Specific heat capacity

physical quantity

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synonym specific heat (Specific heat) generally refers to specific heat capacity

Specific Heat Capacity, denoted by the symbol c, is also called specific heat, which is the specific heat of a substance per unit mass Heat capacity That is, the heat absorbed or released when the unit mass object changes the unit temperature.

Using the concept of specific heat capacity, we can infer the molar heat capacity that represents the heat required for 1 mol of material to increase 1 K. Molar heat capacity C under isobaric conditions p It is called molar heat capacity at constant pressure. The molar heat capacity Cv at constant volume is called the molar heat capacity at constant volume. Usually, the relationship between molar heat capacity at constant pressure and temperature is related to polynomial 。^[1]

Chinese name: Specific heat capacity
Foreign name: specific heat capacity^[2]
Discipline: thermodynamics
Alias: Specific heat capacity

Abbreviation: specific heat
Symbol: c
Calculation formula: c=Q/m(t-t zero )
International (SI) units: Joule per kg Kelvin [Jkg -1 K -1 ]^[2]
CUoM: Joule per kg ℃ [J/(kg · ℃)]

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seven material
eight historical background

application

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The specific heat capacity of water is large, and it is widely used in industrial and agricultural production and daily life. This application mainly considers two aspects. First, water of a certain quality absorbs (or emits) a lot of heat while its own temperature changes little, which is conducive to regulating the climate; Second, a certain quality of water increases (or decreases) a certain temperature, absorbing (or releasing) a lot of heat, which is conducive to water use coolant Or heating.

Adjust the climate

The specific heat capacity of water is large, which has a significant impact on climate change. Under the same condition of heating or cooling, the temperature of water changes little, and this characteristic of water Climate impact Very big, daytime coastal areas The temperature rises more slowly than that in inland areas, and the coastal temperature decreases less at night. Therefore, the temperature in coastal areas changes little in a day, while the inland temperature changes greatly. In summer, the inland is hotter than the coastal area, and in winter, the inland is colder than the coastal area. When ambient temperature When the change is fast, the temperature of water changes relatively slowly. organism Internal water The proportion of is very high, which helps to regulate the temperature of the organism itself, so as to avoid serious damage to the organism caused by too fast temperature change. Sea land breeze The causes are similar.

1. Impact on temperature

According to Xinhua News Agency, Three Gorges Reservoir After the impoundment, the world's largest Man-made lake Will become a natural "air conditioner" Mountain City Chongqing's climate is warm in winter and cool in summer. It is estimated that the temperature may drop by 5 ℃ in summer and rise by 3 to 4 ℃ in winter.

2． heat island effect Mitigation of

A sunny, windless summer day on an island Ground temperature , which is higher than the ambient sea temperature, and thus forms the sea breeze circulation and Cumulus convection This is the performance of the ocean heat island effect. The specific heat capacity of water is more than 4 times that of sand. Water and sand with the same quality will absorb more heat if they rise to the same temperature; If the heat absorbed or released is the same, the temperature change of water is much smaller than that of sand. In summer, the sun shines on the sea. Although the sea water absorbs a lot of heat, its specific heat capacity is large, so the temperature of the sea water does not change much, and the temperature at the seaside does not change much. And in desert Because the specific heat capacity of sand and stone is small, the temperature will rise a lot when it absorbs the same heat, so the temperature difference between day and night in the desert is large. The diurnal temperature difference of the coast is smaller than that of the desert, which is suitable for living. Since 2010~2013, due to urban population Concentrated, developed industry, traffic congestion, serious air pollution, and most buildings in the city are made of stone and concrete. In terms of spatial distribution of temperature, the city is like a warm island, thus forming Urban heat island effect 。 In mitigation heat island effect Experts estimate that a medium-sized city is surrounded by cities Green belt After the seedlings grow into shade, the green belt can conserve water all the year round, which is equivalent to a building with a volume of 1.14 × 10m three Due to the large specific heat capacity of water, the medium reservoirs in the urban area can reduce the high temperature in summer by more than 1 ℃, effectively alleviating the increasingly serious "heat island effect".

