Heat capacity- this is the amount of heat absorbed by the body when heated by 1 degree.
The heat capacity of a body is indicated by capital Latin letter WITH.
What does the heat capacity of a body depend on? First of all, from its mass. It is clear that to heat, for example, 1 kilogram of water will be required more heat than for 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 large quantity it receives heat from the burner.
Thus, different amounts of heat are required to heat the same mass of different substances to the same temperature. 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 by 1°C sunflower oil the amount of heat required is 1700 J.
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, 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).
22) Melting: definition, calculation of the amount of heat for melting or solidification, specific heat of fusion, graph of t 0 (Q).
Thermodynamics
Chapter molecular physics, which studies the transfer of energy, the patterns of transformation of some types of energy into others. Unlike molecular kinetic theory, thermodynamics does not take into account internal structure substances and microparameters.
Thermodynamic system
It is a collection of bodies that exchange energy (in the form of work or heat) with each other or with environment. For example, the water in the kettle cools down, and heat is exchanged between the water and the kettle and the heat of the kettle with the environment. A cylinder with gas under the piston: the piston performs work, as a result of which the gas receives energy and its macroparameters change.
Quantity of heat
This energy, which the system receives or releases during the heat exchange process. Denoted by the symbol Q, it is measured, like any energy, in Joules.
As a result of various heat exchange processes, the energy that is transferred is determined in its own way.
Heating and cooling
This process is characterized by a change in the temperature of the system. The amount of heat is determined by the formula
Specific heat capacity of a substance with measured by the amount of heat required to warm up units of mass of this substance by 1K. Heating 1kg of glass or 1kg of water requires different amounts of energy. Specific heat capacity is a known quantity, already calculated for all substances; see the value in physical tables.
Heat capacity of substance C- this is the amount of heat that is necessary to heat a body without taking into account its mass by 1K.
Melting and crystallization
Melting is the transition of a substance from a solid to a liquid state. The reverse transition is called crystallization.
Energy that is spent on destruction crystal lattice substances, determined by the formula
The specific heat of fusion is a known value for each substance; see the value in physical tables.
Vaporization (evaporation or boiling) and condensation
Vaporization is the transition of a substance from a liquid (solid) state to a gaseous state. Reverse process called condensation.
The specific heat of vaporization is a known value for each substance; see the value in physical tables.
Combustion
The amount of heat released when a substance burns
The specific heat of combustion is a known value for each substance; see the value in physical tables.
For a closed and adiabatically isolated system of bodies, the heat balance equation is satisfied. The algebraic sum of the amounts of heat given and received by all bodies participating in heat exchange is equal to zero:
Q 1 +Q 2 +...+Q n =0
23) The structure of liquids. Surface layer. Surface tension force: examples of manifestation, calculation, surface tension coefficient.
From time to time, any molecule may move to a nearby vacant location. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely located molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called close order(Fig. 3.5.1).
The coefficient β is called temperature coefficient of volumetric expansion . This coefficient for liquids is tens of times greater than for solids. For water, for example, at a temperature of 20 °C β in ≈ 2 10 – 4 K – 1, for steel β st ≈ 3.6 10 – 5 K – 1, for quartz glass β kv ≈ 9 10 – 6 K - 1 .
The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands as the temperature decreases (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.
When water freezes, it expands, so ice remains floating on the surface of a freezing body of water. The temperature of freezing water under the ice is 0 °C. In denser layers of water at the bottom of the reservoir, the temperature is about 4 °C. Thanks to this, life can exist in the water of freezing reservoirs.
The most interesting feature of liquids is the presence free surface . Liquid, unlike gases, does not fill the entire volume of the container into which it is poured. An interface is formed between liquid and gas (or vapor), which is located in special conditions compared to the rest of the liquid mass.. It should be borne in mind that due to the extremely low compressibility, the presence of a more densely packed surface layer does not lead to any noticeable change in the volume of the liquid. If a molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depths of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must perform positive work Δ A external, proportional to the change Δ S surface area:
It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, a free drop of liquid takes on a spherical shape. The liquid behaves as if forces acting tangentially to its surface are contracting (pulling) this surface. These forces are called surface tension forces .
The presence of surface tension forces makes the surface of a liquid look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area of the liquid.
Some liquids, such as soapy water, have the ability to form thin films. Well-known soap bubbles have a regular spherical shape - this also shows the effect of surface tension forces. If a wire frame, one of whose sides is movable, is lowered into a soap solution, then the entire frame will be covered with a film of liquid (Fig. 3.5.3).
