What do you think heats up faster on the stove: a liter of water in a saucepan or the saucepan itself weighing 1 kilogram? The mass of the bodies is the same, it can be assumed that heating will occur at the same rate.
But it wasn't there! You can do an experiment - put an empty saucepan on the fire for a few seconds, just do not burn it, and remember to what temperature it has heated up. And then pour water into the pan of exactly the same weight as the weight of the pan. In theory, the water should heat up to the same temperature as an empty pan in twice the time, since in this case they both heat up - both water and a pan.
However, even if you wait three times as long, make sure that the water is still less heated. It takes almost ten times longer for water to heat up to the same temperature as a pot of the same weight. Why is this happening? What stops water from heating up? Why should we waste extra gas to heat water when cooking? Because there is a physical quantity called the specific heat capacity of a substance.
Specific heat capacity of a substance
This value shows how much heat must be transferred to a body with a mass of one kilogram so that its temperature increases by one degree Celsius. It is measured in J / (kg * ˚С). This value exists not on a whim, but because of the difference in the properties of various substances.
The specific heat of water is about ten times that of iron, so the pot will heat up ten times faster than the water in it. It's curious that specific heat ice is half the heat capacity of water. Therefore, ice will heat up twice as fast as water. Melting ice is easier than heating water. As strange as it sounds, it is a fact.
Calculation of the amount of heat
The specific heat capacity is denoted by the letter c and used in the formula for calculating the amount of heat:
Q = c*m*(t2 - t1),
where Q is the amount of heat,
c - specific heat capacity,
m - body weight,
t2 and t1 are, respectively, the final and initial temperatures of the body.
Specific heat formula: c = Q / m*(t2 - t1)
You can also express from this formula:
- m = Q / c*(t2-t1) - body weight
- t1 = t2 - (Q / c*m) - initial body temperature
- t2 = t1 + (Q / c*m) - final body temperature
- Δt = t2 - t1 = (Q / c*m) - temperature difference (delta t)
What about the specific heat capacity of gases? Everything is more confusing here. With solids and liquids, the situation is much simpler. Their specific heat capacity is a constant, known, easily calculated value. As for the specific heat capacity of gases, this value is very different in different situations. Let's take air as an example. The specific heat capacity of air depends on the composition, humidity, and atmospheric pressure.
At the same time, with an increase in temperature, the gas increases in volume, and we need to introduce one more value - a constant or variable volume, which will also affect the heat capacity. Therefore, when calculating the amount of heat for air and other gases, special graphs of the values of the specific heat capacity of gases are used depending on various factors and conditions.
The amount of energy that must be supplied to 1 g of a substance in order to raise its temperature by 1 ° C. By definition, it takes 4.18 J to raise the temperature of 1 gram of water by 1°C. encyclopedic Dictionary.… … Ecological dictionary
specific heat- - [A.S. Goldberg. English Russian Energy Dictionary. 2006] Topics energy in general EN specific heatSH …
SPECIFIC HEAT- physical. a quantity measured by the amount of heat required to heat 1 kg of a substance by 1 K (see). The unit of specific heat capacity in SI (see) per kilogram kelvin (J kg ∙ K)) ... Great Polytechnic Encyclopedia
specific heat- savitoji šiluminė talpa statusas T sritis fizika atitikmenys: engl. heat capacity per unit mass; mass heat capacity; specific heat capacity vok. Eigenwarme, f; spezifice Wärme, f; spezifische Wärmekapazität, f rus. mass heat capacity, f;… … Fizikos terminų žodynas
See heat capacity... Great Soviet Encyclopedia
specific heat- specific heat... Dictionary of chemical synonyms I
specific heat capacity of gas- — Topics oil and gas industry EN gas specific heat … Technical Translator's Handbook
specific heat capacity of oil- — Topics oil and gas industry EN oil specific heat … Technical Translator's Handbook
specific heat capacity at constant pressure- - [A.S. Goldberg. English Russian Energy Dictionary. 2006] Topics energy in general EN specific heat at constant pressurecpconstant pressure specific heat … Technical Translator's Handbook
specific heat capacity at constant volume- - [A.S. Goldberg. English Russian Energy Dictionary. 2006] Topics energy in general EN specific heat at constant volumeconstant volume specific heatCv … Technical Translator's Handbook
Books
- Physical and geological foundations for studying the movement of water in deep horizons, Trushkin V.V. In general, the book is devoted to the law of autoregulation of water temperature with a host body, discovered by the author in 1991. At the beginning of the book, a review of the state of knowledge of the problem of movement of deep ...
