The internal energy of a body can be changed by work external forces. To characterize the change internal energy during heat exchange, a quantity is introduced, called the amount of heat and denoted by Q.

AT international system the unit of heat quantity, as well as work and energy, is the joule: = = = 1 J.

In practice, an off-system unit of the amount of heat is sometimes used - a calorie. 1 cal. = 4.2 J.

It should be noted that the term "quantity of heat" is unfortunate. It was introduced at a time when it was believed that bodies contained some weightless, elusive liquid - caloric. The process of heat transfer allegedly consists in the fact that caloric, pouring from one body into another, carries with it a certain amount of heat. Now, knowing the basics of the molecular-kinetic theory of the structure of matter, we understand that there is no caloric in bodies, the mechanism for changing the internal energy of a body is different. However, the power of tradition is great and we continue to use the term, introduced on the basis of incorrect ideas about the nature of heat. At the same time, understanding the nature of heat transfer, one should not completely ignore misconceptions about it. On the contrary, by drawing an analogy between the flow of heat and the flow of a hypothetical liquid of caloric, the amount of heat and the amount of caloric, it is possible to visualize the ongoing processes in solving certain classes of problems and solve problems correctly. In the end, the correct equations describing the processes of heat transfer were obtained at one time on the basis of incorrect ideas about caloric as a heat carrier.

Let us consider in more detail the processes that can occur as a result of heat transfer.

Pour some water into a test tube and close it with a cork. Hang the test tube to a rod fixed in a tripod and bring an open flame under it. From the flame, the test tube receives a certain amount of heat and the temperature of the liquid in it rises. As the temperature rises, the internal energy of the liquid increases. There is an intensive process of its vaporization. Expanding liquid vapors mechanical work pushing the stopper out of the tube.

Let's conduct another experiment with a model gun made from a piece of brass tube, which is mounted on a trolley. On one side, the tube is tightly closed with an ebonite plug, through which a pin is passed. Wires are soldered to the stud and tube, ending in terminals that can be energized from the lighting network. The gun model is thus a kind of electric boiler.

Pour some water into the cannon barrel and close the tube with a rubber stopper. Connect the gun to a power source. Electricity, passing through the water, heats it. Water boils, which leads to its intense vaporization. The pressure of water vapor increases and, finally, they do the work of pushing the cork out of the gun barrel.

The gun, due to recoil, rolls back in the direction opposite to the cork launch.

Both experiences are united by the following circumstances. In the process of heating the liquid in various ways, the temperature of the liquid and, accordingly, its internal energy increased. In order for the liquid to boil and evaporate intensively, it was necessary to continue heating it.

The vapors of the liquid, due to their internal energy, performed mechanical work.

We investigate the dependence of the amount of heat required to heat the body on its mass, temperature changes and the type of substance. To study these dependencies, we will use water and oil. (To measure the temperature in the experiment, an electric thermometer is used, made of a thermocouple connected to a mirror galvanometer. One thermocouple junction is lowered into a vessel with cold water to ensure its temperature is constant. The other thermocouple junction measures the temperature of the liquid under study).

The experience consists of three series. In the first series, for a constant mass of a particular liquid (in our case, water), the dependence of the amount of heat required to heat it on temperature changes is studied. The amount of heat received by the liquid from the heater (electric stove) will be judged by the heating time, assuming that there is a directly proportional relationship between them. In order for the result of the experiment to correspond to this assumption, it is necessary to ensure a steady flow of heat from the electric stove to the heated body. To do this, the electric stove was connected to the network in advance, so that by the beginning of the experiment the temperature of its surface would cease to change. For more uniform heating of the liquid during the experiment, we will stir it with the help of the thermocouple itself. We will record the readings of the thermometer at regular intervals until the light spot reaches the edge of the scale.

Let us conclude: there is a direct proportional relationship between the amount of heat required to heat a body and a change in its temperature.

In the second series of experiments, we will compare the amount of heat required to heat the same liquids of different masses when their temperature changes by the same amount.

