What will heat up faster on the stove - a kettle or a bucket of water? The answer is obvious - a teapot. Then the second question is why?
The answer is no less obvious - because the mass of water in the kettle is less. Great. And now you can do a real physical experience yourself at home. To do this, you will need two identical small saucepans, an equal amount of water and vegetable oil, for example, half a liter each and a stove. Place saucepans with oil and water on the same heat. Now just watch what will heat up faster. If you have a thermometer for liquids, you can use it; if not, you can simply test the temperature with your finger from time to time, just be careful not to get burned. In any case, you will soon see that the oil heats up much faster than water. And one more question, which can also be implemented in the form of experience. What will boil faster - warm water or cold? Everything is obvious again - the warm one will be first at the finish line. Why all these strange questions and experiments? To determine the physical quantity called “amount of heat”.
Quantity of heat
The amount of heat is the energy that a body loses or gains during heat transfer. This is clear from the name. When cooling, the body will lose a certain amount of heat, and when heating, it will absorb. And the answers to our questions showed us What does the amount of heat depend on? Firstly, the greater the mass of a body, the greater the amount of heat that must be expended to change its temperature by one degree. Secondly, the amount of heat required to heat a body depends on the substance of which it consists, that is, on the type of substance. And thirdly, the difference in body temperature before and after heat transfer is also important for our calculations. Based on the above, we can determine the amount of heat using the formula:
where Q is the amount of heat,
m - body weight,
(t_2-t_1) - the difference between the initial and final body temperatures,
c is the specific heat capacity of the substance, found from the corresponding tables.
Using this formula, you can calculate the amount of heat that is necessary to heat any body or that this body will release when cooling.
The amount of heat is measured in joules (1 J), like any type of energy. However, this value was introduced not so long ago, and people began measuring the amount of heat much earlier. And they used a unit that is widely used in our time - calorie (1 cal). 1 calorie is the amount of heat required to heat 1 gram of water by 1 degree Celsius. Guided by these data, those who like to count calories in the food they eat can, for fun, calculate how many liters of water can be boiled with the energy they consume with food during the day.
(or heat transfer).
Specific heat capacity of a substance.
Heat capacity- this is the amount of heat absorbed by a body when heated by 1 degree.
The heat capacity of a body is indicated by a capital Latin letter WITH.
What does the heat capacity of a body depend on? First of all, from its mass. It is clear that heating, for example, 1 kilogram of water will require more heat than heating 200 grams.
What about the type of substance? Let's do an experiment. Let's take two identical vessels and, having poured water weighing 400 g into one of them, and vegetable oil weighing 400 g into the other, we will begin to heat them using identical burners. By observing the thermometer readings, we will see that the oil heats up quickly. To heat water and oil to the same temperature, the water must be heated longer. But the longer we heat the water, the more heat it receives from the burner.
Thus, heating the same mass of different substances to the same temperature requires different amounts of heat. The amount of heat required to heat a body and, therefore, its heat capacity depend on the type of substance of which the body is composed.
So, for example, to increase the temperature of water weighing 1 kg by 1°C, an amount of heat equal to 4200 J is required, and to heat the same mass of sunflower oil by 1°C, an amount of heat equal to 1700 J is required.
A physical quantity showing how much heat is required to heat 1 kg of a substance by 1 ºС is called specific heat capacity of this substance.
Each substance has its own specific heat capacity, which is denoted by the Latin letter c and measured in joules per kilogram degree (J/(kg °C)).
The specific heat capacity of the same substance in different states of aggregation (solid, liquid and gaseous) is different. For example, the specific heat capacity of water is 4200 J/(kg °C), and the specific heat capacity of ice is 2100 J/(kg °C); aluminum in the solid state has a specific heat capacity of 920 J/(kg - °C), and in the liquid state - 1080 J/(kg - °C).
Note that water has a very high specific heat capacity. Therefore, water in the seas and oceans, heating up in summer, absorbs a large amount of heat from the air. Thanks to this, in those places that are located near large bodies of water, summer is not as hot as in places far from the water.
Calculation of the amount of heat required to heat a body or released by it during cooling.
From the above it is clear that the amount of heat required to heat a body depends on the type of substance of which the body consists (i.e., its specific heat capacity) and on the mass of the body. It is also clear that the amount of heat depends on how many degrees we are going to increase the body temperature.