In today's lesson, we will introduce such a physical concept as the specific heat capacity of a substance. We know that it depends on chemical properties substances, and its value, which can be found in the tables, is different for different substances. Then we will find out the units of measurement and the formula for finding the specific heat capacity, and also learn how to analyze the thermal properties of substances by the value of their specific heat capacity.
Calorimeter(from lat. calories- warm and metor- measure) - a device for measuring the amount of heat released or absorbed in any physical, chemical or biological process. The term "calorimeter" was proposed by A. Lavoisier and P. Laplace.
The calorimeter consists of a cover, internal and external glass. It is very important in the design of the calorimeter that there is a layer of air between the smaller and larger vessels, which, due to low thermal conductivity, provides poor heat transfer between the contents and the external environment. This design makes it possible to consider the calorimeter as a kind of thermos and practically get rid of the influence of the external environment on the course of heat transfer processes inside the calorimeter.
The calorimeter is intended for more accurate measurements of specific heat capacities and other thermal parameters of bodies than indicated in the table.
Comment. It is important to note that such a concept as the amount of heat, which we use very often, should not be confused with the internal energy of the body. The amount of heat determines precisely the change in internal energy, and not its specific value.
Note that the specific heat capacity of different substances is different, which can be seen from the table (Fig. 3). For example, gold has a specific heat capacity. As we have already pointed out earlier, the physical meaning of this specific heat capacity means that in order to heat 1 kg of gold by 1 °C, it needs to be supplied with 130 J of heat (Fig. 5).
Rice. 5. Specific heat capacity of gold
In the next lesson, we will discuss how to calculate the amount of heat.
Listliterature
- Gendenstein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
- Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
- Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.
- Internet portal "vactekh-holod.ru" ()
Homework
Heat capacity is the ability to absorb some amount of heat during heating or give it away when cooled. The heat capacity of a body is the ratio of an infinitesimal amount of heat that a body receives to the corresponding increase in its temperature indicators. The value is measured in J/K. In practice, a slightly different value is used - specific heat capacity.
Definition
What does specific heat capacity mean? This is a quantity related to a single amount of a substance. Accordingly, the amount of a substance can be measured in cubic meters, kilograms, or even in moles. What does it depend on? In physics, the heat capacity depends directly on which quantitative unit it refers to, which means that they distinguish between molar, mass and volumetric heat capacity. In the construction industry, you will not meet with molar measurements, but with others - all the time.
What affects specific heat capacity?
You know what heat capacity is, but what values \u200b\u200baffect the indicator is not yet clear. The value of specific heat capacity is directly affected by several components: the temperature of the substance, pressure and other thermodynamic characteristics.
As the temperature of the product rises, its specific heat capacity increases, however, certain substances differ in a completely non-linear curve in this dependence. For example, with an increase in temperature indicators from zero to thirty-seven degrees, the specific heat capacity of water begins to decrease, and if the limit is between thirty-seven and one hundred degrees, then the indicator, on the contrary, will increase.
It is worth noting that the parameter also depends on how the thermodynamic characteristics of the product (pressure, volume, and so on) are allowed to change. For example, the specific heat at a stable pressure and at a stable volume will be different.
How to calculate the parameter?
Are you interested in what is the heat capacity? The calculation formula is as follows: C \u003d Q / (m ΔT). What are these values? Q is the amount of heat that the product receives when heated (or released by the product during cooling). m is the mass of the product, and ΔT is the difference between the final and initial temperatures of the product. Below is a table of the heat capacity of some materials.
What can be said about the calculation of heat capacity?
Calculating the heat capacity is not an easy task, especially if only thermodynamic methods are used, it is impossible to do it more precisely. Therefore, physicists use the methods of statistical physics or knowledge of the microstructure of products. How to calculate for gas? The heat capacity of a gas is calculated from the calculation of the average energy of thermal motion of individual molecules in a substance. The movement of molecules can be translational and rotary type, and inside a molecule there can be a whole atom or a vibration of atoms. Classical statistics says that for each degree of freedom of rotational and translational movements, there is a molar value, which is equal to R / 2, and for each vibrational degree of freedom, the value is equal to R. This rule is also called the equipartition law.