With the establishment of the reservoir, the water increases, and the specific heat capacity of the water is large, the temperature changes less when it is also cold and heated, so that the temperature in summer will not rise higher than in the past, and the temperature in winter will not fall lower than in the past, so that the temperature remains relatively stable, and the reservoir will become a huge "natural air conditioner".

Field of application

1. Application of water cooling system

People began to use water to cool early fever On the computer CPU Can be used in heat dissipation Heat sink And CPU core Contact, so that the heat generated by the CPU passes through heat conduction The heat is transmitted to the heat sink, and then the heat emitted into the air is taken away by the fan. However, the specific heat capacity of water is far greater than that of air, so water can be used instead of air as the heat dissipation medium, and the water with increased internal energy can be taken away by the pump to form a water cooling system. In this way, the temperature of the water will not rise significantly after the heat generated by the CPU is transferred to the water, Thermal performance It is superior to the above systems that directly use air and fans.

Weapons: the First World War and the Second World War Some weapons use water-cooled barrels; Some fighter and bomber engines use water-cooled engines.

heat engine (such as the engine of a car, the generator of a power plant, etc.) Cooling system And water Coolant It also takes advantage of the large specific heat capacity of water.

2． agricultural production Applications on

Rice is a warm crop. In order to prevent frost, farmers generally adopt“ Frequent irrigation in shallow water ”That is, in the evening, pour some water into the rice field for the night, and then drain the water from the rice field when the sun rises the next day. According to the characteristics of large specific heat capacity of water, when the temperature drops at night, the temperature of seedlings does not change much, which plays a role in heat preservation of seedlings.

3. Hot water heating

Radiators for heating in winter Warm water bag 。 The "heating" water in buildings in the north of China is used as a medium to bring the heat generated when the fuel is burned to the house for heating.

4. Others

For example, in the hot summer, the ancient royal family used water to flow down from the roof Heatstroke prevention effect.

See entry for other information Specific heat capacity at constant pressure 、 Specific heat capacity at constant volume 。

concept

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Specific heat capacity yes thermodynamics One commonly used in physical quantity , indicating the ability of a substance to increase the heat required for temperature, rather than the ability to absorb or dissipate heat. It refers to the heat absorbed (or released) by a certain substance per unit mass when it rises (or falls) per unit temperature. Its SI units are joule Per kilogram Kelvin [Jkg -1 K -1 ], that is, the heat required to raise the temperature of 1kg material by 1 Kelvin. According to this theorem, the following formula can be basically obtained:

The greater the specific heat capacity of the material, the same mass and temperature rise More heat energy is needed. Taking water and oil as examples, the specific heat capacities of water and oil are about 4200 Jkg respectively -1 K -1 And 2000 Jkg -1 K -1 That is, the heat energy of heating water with the same quality is about twice as much as that of oil. If water and oil of the same quality are heated with the same heat energy, the temperature of the oil Liter will Greater than the temperature rise of water.

Carnot theorem It is pointed out that, Reversible cycle The efficiency of heat source Is related to the temperature of the working substance (working medium) or Work Path And other factors.

Thermodynamic temperature Also called absolute temperature, it is thermodynamics And one of the important parameters in statistical physics. Generally speaking Absolute zero Is corresponding -273.15 ℃ 。

Discipline definition

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The ratio of the heat absorbed by a substance of a certain mass when the temperature rises to the product of the mass of the substance and the rising temperature is called the specific heat capacity (specific heat) of the substance, which is represented by the symbol c. his International System of Units The units in are joule Per kilogram Kelvin [J/(kg · K)], one of its common units is joules per kilogram of Celsius [J/(kg · C)]. J refers to joules, K refers to Thermodynamic temperature scale , that is, the energy required to raise (or lower) the temperature of 1 kg of material by 1 kelvin. According to this theorem, the following formula can be obtained:

Q is the heat absorbed (or released); M is Mass of object Δ T is the change of temperature after heat absorption (or heat release). In junior high school textbooks, Δ T is written as Δ t, which is not standard in fact (we often use ℃ as the unit of temperature in our life, rarely use K, and Δ T=Δ t, so we use Δ t in middle school, but still use Δ T in international or higher scientific fields).