So, to determine the amount of heat required to heat a body or released by it during cooling, you need to multiply the specific heat capacity of the body by its mass and by the difference between its final and initial temperatures:
Q = cm (t 2 - t 1 ) ,
Where Q- quantity of heat, c— specific heat capacity, m- body mass , t 1 — initial temperature, t 2 — final temperature.
When the body heats up t 2 > t 1 and therefore Q > 0 . When the body cools down t 2i< t 1 and therefore Q< 0 .
If the heat capacity of the entire body is known WITH, Q determined by the formula:
Q = C (t 2 - t 1 ) .
The process of transferring energy from one body to another without doing work is called heat exchange or heat transfer. Heat exchange occurs between bodies having different temperatures. When contact is established between bodies with different temperatures, part of the internal energy is transferred from a body with a higher temperature to a body with a lower temperature. The energy transferred to a body as a result of heat exchange is called amount of heat.
Specific heat capacity of a substance:
If the heat transfer process is not accompanied by work, then, based on the first law of thermodynamics, the amount of heat is equal to the change in the internal energy of the body: .
The average energy of the random translational motion of molecules is proportional to the absolute temperature. The change in the internal energy of a body is equal to the algebraic sum of the changes in the energy of all atoms or molecules, the number of which is proportional to the mass of the body, therefore the change in internal energy and, therefore, the amount of heat is proportional to the mass and the change in temperature:
The proportionality factor in this equation is called specific heat capacity of a substance. Specific heat capacity shows how much heat is needed to heat 1 kg of a substance by 1 K.
Work in thermodynamics:
In mechanics, work is defined as the product of the moduli of force and displacement and the cosine of the angle between them. Work is done when a force acts on a moving body and is equal to the change in its kinetic energy.
In thermodynamics, the movement of a body as a whole is not considered; we are talking about the movement of parts of a macroscopic body relative to each other. As a result, the volume of the body changes, but its speed remains equal to zero. Work in thermodynamics is defined in the same way as in mechanics, but is equal to the change not in the kinetic energy of the body, but in its internal energy.
When work is performed (compression or expansion), the internal energy of the gas changes. The reason for this is: during elastic collisions of gas molecules with a moving piston, their kinetic energy changes.
Let us calculate the work done by the gas during expansion. The gas acts on the piston with a force where is the gas pressure and is the surface area of the piston. When the gas expands, the piston moves in the direction of force over a small distance. If the distance is small, then the gas pressure can be considered constant. The work done by the gas is:
Where is the change in gas volume.
In the process of gas expansion, it does positive work, since the direction of the force and displacement coincide. During the expansion process, the gas releases energy to surrounding bodies.
The work done by external bodies on the gas differs from the work of the gas only in sign, since the force acting on the gas is opposite to the force with which the gas acts on the piston and is equal to it in absolute value (Newton’s third law); and the movement remains the same. Therefore, the work of external forces is equal to:
First law of thermodynamics:
The first law of thermodynamics is the law of conservation of energy, extended to thermal phenomena. Law of energy conservation: Energy in nature does not arise from nothing and does not disappear: the amount of energy is unchanged, it only passes from one form to another.
Thermodynamics considers bodies whose center of gravity remains virtually unchanged. The mechanical energy of such bodies remains constant, and only the internal energy can change.
Internal energy can change in two ways: heat transfer and work. In the general case, internal energy changes both due to heat transfer and due to work done. The first law of thermodynamics is formulated precisely for such general cases:
The change in the internal energy of a system during its transition from one state to another is equal to the sum of the work of external forces and the amount of heat transferred to the system:
If the system is isolated, then no work is done on it and it does not exchange heat with surrounding bodies. According to the first law of thermodynamics the internal energy of an isolated system remains unchanged.
Considering that , the first law of thermodynamics can be written as follows:
The amount of heat transferred to the system goes to change its internal energy and to perform work on external bodies by the system.
Second law of thermodynamics: It is impossible to transfer heat from a colder system to a hotter one in the absence of other simultaneous changes in both systems or in surrounding bodies.
« Physics - 10th grade"
In what processes do aggregate transformations of matter occur?
How can you change the state of aggregation of a substance?
You can change the internal energy of any body by doing work, heating or, conversely, cooling it.
So, when forging a metal, work is done and it heats up, at the same time the metal can be heated over a burning flame.
Also, if the piston is fixed (Fig. 13.5), then the volume of gas does not change when heated and no work is done. But the temperature of the gas, and therefore its internal energy, increases.
Internal energy can increase and decrease, so the amount of heat can be positive or negative.
