation of absolute zero.

When measuring thermodynamic temperature, one of the strict consequences of the second law of thermodynamics is usually used, which connects a conveniently measured thermodynamic property with thermodynamic temperature. Among such relationships: the laws of an ideal gas, the laws of black body radiation, etc. Over a wide range of temperatures, roughly from the boiling point of helium to the solidification point of gold, the most accurate thermodynamic temperature measurements are provided by a gas thermometer.

In practice, measuring temperature on a thermodynamic scale is difficult. The value of this temperature is usually marked on a convenient secondary thermometer, which is more stable and sensitive than instruments reproducing the thermodynamic scale. Secondary thermometers are calibrated according to highly stable reference points, the temperatures of which, according to the thermodynamic scale, are found in advance by extremely accurate measurements.

In this work, a thermocouple (contact of two different metals) is used as a secondary thermometer, and melting and boiling temperatures are used as reference points. various substances. The thermometric property of a thermocouple is the contact potential difference.

A thermocouple is called a closed electrical circuit containing two junctions of two different metal conductors. If the temperature of the junctions is different, then the circuit will go due to the thermoelectromotive force electricity. The value of thermoelectromotive force e is proportional to the temperature difference:

where k is const if the temperature difference is not very large.

The value of k usually does not exceed several tens of microvolts per degree and depends on the materials from which the thermocouple is made.

Exercise 1. Thermocouple manufacturing

For this task, you can get 2 points on the exam in 2020

Task 11 of the USE in physics is devoted to the basics of thermodynamics and molecular kinetic theory. The general theme of this ticket is the explanation of various phenomena.

Task 11 of the Unified State Examination in physics is always constructed in the same way: the student will be offered a graph or a description of any dependence (the release of thermal energy when a body is heated, a change in gas pressure depending on its temperature or density, any processes in ideal gas). After that, five statements are given, directly or indirectly related to the topic of the ticket and representing a textual description of thermodynamic laws. Of these, the student must select two statements that he considers true, corresponding to the condition.

Task 11 of the Unified State Examination in Physics usually scares students, because it contains a lot of digital data, tables, and graphs. In fact, it is theoretical, and the student will not have to calculate anything when answering the question. Therefore, in fact, this question usually does not cause any special difficulties. However, the student must adequately assess his abilities and it is not recommended to “stay up” on the eleventh task, because the time to complete the entire test is limited to a certain number of minutes.