The specific heat capacity of the material is related to the process. stay engineering application The commonly used ones are Specific heat capacity at constant pressure Cp、 Specific heat capacity at constant volume Cv and Saturation state There are three specific heat capacities.

Constant pressure specific heat capacity Cp: refers to the energy absorbed or released by a unit mass of material when the temperature rises or falls by 1 ℃ or 1K under the condition of constant pressure.

Constant specific heat capacity Cv: refers to the energy absorbed or released by a unit mass of material when the temperature rises or falls by 1 ℃ or 1K under the condition of constant volume (volume).

Saturation state Specific heat capacity: refers to the heat absorbed or released by a unit mass of material in a saturated state when the temperature rises or falls by 1 ℃ or 1K.

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The unit of specific heat capacity is the composite unit.

stay International System of Units Medium, energy, work, heat Primary unit Unify as joule , the primary unit of temperature is Kelvin Therefore, the SI unit of specific heat capacity is J·kg -1 ·K -1 , read as "joules per kilogram of kelvin".

CUoM ： J/（kg·℃） , J/(g ·℃), kJ/(kg ·℃), cal/(kg ·℃), kcal/(kg ·℃), etc. Note that Celsius and Kelvin are only Temperature scale There are some differences in the expression, and they are equivalent in the magnitude meaning of the temperature difference. Therefore, ℃ and K in these units can be replaced each other arbitrarily. For example“ Coke/kg ℃ ”And“ Joule per kilogram open ”Is equivalent.

calculation

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If an object with a mass of m absorbs (or emits) heat Δ Q in a process, and the temperature increases (or decreases) Δ T, then Δ Q/Δ T is called the object's heat capacity Quantity (heat capacity for short), expressed in C, namely

Divide heat capacity by mass to get specific heat capacity

For heat capacity and specific heat capacity of micro process

Therefore, the heat absorbed (or released) in the limited process of temperature change from T1 to T2

Generally, the heat capacity and specific heat capacity are functions of temperature, but they are not very large in the temperature range Large time , can be regarded as a constant approximately. So there is

If the temperature changes

Then there are:

This is the basic formula for calculating heat with specific heat capacity in middle school.

In English, the specific heat capacity is called Specific Heat Capacity (SHC).

The formula for calculating heat energy with specific heat capacity is: energy=mass × specific heat × temperature change

It can be abbreviated as: Energy=SHC × Mass × Temp Ch,

Specific heat related Heat calculation formula : Q=cm Δ T, that is, Q absorption (discharge)=cm (T initial - T final), where c is the specific heat, m is the mass, and Q is the energy heat. Q=cm Δ T rise (use the actual temperature rise to reduce the object Initial temperature ）Q=cm Δ T drop (after actual initial temperature drop) during heat release. Or Q=cm Δ T=cm (T end - T beginning), Q ＞ 0 is endothermic, Q ＜ 0 is exothermic.

Specific heat capacity Calculation formula Generally

C is the specific heat capacity

M is the mass of the object

t o Indicates the initial temperature of the object

T is the final temperature of the object

(△ t: object change temperature, i.e. t-t zero )

This is the formula used to calculate the temperature rise of an object. If the object is lowered, the final temperature is subtracted from the initial temperature of the object.

The calculation formula of specific heat capacity is also written

Liquid water data

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During the experiment, Liquid water Of Specific heat at constant pressure The volume is often used to calculate the heat absorbed or released. As the most common substance, the specific heat data of water is easy to obtain. When the accuracy of the experiment is not high, the specific heat of water at constant pressure can be approximately considered as 4.2kJ/(kg · ℃).