In this case, a particle of a monatomic gas differs by only three translational degrees of freedom, and therefore its heat capacity should be equal to 3R/2, which is in excellent agreement with experiment. Each diatomic gas molecule has three translational, two rotational and one vibrational degrees of freedom, which means that the equipartition law will be 7R/2, and experience has shown that the heat capacity of a mole of a diatomic gas at ordinary temperature is 5R/2. Why was there such a discrepancy in theory? Everything is due to the fact that when establishing the heat capacity, it will be necessary to take into account various quantum effects, in other words, to use quantum statistics. As you can see, heat capacity is a rather complicated concept.
Quantum mechanics says that any system of particles that oscillate or rotate, including a gas molecule, can have certain discrete energy values. If the energy of thermal motion in the installed system is insufficient to excite oscillations of the required frequency, then these oscillations do not contribute to the heat capacity of the system.
In solids, the thermal motion of atoms is a weak oscillation near certain equilibrium positions, this applies to nodes crystal lattice. An atom has three vibrational degrees of freedom and, according to the law, the molar heat capacity of a solid is equal to 3nR, where n is the number of atoms present in the molecule. In practice, this value is the limit to which the heat capacity of the body tends at high temperatures. The value is achieved with normal temperature changes in many elements, this applies to metals, as well as simple compounds. The heat capacity of lead and other substances is also determined.
What can be said about low temperatures?
We already know what heat capacity is, but if we talk about low temperatures, then how will the value be calculated then? If a we are talking about low temperature indicators, then the heat capacity of a solid body then turns out to be proportional T 3 or the so-called Debye's law of heat capacity. The main criterion for distinguishing high performance temperatures from low, it is common to compare them with a parameter characteristic of a particular substance - this may be the characteristic or Debye temperature q D . The presented value is set by the vibration spectrum of atoms in the product and depends significantly on the crystal structure.
In metals, conduction electrons make a certain contribution to the heat capacity. This part of the heat capacity is calculated using the Fermi-Dirac statistics, which takes electrons into account. The electronic heat capacity of a metal, which is proportional to the usual heat capacity, is a relatively small value, and it contributes to the heat capacity of the metal only at temperatures close to absolute zero. Then the lattice heat capacity becomes very small and can be neglected.
Mass heat capacity
Mass specific heat capacity is the amount of heat that is required to be brought to a unit mass of a substance in order to heat the product per unit temperature. This value is denoted by the letter C and it is measured in joules divided by a kilogram per kelvin - J / (kg K). This is all that concerns the heat capacity of the mass.
What is volumetric heat capacity?
Volumetric heat capacity is a certain amount of heat that needs to be brought to a unit volume of production in order to heat it per unit temperature. This indicator is measured in joules divided by a cubic meter per kelvin or J / (m³ K). In many building reference books, it is the mass specific heat capacity in work that is considered.
Practical application of heat capacity in the construction industry
Many heat-intensive materials are actively used in the construction of heat-resistant walls. This is extremely important for houses that are characterized by periodic heating. For example, oven. Heat-intensive products and walls built from them perfectly accumulate heat, store it during heating periods of time and gradually release heat after the system is turned off, thus allowing you to maintain an acceptable temperature throughout the day.
So, the more heat is stored in the structure, the more comfortable and stable the temperature in the rooms will be.
It should be noted that ordinary brick and concrete used in housing construction have a significantly lower heat capacity than expanded polystyrene. If we take ecowool, then it is three times more heat-consuming than concrete. It should be noted that in the formula for calculating the heat capacity, it is not in vain that there is mass. Due to the large huge mass of concrete or brick, in comparison with ecowool, it allows accumulating huge amounts of heat in the stone walls of structures and smoothing out all daily temperature fluctuations. Only a small mass of insulation in all frame houses, despite the good heat capacity, is the weakest area for all frame technologies. To solve this problem, impressive heat accumulators are installed in all houses. What it is? These are structural parts that are characterized by a large mass with a fairly good heat capacity index.