The process of transferring energy from one body to another without doing work is called heat exchange.
The quantitative measure of the change in internal energy during heat transfer is called amount of heat.
Molecular picture of heat transfer.
During heat exchange at the boundary between bodies, the interaction of slowly moving molecules of a cold body with fast moving molecules of a hot body occurs. As a result, the kinetic energies of the molecules are equalized and the speeds of the molecules of a cold body increase, and those of a hot body decrease.
During heat exchange, energy is not converted from one form to another; part of the internal energy of a more heated body is transferred to a less heated body.
Amount of heat and heat capacity.
You already know that to heat a body of mass m from temperature t 1 to temperature t 2 it is necessary to transfer an amount of heat to it:
Q = cm(t 2 - t 1) = cm Δt. (13.5)
When a body cools, its final temperature t 2 turns out to be less than the initial temperature t 1 and the amount of heat given off by the body is negative.
The coefficient c in formula (13.5) is called specific heat capacity substances.
Specific heat- this is a quantity numerically equal to the amount of heat that a substance weighing 1 kg receives or releases when its temperature changes by 1 K.
The specific heat capacity of gases depends on the process by which heat transfer occurs. If you heat a gas at constant pressure, it will expand and do work. To heat a gas by 1 °C at constant pressure, it needs to transfer more heat than to heat it at a constant volume, when the gas will only heat up.
Liquids and solids expand slightly when heated. Their specific heat capacities at constant volume and constant pressure differ little.
Specific heat of vaporization.
To transform a liquid into steam during the boiling process, a certain amount of heat must be transferred to it. The temperature of a liquid does not change when it boils. The transformation of a liquid into vapor at a constant temperature does not lead to an increase in the kinetic energy of the molecules, but is accompanied by an increase in the potential energy of their interaction. After all, the average distance between gas molecules is much greater than between liquid molecules.
A quantity numerically equal to the amount of heat required to convert a liquid weighing 1 kg into steam at a constant temperature is called specific heat of vaporization.
The process of evaporation of a liquid occurs at any temperature, while the fastest molecules leave the liquid, and it cools during evaporation. The specific heat of evaporation is equal to the specific heat of vaporization.
This value is denoted by the letter r and expressed in joules per kilogram (J/kg).
The specific heat of vaporization of water is very high: r H20 = 2.256 10 6 J/kg at a temperature of 100 °C. For other liquids, for example alcohol, ether, mercury, kerosene, the specific heat of vaporization is 3-10 times less than that of water.
To convert a liquid of mass m into vapor, an amount of heat is required equal to:
Q p = rm. (13.6)
When steam condenses, the same amount of heat is released:
Q k = -rm. (13.7)
Specific heat of fusion.
When a crystalline body melts, all the heat supplied to it goes to increase the potential energy of interaction between molecules. The kinetic energy of the molecules does not change, since melting occurs at a constant temperature.
A value numerically equal to the amount of heat required to transform a crystalline substance weighing 1 kg at the melting point into a liquid is called specific heat of fusion and are denoted by the letter λ.
When a substance weighing 1 kg crystallizes, exactly the same amount of heat is released as is absorbed during melting.
The specific heat of melting of ice is quite high: 3.34 10 5 J/kg.
“If ice did not have a high heat of fusion, then in the spring the entire mass of ice would have to melt in a few minutes or seconds, since heat is continuously transferred to the ice from the air. The consequences of this would be dire; after all, even in the current situation, large floods and strong flows of water arise when large masses of ice or snow melt.” R. Black, XVIII century.
In order to melt a crystalline body of mass m, an amount of heat is required equal to:
Qpl = λm. (13.8)
The amount of heat released during crystallization of a body is equal to:
Q cr = -λm (13.9)
Heat balance equation.
Let us consider the heat exchange within a system consisting of several bodies that initially have different temperatures, for example, the heat exchange between water in a vessel and a hot iron ball lowered into the water. According to the law of conservation of energy, the amount of heat given off by one body is numerically equal to the amount of heat received by another.
The amount of heat given is considered negative, the amount of heat received is considered positive. Therefore, the total amount of heat Q1 + Q2 = 0.
If heat exchange occurs between several bodies in an isolated system, then
Q 1 + Q 2 + Q 3 + ... = 0. (13.10)
Equation (13.10) is called heat balance equation.
Here Q 1 Q 2, Q 3 are the amounts of heat received or given off by bodies. These amounts of heat are expressed by formula (13.5) or formulas (13.6)-(13.9), if various phase transformations of the substance (melting, crystallization, vaporization, condensation) occur during the heat exchange process.