The data of constant pressure specific heat capacity Cp of liquid water at different pressures and temperatures (unit: kJ/(kg · ℃)) are given below

Constant pressure specific heat capacity of liquid water under different pressures and temperatures
pressure x10^5 Pa	Temperature (℃)
pressure x10^5 Pa	zero	twenty	fifty	one hundred	one hundred and fifty	two hundred	two hundred and fifty	three hundred	three hundred and fifty
one	four point two one seven	four point one eight two	four point one eight one	-	-	-	-	-	-
five	four point two one five	four point one eight one	four point one eight zero	four point two one five	four point three one zero	-	-	-	-
ten	four point two one two	four point one seven nine	four point one seven nine	four point two one four	four point three zero eight	-	-	-	-
fifty	four point one nine one	four point one six six	four point one seven zero	four point two zero five	four point two nine six	four point four seven seven	four point eight five five	three point two nine nine	-
one hundred	four point one six five	four point one five one	four point one five eight	four point one nine four	four point two eight one	four point four five zero	four point seven nine one	five point seven zero three	four point zero four two
one hundred and fifty	four point one four one	four point one three seven	four point one four eight	four point one eight three	four point two five one	four point four two five	four point seven three five	five point four nine five	eight point eight six three
two hundred	four point one one seven	four point one two three	four point one three seven	four point one seven three	four point two five two	four point four zero two	four point six eight five	five point three three two	eight point one zero three
two hundred and fifty	four point zero nine five	four point one zero nine	four point one two seven	four point one six three	four point two three nine	four point three seven nine	four point six three nine	five point two zero one	seven point zero one seven
three hundred	four point zero seven three	four point zero nine seven	four point one one seven	four point one five three	four point two two six	four point three five eight	four point five nine eight	five point zero nine one	six point four five one

material

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The heat released by a substance of unit mass when its temperature decreases by one degree is equal to the heat absorbed by it when its temperature rises by one degree, and is numerically equal to its specific heat capacity.

Specific heat capacity

material	Chemical symbol	Model	state	Specific heat capacity (basic) J/(kg · ℃)	Specific heat capacity (25 ℃) J/(kg · K)
hydrogen	H	two	gas	fourteen thousand	fourteen thousand and three hundred
helium	He	one	gas	five thousand one hundred and ninety	five thousand one hundred and ninety-three point two
ammonia	NH three	four	gas	two thousand and fifty-five	two thousand and fifty
neon	Ne	one	gas	one thousand and thirty	one thousand and thirty point one
lithium	Li	one	solid	three thousand five hundred and eighty	three thousand five hundred and eighty-two
ethanol	C two H five OH	nine	liquid	two thousand four hundred and sixty	two thousand four hundred and forty
gasoline	mix	mix	liquid	two thousand and two hundred	two thousand two hundred and twenty
paraffin wax	CnH two n+2	62 to 122	solid	two thousand and two hundred	two thousand and five hundred
methane	CH four	five	gas	two thousand one hundred and sixty	two thousand one hundred and fifty-six
oil	mix	mix	liquid	two thousand	two thousand
Cork stopper	mix	mix	solid	two thousand	two thousand
ethane	C two H six	eight	gas	one thousand seven hundred and thirty	one thousand seven hundred and twenty-nine
nylon	mix	mix	solid	one thousand and seven hundred	one thousand seven hundred and twenty
acetylene	C two H two	four	gas	one thousand and five hundred	one thousand five hundred and eleven
polystyrene	CH two	three	solid	one thousand and three hundred	one thousand and three hundred
hydrogen sulfide	H two S	three	gas	one thousand and one hundred	one thousand one hundred and five
nitrogen	N	two	gas	one thousand and forty	one thousand and forty-two
Air (room temperature)	mix	mix	gas	one thousand and thirty	one thousand and twelve
Air (sea level, dry, 0 ℃)	mix	mix	gas	one thousand and five	one thousand and thirty-five
oxygen	O	two	gas	nine hundred and twenty	nine hundred and eighteen
carbon dioxide	CO two	three	gas	eight hundred and forty	eight hundred and thirty-nine
carbon monoxide	CO	two	gas	one thousand and forty	one thousand and forty-two
aluminum	Al	one	solid	nine hundred	eight hundred and ninety-seven
Asbestos	mix	mix	solid	eight hundred and forty	eight hundred and forty-seven
ceramics	mix	mix	solid	eight hundred and forty	eight hundred and thirty-seven
fluorine	F	two	gas	eight hundred and twenty	eight hundred and twenty-three point nine
brick	mix	mix	solid	seven hundred and fifty	seven hundred and fifty
graphite	C	one	solid	seven hundred and twenty	seven hundred and ten
Tetrafluoromethane	CF four	five	gas	six hundred and sixty	six hundred and fifty-nine point one
sulfur dioxide	SO two	three	gas	six hundred	six hundred and twenty
Glass	mix	mix	solid	six hundred	eight hundred and forty
chlorine	Cl two	two	gas	five hundred and twenty	five hundred and twenty
Diamonds	C	one	solid	five hundred and two	five hundred and nine point one
steel	mix	mix	solid	four hundred and sixty	four hundred and fifty
iron	Fe	one	solid	four hundred and sixty	four hundred and forty-four
brass	Cu,Zn	mix	solid	three hundred and eighty	three hundred and seventy-seven
copper	Cu	one	solid	three hundred and eighty-five	three hundred and eighty-six
silver	Ag	one	solid	two hundred and thirty-five	two hundred and thirty-three
mercury	Hg	one	liquid	one hundred and thirty-nine	one hundred and forty
platinum	Pt	one	solid	one hundred and thirty-five	one hundred and thirty-five
gold	Au	one	solid	one hundred and twenty-nine	one hundred and twenty-six
lead	Pb	one	solid	one hundred and twenty-five	one hundred and twenty-eight
steam （ water ）	H two O	three	gas	one thousand eight hundred and fifty	one thousand eight hundred and fifty
water	H two O	three	liquid	four thousand and two hundred	four thousand one hundred and eighty-six
Ice (solid water ）	H two O	three	solid	two thousand and one hundred	2050 (-10℃）