Examples of heat accumulators in life
What could it be? For example, some internal brick walls, a large stove or fireplace, concrete screeds.
Furniture in any house or apartment is an excellent heat accumulator, because plywood, chipboard and wood can actually store heat only per kilogram of weight three times more than the notorious brick.
Are there any drawbacks to thermal storage? Of course, the main disadvantage of this approach is that the heat accumulator needs to be designed at the stage of creating a frame house layout. All due to the fact that it is very heavy, and this will need to be taken into account when creating the foundation, and then imagine how this object will be integrated into the interior. It is worth saying that it is necessary to take into account not only the mass, it will be necessary to evaluate both characteristics in the work: mass and heat capacity. For example, if you use gold with an incredible weight of twenty tons per cubic meter as a heat storage device, then the product will function as it should only twenty-three percent better than a concrete cube, which weighs two and a half tons.
Which substance is most suitable for a heat storage?
best product for a heat accumulator is not concrete and brick at all! Copper, bronze and iron do a good job of this, but they are very heavy. Oddly enough, but the best heat accumulator is water! The liquid has an impressive heat capacity, the largest among the substances available to us. Only helium gases (5190 J / (kg K) and hydrogen (14300 J / (kg K)) have more heat capacity, but they are problematic to apply in practice. If you wish and need, see the heat capacity table of the substances you need.
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Initial value
Converted value
joule per kilogram per kelvin joule per kilogram per °C joule per gram per °C kilojoule per kilogram per kelvin kilojoule per kilogram per °C calorie (IT) per gram per °C calorie (IT) per gram per °F calorie ( thr.) per gram per °C kilocalorie (th.) per kg per °C cal. (th.) per kg per °C kilocalorie (th.) per kg per kelvin kilocalorie (th.) per kg per kelvin kilogram per kelvin pound-force foot per pound per °Rankine BTU (th) per pound per °F BTU (th) per pound per °F BTU (th) per pound per °Rankine BTU (th) per pound per °Rankine BTU (IT) per pound per °C centigrade warm units per pound per °C
More about specific heat capacity
General information
Molecules move under the influence of heat - this movement is called molecular diffusion. The higher the temperature of a substance, the faster the molecules move and the more intense diffusion occurs. The movement of molecules is affected not only by temperature, but also by pressure, the viscosity of a substance and its concentration, diffusion resistance, the distance that molecules travel during their movements, and their mass. For example, if we compare how the diffusion process occurs in water and in honey, when all other variables, except for viscosity, are equal, then it is obvious that the molecules in water move and diffuse faster than in honey, since honey has a higher viscosity.
Molecules need energy to move, and the faster they move, the more energy they need. Heat is one of the types of energy used in this case. That is, if a certain temperature is maintained in a substance, then the molecules will move, and if the temperature is increased, then the movement will accelerate. Energy in the form of heat is obtained by burning fuels such as natural gas, coal, or wood. If several substances are heated using the same amount of energy, then some substances are likely to heat up faster than others due to more intense diffusion. Heat capacity and specific heat capacity describe just these properties of substances.
Specific heat determines how much energy (that is, heat) is required to change the temperature of a body or substance of a certain mass by a certain amount. This property is different from heat capacity, which determines the amount of energy required to change the temperature of an entire body or substance to a certain temperature. Heat capacity calculations, unlike specific heat capacity, do not take into account mass. Heat capacity and specific heat capacity are calculated only for substances and bodies in a stable state of aggregation, for example, for solids. This article discusses both of these concepts, as they are interrelated.
Heat capacity and specific heat capacity of materials and substances
Metals
Metals have a very strong molecular structure, since the distance between molecules in metals and other solids is much smaller than in liquids and gases. Due to this, the molecules can only move over very small distances, and, accordingly, much less energy is needed to make them move at a higher speed than for the molecules of liquids and gases. Due to this property, their specific heat capacity is low. This means that it is very easy to raise the temperature of the metal.
Water
On the other hand, water has a very high specific heat capacity, even compared to other liquids, so it takes much more energy to heat one unit mass of water by one degree, compared to substances that have a lower specific heat capacity. Water has a high heat capacity due to the strong bonds between the hydrogen atoms in the water molecule.