Learning Objective: Introduce the concepts of heat quantity and specific heat capacity.
Developmental goal: To cultivate attentiveness; teach to think, draw conclusions.
1. Updating the topic
2. Explanation of new material. 50 min.
You already know that the internal energy of a body can change both by doing work and by heat transfer (without doing work).
The energy that a body gains or loses during heat transfer is called the amount of heat. (write in notebook)
This means that the units for measuring the amount of heat are also Joules ( J).
We conduct an experiment: two glasses in one with 300 g of water, and in the other 150 g, and an iron cylinder weighing 150 g. Both glasses are placed on the same tile. After some time, thermometers will show that the water in the vessel in which the body is located heats up faster.
This means that heating 150 g of iron requires less heat than heating 150 g of water.
The amount of heat transferred to a body depends on the type of substance from which the body is made. (write in notebook)
We propose the question: is the same amount of heat required to heat bodies of equal mass, but consisting of different substances, to the same temperature?
We conduct an experiment with Tyndall's device to determine specific heat capacity.
We conclude: bodies made of different substances, but of the same mass, give up when cooled and require different amounts of heat when heated by the same number of degrees.
We draw conclusions:
1. To heat bodies of equal mass, consisting of different substances, to the same temperature, different amounts of heat are required.
2. Bodies of equal mass, consisting of different substances and heated to the same temperature. When cooled by the same number of degrees, different amounts of heat are released.
We conclude that the amount of heat required to heat a unit mass of different substances by one degree will vary.
We give the definition of specific heat capacity.
A physical quantity numerically equal to the amount of heat that must be transferred to a body weighing 1 kg in order for its temperature to change by 1 degree is called the specific heat capacity of a substance.
Enter the unit of measurement for specific heat capacity: 1J/kg*degree.
Physical meaning of the term : Specific heat capacity shows by what amount the internal energy of 1g (kg) of a substance changes when it is heated or cooled by 1 degree.
Let's look at the table of specific heat capacities of some substances.
We solve the problem analytically
How much heat is required to heat a glass of water (200 g) from 20 0 to 70 0 C.
To heat 1 g per 1 g, 4.2 J is required.
And to heat 200 g by 1 g it will take 200 more - 200 * 4.2 J.
And to heat 200 g by (70 0 -20 0) it will take another (70-20) more - 200 * (70-20) * 4.2 J
Substituting the data, we get Q = 200 * 50 * 4.2 J = 42000 J.
Let us write the resulting formula in terms of the corresponding quantities
4. What determines the amount of heat received by a body when heated?
Please note that the amount of heat required to heat any body is proportional to the mass of the body and the change in its temperature.,
There are two cylinders of the same mass: iron and brass. Is the same amount of heat required to heat them the same number of degrees? Why?
What amount of heat is needed to heat 250 g of water from 20 o to 60 0 C.
What is the relationship between calorie and joule?
A calorie is the amount of heat required to heat 1 g of water by 1 degree.
1 cal = 4.19 = 4.2 J
1kcal=1000cal
1kcal=4190J=4200J
3. Problem solving. 28 min.
If cylinders of lead, tin and steel weighing 1 kg heated in boiling water are placed on ice, they will cool and part of the ice under them will melt. How will the internal energy of the cylinders change? Under which of the cylinders will more ice melt, under which less?
A heated stone weighing 5 kg. Cooling in water by 1 degree, it transfers 2.1 kJ of energy to it. What is the specific heat capacity of the stone?
When hardening a chisel, it was first heated to 650 0, then lowered into oil, where it cooled to 50 0 C. What amount of heat was released if its mass was 500 grams.
How much heat was used to heat a steel blank for the compressor crankshaft weighing 35 kg from 20 0 to 1220 0 C.
Independent work
What type of heat transfer?
Students fill out the table.
- The air in the room is heated through the walls.
- Through an open window into which warm air enters.
- Through glass that lets in the sun's rays.
- The earth is heated by the sun's rays.
- The liquid is heated on the stove.
- The steel spoon is heated by the tea.
- The air is heated by the candle.
- The gas moves near the fuel-generating parts of the machine.
- Heating a machine gun barrel.
- Boiling milk.
5. Homework: Peryshkin A.V. “Physics 8” § §7, 8; collection of problems 7-8 Lukashik V.I. No. 778-780, 792,793 2 min.