For the convenience of comparing the obtained values, the mass of water for the second experiment will be taken two times less than in the first experiment.

Again, we will record the thermometer readings at regular intervals.

Comparing the results of the first and second experiments, we can draw the following conclusions.

In the third series of experiments, we will compare the amounts of heat required to heat equal masses of different liquids when their temperature changes by the same amount.

We will heat oil on an electric stove, the mass of which is equal to the mass of water in the first experiment. We will record the thermometer readings at regular intervals.

The result of the experiment confirms the conclusion that the amount of heat necessary to heat the body is directly proportional to the change in its temperature and, in addition, indicates the dependence of this amount of heat on the type of substance.

Since oil was used in the experiment, the density of which is less than the density of water, and a smaller amount of heat was required to heat the oil to a certain temperature than to heat water, it can be assumed that the amount of heat required to heat the body depends on its density.

To test this assumption, we will simultaneously heat on the heater constant power equal masses of water, paraffin and copper.

After the same time, the temperature of copper is about 10 times, and paraffin is about 2 times higher than the temperature of water.

But copper has a greater and paraffin less density than water.

Experience shows that the quantity that characterizes the rate of change in the temperature of the substances from which the bodies involved in heat exchange are made is not the density. This quantity is called the specific heat capacity of the substance and is denoted by the letter c.

To compare specific heat capacities various substances is a special device. The device consists of racks in which a thin paraffin plate and a bar with rods passed through it are attached. Aluminum, steel and brass cylinders of equal mass are fixed at the ends of the rods.

We heat the cylinders to the same temperature by immersing them in a vessel of water standing on a hot electric stove. Let's fix the hot cylinders on the racks and release them from the fasteners. The cylinders simultaneously touch the paraffin plate and, melting the paraffin, begin to sink into it. The depth of immersion of cylinders of the same mass into a paraffin plate, when their temperature changes by the same amount, turns out to be different.

Experience shows that the specific heat capacities of aluminum, steel and brass are different.

Having done the corresponding experiments with melting solids, vaporization of liquids, combustion of fuel, we obtain the following quantitative dependences.

To obtain units of specific quantities, they must be expressed from the corresponding formulas and substitute the units of heat - 1 J, mass - 1 kg, and for specific heat - and 1 K into the resulting expressions.

We get units: specific heat capacity - 1 J/kg·K, other specific heats: 1 J/kg.

and this released heat equal to 10 MJ? 3. How much heat is needed to turn 250g of ether into vapor at a temperature of 35C? 4. How much energy is required to heat and melt lead weighing 0.4 kg with an initial temperature of 17C? 5. Dry pine firewood with a volume of 2 m and coal weighing 1.5 tons were prepared for winter. How much heat is released in the furnace with the complete combustion of this fuel? 6. Calculate the amount of heat required to turn 200 g of alcohol into vapor. at a temperature of 28C? 7. What will be the final temperature if ice weighing 500 g at a temperature of 0C is immersed in water with a volume of 4 liters at a temperature of 30C? 8. How much pine wood must be used to turn 1500 kg of snow taken at a temperature of -10C into water at a temperature of 5C? Heat losses can be neglected

2. At a voltage of 450 V, the current in the motor is 90 A. Determine the current power in the motor winding and its resistance.

3. What is the energy consumption for 40 s in a car light bulb, designed for a voltage of 12 V at a current of 3 A?

LevelII

4. For how long will an electric iron release an amount of heat of 800 J if the current in the spiral is 3 A and the voltage in the network is 220 V?

5. Determine the power consumed by the second lamp (Fig. 126), if the voltmeter reading is 6 V.

6. Determine the power of the electric kettle if 1 kg of water in it heats up from 20 to 80 °C in 5 minutes. Ignore energy losses.

Test No. 4. Work and current power.

Option 3

LevelI

1. What work will the current do in the electric motor in 90 s if, at a voltage of 220 V, the current in the motor winding is 0.2 A?