Water is one of the main components of all living organisms and plants on Earth, therefore its specific heat capacity plays an important role for life on our planet. Due to the high specific heat capacity of water, the temperature of the fluid in plants and the temperature of the cavity fluid in the body of animals change little even on very cold or very hot days.
Water provides a system for maintaining the thermal regime both in animals and plants, and on the surface of the Earth as a whole. A huge part of our planet is covered with water, so it is water that plays a big role in regulating weather and climate. Even with a large amount of heat coming from the impact of solar radiation on the Earth's surface, the temperature of the water in the oceans, seas and other bodies of water increases gradually, and the ambient temperature also changes slowly. On the other hand, the effect on temperature of the intensity of heat from solar radiation is large on planets where there are no large surfaces covered with water, such as the Earth, or in regions of the Earth where water is scarce. This is especially noticeable when looking at the difference between day and night temperatures. So, for example, near the ocean, the difference between day and night temperatures is small, but in the desert it is huge.
The high heat capacity of water also means that water not only heats up slowly, but also cools slowly. Due to this property, water is often used as a refrigerant, that is, as a coolant. In addition, the use of water is beneficial due to its low price. In countries with cold climates hot water circulates in pipes for heating. Mixed with ethylene glycol, it is used in car radiators to cool the engine. Such liquids are called antifreeze. The heat capacity of ethylene glycol is lower than the heat capacity of water, so the heat capacity of such a mixture is also lower, which means that the efficiency of a cooling system with antifreeze is also lower than systems with water. But this has to be put up with, since ethylene glycol does not allow water to freeze in winter and damage the channels of the car's cooling system. More ethylene glycol is added to coolants designed for colder climates.
Heat capacity in everyday life
Other things being equal, the heat capacity of materials determines how quickly they heat up. The higher the heat capacity, the more energy is needed to heat this material. That is, if two materials with different heat capacities are heated with the same amount of heat and under the same conditions, then a substance with a lower heat capacity will heat up faster. Materials with high heat capacity, on the contrary, heat up and give off heat back to environment slower.
Kitchen utensils and utensils
Most often, we choose materials for dishes and kitchen utensils based on their heat capacity. This mainly applies to items that are in direct contact with heat, such as pots, plates, baking dishes, and other similar utensils. For example, for pots and pans, it is better to use materials with a low heat capacity, such as metals. This helps the heat to transfer more easily and quickly from the heater through the pot to the food and speeds up the cooking process.
On the other hand, since materials with a high heat capacity retain heat for a long time, they are good to use for insulation, that is, when it is necessary to keep the heat of the products and prevent it from escaping into the environment or, conversely, to prevent the heat of the room from heating the chilled products. Most often, such materials are used for plates and cups in which hot or, conversely, very cold food and drinks are served. They help not only to keep the temperature of the product, but also prevent people from getting burned. Ceramic and expanded polystyrene cookware are good examples of the use of such materials.
Heat insulating food
Depending on a number of factors, such as the content of water and fat in products, their heat capacity and specific heat capacity can be different. In cooking, knowledge of the heat capacity of foods makes it possible to use some foods for insulation. If you cover other food with insulating products, they will help this food to keep warm longer under them. If the dishes under these heat-insulating products have a high heat capacity, then they slowly release heat into the environment anyway. After they warm up well, they lose heat and water even more slowly thanks to the insulating products on top. Therefore, they stay hot longer.
An example of a thermal insulating product is cheese, especially on pizza and other similar dishes. Until it melts, it allows water vapor to pass through, which allows the food underneath to cool quickly, as the water it contains evaporates and in doing so cools the food it contains. The melted cheese covers the surface of the dish and insulates the food underneath. Often under the cheese are foods with a high water content, such as sauces and vegetables. Because of this, they have a high heat capacity and keep warm for a long time, especially because they are under melted cheese, which does not release water vapor to the outside. That's why pizza out of the oven is so hot that you can easily burn yourself with sauce or vegetables, even when the dough around the edges has cooled down. The surface of the pizza under the cheese does not cool for a long time, which makes it possible to deliver the pizza to your home in a well-insulated thermal bag.
Some recipes use sauces in the same way as cheese to insulate the food underneath. The higher the fat content in the sauce, the better it isolates the products - sauces based on butter or cream are especially good in this case. This is again due to the fact that fat prevents the evaporation of water and, therefore, the removal of the heat required for evaporation.