Surface tension forces tend to reduce the surface of the film. To balance the moving side of the frame, you need to apply external force If, under the influence of a force, the crossbar moves by Δ x, then work Δ will be performed A vn = F vn Δ x = Δ E p = σΔ S, where Δ S = 2LΔ x– increment in the surface area of both sides of the soap film. Since the moduli of forces and are the same, we can write:
|
Thus, the surface tension coefficient σ can be defined as modulus of the surface tension force acting per unit length of the line bounding the surface.
Due to the action of surface tension forces in liquid droplets and inside soap bubbles excess pressure Δ occurs p. If you mentally cut a spherical drop of radius R into two halves, then each of them must be in equilibrium under the action of surface tension forces applied to the cut boundary of length 2π R and strength overpressure, acting on area π R 2 sections (Fig. 3.5.4). The equilibrium condition is written as
If these forces are greater than the forces of interaction between the molecules of the liquid itself, then the liquid wets surface of a solid. In this case, the liquid approaches the surface of the solid at a certain acute angle θ, characteristic of a given liquid-solid pair. The angle θ is called contact angle . If the forces of interaction between liquid molecules exceed the forces of their interaction with solid molecules, then the contact angle θ turns out to be obtuse (Fig. 3.5.5). In this case they say that the liquid does not wet surface of a solid. At complete wettingθ = 0, at complete non-wettingθ = 180°.
Capillary phenomena called the rise or fall of liquid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.
In Fig. 3.5.6 shows a capillary tube of a certain radius r, lowered at the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of liquid in the capillary continues until the force of gravity acting on the column of liquid in the capillary becomes equal in magnitude to the resultant F n surface tension forces acting along the boundary of contact of the liquid with the surface of the capillary: F t = F n, where F t = mg = ρ hπ r 2 g, F n = σ2π r cos θ.
This implies:
With complete non-wetting θ = 180°, cos θ = –1 and, therefore, h < 0. Уровень несмачивающей жидкости в капилляре опускается ниже уровня жидкости в сосуде, в которую опущен капилляр.
Water almost completely wets the clean glass surface. On the contrary, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary drops below the level in the vessel.
24) Vaporization: definition, types (evaporation, boiling), calculation of the amount of heat for vaporization and condensation, specific heat of vaporization.
Evaporation and condensation. Explanation of the phenomenon of evaporation based on ideas about the molecular structure of matter. Specific heat of vaporization. Its units.
The phenomenon of turning a liquid into vapor is called vaporization.
Evaporation - the process of vaporization occurring from an open surface.
Liquid molecules move with at different speeds. If any molecule ends up at the surface of a liquid, it can overcome the attraction of neighboring molecules and fly out of the liquid. The ejected molecules form steam. The remaining molecules of the liquid change speed upon collision. At the same time, some molecules acquire a speed sufficient to fly out of the liquid. This process continues so the liquids evaporate slowly.
*The rate of evaporation depends on the type of liquid. Those liquids whose molecules are attracted with less force evaporate faster.
*Evaporation can occur at any temperature. But when high temperatures evaporation occurs faster .
*The rate of evaporation depends on its surface area.
*With wind (air flow), evaporation occurs faster.
During evaporation, the internal energy decreases, because During evaporation, the liquid leaves fast molecules, therefore, the average speed of the remaining molecules decreases. This means that if there is no influx of energy from outside, then the temperature of the liquid decreases.
The phenomenon of vapor turning into liquid is called condensation.
It is accompanied by the release of energy.
Steam condensation explains the formation of clouds. Water vapor rising above the ground forms clouds in the upper cold layers of air, which consist of tiny drops of water.
Specific heat of vaporization – physical a value showing how much heat is needed to convert a liquid weighing 1 kg into steam without changing temperature.
Ud. heat of vaporization denoted by the letter L and measured in J/kg
Ud. heat of vaporization of water: L=2.3×10 6 J/kg, alcohol L=0.9×10 6
Amount of heat required to convert liquid into vapor: Q = Lm
As is known, at different mechanical processes change occurs mechanical energy W meh. A measure of the change in mechanical energy is the work of forces applied to the system:
\(~\Delta W_(meh) = A.\)
During heat exchange, a change in the internal energy of the body occurs. A measure of the change in internal energy during heat transfer is the amount of heat.