Theoretically, the specific heat capacity of water is the largest among common liquid and solid substances

Explanation of the values in the table above:

(1) The unit of specific heat in this table is kJ/(kg · ℃)/J/(kg · ℃), and the two units are thousands Base 1kJ/(kg·℃)/=1×10³J/(kg·℃)

⑵ The specific heat of water is larger, and that of metal is smaller

⑶ c aluminum>c steel>c iron>c lead

Supplementary notes ：

1. Different substances have different specific heat capacities, Specific heat capacity is a kind of substance Therefore, different substances can be identified (roughly) with different specific heats (note that some substances have similar specific heats).

two The specific heat of the same substance generally does not follow The quality and shape change. For example, a cup of water and a bucket of water have the same specific heat, that is, the specific heat capacity is Strength properties 。

3. For the same substance calorific value And State of matter of The specific heat of the same substance in the same state is certain (ignored Temperature contrast But the specific heat is different in different states. For example, the specific heat of water is different from that of ice.

4. When the temperature changes, the specific heat capacity also has a small change, but generally can be ignored. The specific heat values given in the specific heat capacity table are those of these substances at room temperature average value 。

5. The specific heat capacity of gas is closely related to the thermal expansion of gas, which is different when the volume is constant and the pressure is constant, so there is Specific heat capacity at constant volume and Specific heat capacity at constant pressure Two concepts. However, there is little difference between solid and liquid, and they are generally no longer distinguished.

Specific heat capacity of common gases

(Unit: kJ/(kg · K))

Cp　 Cv

Oxygen 0.909 0.649

Hydrogen 14.05 9.934

steam 1.842　1.381

Nitrogen 1.038 0.741

historical background

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Originally in the 18th century, Scottish physicist and chemist Joseph Black It is found that different substances with the same mass need different heat to rise to the same temperature, so the concept of specific heat capacity is proposed. Almost any substance can measure the specific heat capacity, such as chemical element , compounds, alloys, solutions, and compound material 。

Historically, the heat was defined as the specific heat capacity of water, and the heat required to increase 1 degree of water per gram was defined as 1 calorie.

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