2. Determine the power of the current in the light bulb, if at a voltage of 5 V the current in it is 100 mA.

3. How much heat will be released in a rheostat with a resistance of 50 ohms in 2 minutes at a current strength of 2 A in the circuit?

LevelII

4. How many degrees in 5 minutes can 1.5 kg of water be heated on an electric stove if, at a voltage of 220 V, the current in it is 5 A? Ignore energy losses.

5. Determine the power consumed by the first lamp (Fig. 127), if the ammeter reading is 2 A.

6. How long does it take to heat 500 g of water in a glass from 20 °C to boiling point using a 500 W electric boiler?

please write

2) what amount of heat is needed to convert a liquid of 1 kg of aluminum and 1 kg of copper having a swimming temperature?

brick fireplace weighing 2 tons from 50 to 20ºС. 3. Calculate the amount of heat required to heat a 500 g iron pan with 2.5 kg of sunflower oil from 20 to 150ºС. 4. To what temperature can 3 kg of lead be heated if an amount of heat equal to 50 kJ is transferred to it and its initial temperature is equal to 10ºС. 5. What is the heat capacity of the metal if 690 kJ of thermal energy was expended to heat 3 kg of this metal from 50 to 300ºС. Make a guess about the name of this metal. Solve all problems

Physics exam for grade 8

4. The amount of heat

The energy that a body gains or loses during heat transfer is called amount of heat. The amount of heat depends

From the mass of the body (the greater the mass of the body, the more heat must be expended in order to heat the body by the same number of degrees);

From the difference in body temperature and depends on;

What substance does the body consist of, that is, from the kind of substance.

The amount of heat is denoted by the letter Q and is measured in joules.

Specific heat

The amount of heat that must be transferred to a body of mass 1 kg in order to heat it by 1 degree C is called specific heat capacity substances. Specific heat capacity is denoted by the letter c and is measured in J / kg * 0 C

It should be remembered that the specific heat capacity of a substance in different states of aggregation is different. The specific heat capacity of water is the largest - 4200 J / kg * 0.

Specific heating value of fuel

When fuel is burned, atoms combine to form molecules, and energy is released.

The physical quantity showing how much heat is released during the complete combustion of fuel weighing 1 kg is called specific heat fuel combustion. The specific heat of combustion is denoted by the letter q. The unit of specific heat of combustion is 1 J/kg. The specific heat of combustion is determined experimentally using rather complex instruments.

Melting and solidification of crystalline bodies

The transition of matter from solid state of aggregation into liquid is called melting.

To melt a body, you must first bring it to a certain temperature.

The temperature at which a substance begins to melt is called melting point substances.

The melting point of substances is different, for example, ice can be melted by bringing it into the room, and iron is melted in special furnaces, where a high temperature is reached.

The transition of a substance from a liquid to a solid state is called crystallization.

In order for a body to crystallize, it must cool down to a certain temperature.

The temperature at which a substance crystallizes is called crystallization temperature.

Experiments show that substances crystallize at the same temperature at which they melt. In order for the body to completely pass from solid state into a liquid, a constant supply of energy is required.

Specific heat of fusion and crystallization

When the body is heated average speed the movement of molecules increases, therefore, increases, and their kinetic energy and temperature. As a result, the range of molecular vibrations increases. When the body is heated to the melting point, the order in the arrangement of particles in the crystals is disturbed. The crystals lose their shape, the body melts.

A physical quantity showing how much heat must be reported crystalline body weighing 1 kg, so that at the melting point it is completely transferred from a solid to a liquid state, is called specific heat of fusion.

The specific heat of fusion is denoted by /\ (lambda). Its unit is 1 J/kg.

At the melting point, the internal energy of a substance in liquid state more than the internal energy of the same mass of matter in the solid state. When a substance solidifies, the same amount of substance is released that was spent on its melting.

The specific heat of fusion is: Q=/\*m.

When a substance solidifies, everything happens in the reverse order:

The average kinetic energy and the speed of molecules in a cooled molten substance decrease. The arrangement of particles becomes ordered - a crystal is formed.