Quantity of heat is a measure of the change in internal energy that a body receives (or gives up) during the process of heat exchange.
Thus, both work and the amount of heat characterize the change in energy, but are not identical to energy. They do not characterize the state of the system itself, but determine the process of energy transition from one type to another (from one body to another) when the state changes and significantly depend on the nature of the process.
The main difference between work and the amount of heat is that work characterizes the process of changing the internal energy of a system, accompanied by the transformation of energy from one type to another (from mechanical to internal). The amount of heat characterizes the process of transfer of internal energy from one body to another (from more heated to less heated), not accompanied by energy transformations.
Experience shows that the amount of heat required to heat a body mass m on temperature T 1 to temperature T 2, calculated by the formula
\(~Q = cm (T_2 - T_1) = cm \Delta T, \qquad (1)\)
Where c- specific heat capacity of the substance;
\(~c = \frac(Q)(m (T_2 - T_1)).\)
The SI unit of specific heat capacity is joule per kilogram Kelvin (J/(kg K)).
Specific heat c is numerically equal to the amount of heat that must be imparted to a body weighing 1 kg in order to heat it by 1 K.
Heat capacity body C T is numerically equal to the amount of heat required to change body temperature by 1 K:
\(~C_T = \frac(Q)(T_2 - T_1) = cm.\)
The SI unit of heat capacity of a body is joule per Kelvin (J/K).
To transform a liquid into steam at a constant temperature, it is necessary to expend an amount of heat
\(~Q = Lm, \qquad (2)\)
Where L- specific heat of vaporization. When steam condenses, the same amount of heat is released.
In order to melt a crystalline body weighing m at the melting point, the body needs to communicate the amount of heat
\(~Q = \lambda m, \qquad (3)\)
Where λ - specific heat of fusion. When a body crystallizes, the same amount of heat is released.
The amount of heat released during complete combustion of a mass of fuel m,
\(~Q = qm, \qquad (4)\)
Where q- specific heat of combustion.
The SI unit of specific heats of vaporization, melting and combustion is joule per kilogram (J/kg).
Literature
Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyhavanne, 2004. - P. 154-155.
« Physics - 10th grade"
In what processes do aggregate transformations of matter occur?
How can I change state of aggregation substances?
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 and 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.
Liquid 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 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 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.
The focus of our article is the amount of heat. We will consider the concept of internal energy, which is transformed when this quantity changes. We will also show some examples of the application of calculations in human activity.
Heat
With any word native language Each person has their own associations. They are determined personal experience and irrational feelings. What do you usually think of when you hear the word “warmth”? Soft blanket, working battery central heating winter, first sunlight spring, cat Or a mother’s look, a friend’s comforting word, timely attention.
Physicists mean a very specific term by this. And very important, especially in some sections of this complex but fascinating science.
Thermodynamics
It is not worth considering the amount of heat in isolation from the simplest processes on which the law of conservation of energy is based - nothing will be clear. Therefore, first let us remind our readers of them.
Thermodynamics considers any thing or object as a combination of a very large number of elementary parts - atoms, ions, molecules. Its equations describe any change in the collective state of the system as a whole and as a part of the whole when macroparameters change. The latter refers to temperature (denoted as T), pressure (P), concentration of components (usually C).
Internal energy
Internal energy is a rather complex term, the meaning of which is worth understanding before talking about the amount of heat. It denotes the energy that changes when the value of the macroparameters of an object increases or decreases and does not depend on the reference system. It is part of the total energy. It coincides with it in conditions when the center of mass of the thing under study is at rest (that is, there is no kinetic component).
When a person feels that an object (say a bicycle) has become hot or cold, it shows that all the molecules and atoms that make up this system, experienced a change in internal energy. However, the constant temperature does not mean the preservation of this indicator.
Work and heat
The internal energy of any thermodynamic system can be transformed in two ways:
- by doing work on it;
- during heat exchange with the environment.
The formula for this process looks like this:
dU=Q-A, where U is internal energy, Q is heat, A is work.
Let the reader not be deceived by the simplicity of the expression. The rearrangement shows that Q=dU+A, however, the introduction of entropy (S) brings the formula to the form dQ=dSxT.
Since in in this case the equation takes the form of a differential one, then the first expression requires the same. Next, depending on the forces acting in the object under study and the parameter that is being calculated, the required ratio is derived.