In cooking, materials that are not suitable for food are also sometimes used for thermal insulation. Cooks in Central America, the Philippines, India, Thailand, Vietnam and many other countries often use banana leaves for this purpose. They can not only be collected in the garden, but also bought in a store or on the market - they are even imported for this purpose in countries where bananas are not grown. Sometimes aluminum foil is used for insulation purposes. Not only does it prevent water from evaporating, but it also helps keep heat inside by preventing heat transfer in the form of radiation. If you wrap the wings and other protruding parts of the bird in foil when baking, the foil will prevent them from overheating and burning.
Cooking food
Foods with a high fat content, such as cheese, have a low heat capacity. They heat up more with less energy than high heat capacity products and reach temperatures high enough for the Maillard reaction to occur. The Maillard reaction is a chemical reaction that occurs between sugars and amino acids and changes the taste and appearance products. This reaction is important in some cooking methods, such as baking bread and flour confectionery, baking foods in the oven, and frying. To increase the temperature of the food to the temperature at which this reaction takes place, high-fat foods are used in cooking.
Sugar in cooking
The specific heat capacity of sugar is even lower than that of fat. Since sugar quickly heats up to temperatures higher than the boiling point of water, working with it in the kitchen requires safety precautions, especially when making caramel or sweets. Extreme care must be taken when melting the sugar to avoid spilling it on bare skin, as the temperature of the sugar reaches 175° C (350° F) and the burn from the melted sugar will be very severe. In some cases it is necessary to check the consistency of the sugar, but this should never be done with bare hands if the sugar is heated. Often people forget how quickly and how much sugar can heat up, which is why they get burned. Depending on what the melted sugar is for, its consistency and temperature can be checked using cold water as described below.
The properties of sugar and sugar syrup change depending on the temperature at which it is cooked. Hot sugar syrup can be thin, like the thinnest honey, thick, or somewhere in between thin and thick. Recipes for sweets, caramels, and sweet sauces usually specify not only the temperature to which the sugar or syrup should be heated, but also the hardness stage of the sugar, such as the "soft ball" stage or the "hard ball" stage. The name of each stage corresponds to the consistency of the sugar. To determine the consistency, the confectioner drops a few drops of syrup into ice water cooling them down. After that, the consistency is checked by touch. So, for example, if the chilled syrup thickens, but does not harden, but remains soft and you can make a ball out of it, then it is considered that the syrup is in the “soft ball” stage. If the shape of the frozen syrup is very difficult, but still can be changed by hand, then it is in the “hard ball” stage. Confectioners often use a food thermometer and also check the consistency of sugar by hand.
food safety
Knowing the heat capacity of foods, you can determine how long they need to be cooled or heated in order to reach a temperature at which they will not spoil and at which bacteria harmful to the body die. For example, to reach a certain temperature, foods with a higher heat capacity take longer to cool or heat than foods with a low heat capacity. That is, the duration of cooking a dish depends on what products are included in it, and also on how quickly water evaporates from it. Evaporation is important because it requires a lot of energy. Often, a food thermometer is used to check the temperature of a dish or the food in it. It is especially convenient to use it during the preparation of fish, meat and poultry.
microwaves
How efficiently food is heated in a microwave oven depends, among other factors, on the specific heat of the food. The microwave radiation generated by the microwave oven's magnetron causes the molecules of water, fat and some other substances to move faster, causing the food to heat up. Fat molecules are easy to move due to their low heat capacity, and therefore fatty foods are heated to higher temperatures than foods containing a lot of water. The temperature reached may be so high that it is sufficient for the Maillard reaction. Products with a high water content do not reach such temperatures due to the high heat capacity of water, and therefore the Maillard reaction does not occur in them.
The high temperatures reached by microwave fat can cause some foods, such as bacon, to be cooked through, but these temperatures can be hazardous when used. microwave ovens, especially if you do not follow the rules for using the oven, described in the instruction manual. For example, when reheating or cooking fatty foods in the oven, you should not use plastic utensils, as even microwave utensils are not designed for the temperatures that fat reaches. Also, do not forget that fatty foods are very hot, and eat them carefully so as not to burn yourself.
Specific heat capacity of materials used in everyday life
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