Let's take a metal ball as an example of a thermodynamic system. If you press on it, throw it up, drop it into a deep well, then this means doing work on it. Outwardly, all these harmless actions will not cause any harm to the ball, but its internal energy will change, albeit very slightly.
The second method is heat exchange. Now we come to the main goal of this article: a description of what the amount of heat is. This is a change in the internal energy of a thermodynamic system that occurs during heat exchange (see formula above). It is measured in joules or calories. Obviously, if the ball is held over a lighter, in the sun, or simply in warm hand, then it will heat up. And then you can use the change in temperature to find the amount of heat that was communicated to him.
Why gas is the best example of a change in internal energy, and why schoolchildren don’t like physics because of this
Above we described changes in the thermodynamic parameters of a metal ball. They are not very noticeable without special devices, and the reader can only take the word about the processes occurring with the object. It's another matter if the system is gas. Press on it - it will be visible, heat it - the pressure will rise, lower it underground - and it can be easily recorded. Therefore, in textbooks, gas is most often used as a visual thermodynamic system.
But, alas, in modern education real experiences not much attention is paid. Scientist who writes Toolkit, understands perfectly what he's talking about we're talking about. It seems to him that using the example of gas molecules, all thermodynamic parameters will be properly demonstrated. But a student who is just discovering this world is bored hearing about an ideal flask with a theoretical piston. If the school had real research laboratories and allocated hours to work in them, things would be different. So far, unfortunately, the experiments are only on paper. And, most likely, this is precisely the reason that people consider this branch of physics to be something purely theoretical, far from life and unnecessary.
Therefore, we decided to use the bicycle already mentioned above as an example. A person presses on the pedals and does work on them. In addition to imparting torque to the entire mechanism (thanks to which the bicycle moves in space), the internal energy of the materials from which the levers are made changes. The cyclist presses the handles to turn, and again does the work.
Internal energy outer covering(plastic or metal) increases. A person rides out into a clearing under the bright sun - the bicycle heats up, its amount of heat changes. Stops to rest in the shade of an old oak tree and the system cools, losing calories or joules. Increases speed - increases energy exchange. However, calculating the amount of heat in all these cases will show a very small, imperceptible value. Therefore, it seems that the manifestations of thermodynamic physics in real life No.
Application of calculations for changes in the amount of heat
The reader will probably say that all this is very educational, but why are we so tormented at school with these formulas? And now we will give examples in which areas of human activity they are directly needed and how this concerns anyone in their everyday life.
First, look around you and count: how many metal objects surround you? Probably more than ten. But before becoming a paper clip, a carriage, a ring or a flash drive, any metal undergoes smelting. Each plant that processes, say, iron ore, must understand how much fuel is required in order to optimize costs. And when calculating this, it is necessary to know the heat capacity of the metal-containing raw material and the amount of heat that needs to be imparted to it in order for everything to happen. technological processes. Since the energy released by a unit of fuel is calculated in joules or calories, the formulas are needed directly.
Or another example: most supermarkets have a department with frozen goods - fish, meat, fruit. Where raw materials from animal meat or seafood are transformed into semi-finished products, they must know how much electricity refrigeration and freezing units will consume per ton or unit of finished product. To do this, you need to calculate how much heat a kilogram of strawberries or squid loses when cooled by one degree Celsius. And in the end, this will show how much electricity a freezer of a certain power will consume.
Planes, ships, trains
Above we showed examples of relatively motionless, static objects to which a certain amount of heat is imparted or from which, on the contrary, a certain amount of heat is taken away. For objects that move in conditions of constantly changing temperature during operation, calculations of the amount of heat are important for another reason.
There is such a thing as “metal fatigue”. It also includes the utmost permissible loads at a certain rate of temperature change. Imagine an airplane taking off from the humid tropics into the frozen upper atmosphere. Engineers have to work hard to ensure that it does not fall apart due to cracks in the metal that appear when the temperature changes. They are looking for an alloy composition that can withstand real loads and have a large margin of safety. And in order not to search blindly, hoping to accidentally stumble upon the desired composition, you have to do a lot of calculations, including those that include changes in the amount of heat.
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 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 significantly faster than water. And one more question, which can also be implemented in the form of experience. Which 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 physical quantity, called the “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:
Q=cm(t_2-t_1) ,
where Q is the amount of heat,
m - body weight,
(t_2-t_1) - difference between 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, just for fun, calculate how many liters of water can be boiled with the energy they consume with food during the day.
